I'm studying about how scanf works.
After scanned other type variable, char variable stores a white-space('\n') by getchar() or scanf("%c").
To prevent this, they should clear buffer. And I did it with rewind(stdin)
though stdin is rewinded previous input value is keeping in buffer.
and I can do something with the previous value normally.(nothing runtime errors)
but if I try scanf again, scanf will scan a new value even there is a normal value in buffer.
how does scanf determine if it should scan a new value?
I found this mechanism with below code.
#include <stdio.h>
#define p stdin
int main() {
int x;
char ch;
void* A, * B, * C, * D, * E;
A = p->_Placeholder;
printf("A : %p\n", A);//first time, it shows 0000
scanf_s("%d", &x);
B = p->_Placeholder;
printf("B : %p\n", B);//after scanned something, I think it's begin point of buffer which is assigned for this process
rewind(stdin);//rewind _Placeholder
C = p->_Placeholder;
printf("C : %p\n", C);//it outputs the same value as B - length of x
D = p->_Placeholder;
printf("D : %c\n", ((char*)D)[0]);//the previous input value is printed successfully without runtime error. it means buffer is not be cleared by scanf
scanf_s("%c", &ch, 1);//BUT scanf knows the _Placeholder is not pointing new input value, so it will scan a new value from console. How??
E = p->_Placeholder;
printf("E : %p\n", E);
printf("ch : %c\n", ch);
}
You have at least three misunderstandings:
"char variable stores a white-space"
rewind(stdin) clears the buffer
_Placeholder tells you something interesting about how scanf handles whitespace
But, I'm sorry, none of these are true.
Let's review how scanf actually handles whitespace. We start with two important pieces of background information:
The newline character, \n, is in most respects an ordinary whitespace character. It occupies space in the input buffer just like any other character. It arrives in the input buffer when you press the Enter key.
When it's done parsing a %-directive, scanf always leaves unparsed input on the input stream.
Suppose you write
int a, b;
scanf("%d%d", &a, &b);
Suppose you run that code and type, as input
12 34
and then hit the Enter key. What happens?
First, the input stream (stdin) now contains six characters:
"12 34\n"
scanf first processes the first of the two %d directives you gave it. It scans the characters 1 and 2, converting them to the integer 12 and storing it in the variable a. It stops reading at the first non-digit character it sees, which is the space character between 2 and 3. The input stream is now
" 34\n"
Notice that the space character is still on the input stream.
scanf next processes the second %d directive. It doesn't immediately find a digit character, because the space character is still there. But that's okay, because like most (but not quite all) scanf format directives, %d has a secret extra power: it automatically skips whitespace characters before reading and converting an integer. So the second %d reads and discards the space character, then reads the characters 3 and 4 and converts them to the integer 34, which it stores in the variable b.
Now scanf is done. The input stream is left containing just the newline:
"\n"
Next, let's look at a slightly different — although, as we'll see, actually very similar — example. Suppose you write
int x, y;
scanf("%d", &x);
scanf("%d", &y);
Suppose you run that code and type, as input
56
78
(where that's on two lines, meaning that you hit Enter twice).
What happens now?
In this case, the input stream will end up containing these six characters:
"56\n78\n"
The first scanf call has a %d directive to process. It scans the characters 5 and 6, converting them to the integer 56 and storing it in the variable x. It stops reading at the first non-digit character it sees, which is the newline after the 6. The input stream is now
"\n78\n"
Notice that the newline character (both newline characters) are still on the input stream.
Now the second scanf call runs. It, too, has a %d directive to process. The first character on the input stream is not a digit: it's a newline. But that's okay, because %d knows how to skip whitespace. So it reads and discards the newline character, then reads the characters 7 and 8 and converts them to the integer 78, which it stores in the variable y.
Now the second scanf is done. The input stream is left containing just the newline:
"\n"
This may all have made sense, may have seemed unsurprising, may have left you feeling, "Okay, so what's the big deal?" The big deal is this: In both examples, the input was left containing that one, last newline character.
Suppose, later in your program, you have some other input to read. We now come to a hugely significant decision point:
If the next input call is another call to scanf, and if it involves one of the (many) format specifiers that has the secret extra power of also skipping whitespace, that format specifier will skip the newline, then do its job of scanning and converting whatever input comes after the newline, and the program will work as you expect.
But if the next input call is not a call to scanf, or if it's a call to scanf that involves one of the few input specifiers that does not have the secret extra power, the newline will not be "skipped", instead it will be read as actual input. If the next input call is getchar, it will read and return the newline character. If the next input call is fgets, it will read and return a blank line. If the next input call is scanf with the %c directive, it will read and return the newline. If the next input call is scanf with the %[^\n] directive, it will read an empty line. (Actually %[^\n] will read nothing in this case, because it leaves the \n still on the input.)
It's in the second case that the "extra" whitespace causes a problem. It's in the second case that you may find yourself wanting to explicitly "flush" or discard the extra whitespace.
But it turns out that the problem of flushing or discarding the extra whitespace left behind by scanf is a remarkably stubborn one. You can't portably do it by calling fflush. You can't portably do it by calling rewind. If you care about correct, portable code, you basically have three choices:
Write your own code to explicitly read and discard "extra" characters (typically, up to and including the next newline).
Don't try to intermix scanf and other calls. Don't call scanf and then, later, try to call getchar or fgets. If you call scanf and then, later, call scanf with one of the directives (such as "%c") that lacks the "secret extra power", insert an extra space before the format specifier to cause whitespace to be skipped. (That is, use " %c" instead of "%c".)
Don't use scanf at all — do all your input in terms of fgets or getchar.
See also What can I use for input conversion instead of scanf?
Addendum: scanf's handling of whitespace can often seem puzzling. If the above explanation isn't sufficient, it may help to look at some actual C code detailing how scanf works inside. (The code I'm going to show obviously isn't the exact code that's behind your system's implementation, but it will be similar.)
When it comes time for scanf to process a %d directive, you might imagine it will do something like this. (Be forewarned: this first piece of code I'm going to show you is incomplete and wrong. It's going to take me three tries to get it right.)
c = getchar();
if(isdigit(c)) {
int intval;
intval = c - '0';
while(isdigit(c = getchar())) {
intval = 10 * intval + (c - '0');
}
*next_pointer_arg = intval;
n_vals_converted++;
} else {
/* saw no digit; processing has failed */
return n_vals_converted;
}
Let's make sure we understand everything that's going on here. We've been told to process a %d directive. We read one character from the input by calling getchar(). If that character is a digit, it's the first of possibly several digits making up an integer. We read characters and, as long as they're digits, we add them to the integer value, intval, we're collecting. The conversion involves subtracting the constant '0', to convert an ASCII character code to a digit value, and successive multiplication by 10. Once we see a character that's not a digit, we're done. We store the converted value into the pointer handed to us by our caller (here schematically but approximately represented by the pointer value next_pointer_arg), and we add one to a variable n_vals_converted keeping count of how many values we've successfully scanned and converted, which will eventually be scanf's return value.
If, on the other hand, we don't even see one digit character, we've failed: we return immediately, and our return value is the number of values we've successfully scanned and converted so far (which may well be 0).
But there is actually a subtle bug here. Suppose the input stream contains
"123x"
This code will successfully scan and convert the digits 1, 2, and 3 to the integer 123, and store this value into *next_pointer_arg. But, it will have read the character x, and after the call to isdigit in the loop while(isdigit(c = getchar())) fails, the x character will have effectively been discarded: it is no longer on the input stream.
The specification for scanf says that it is not supposed to do this. The specification for scanf says that unparsed characters are supposed to be left on the input stream. If the user had actually passed the format specifier "%dx", that would mean that, after reading and parsing an integer, a literal x is expected in the input stream, and scanf is going to have to explicitly read and match that character. So it can't accidentally read and discard the x in the process of parsing a %d directive.
So we need to modify our hypothetical %d code slightly. Whenever we read a character that turns out not to be an integer, we have to literally put it back on the input stream, for somebody else to maybe read later. There's actually a function in <stdio.h> to do this, sort of the opposite of getc, called ungetc. Here is a modified version of the code:
c = getchar();
if(isdigit(c)) {
int intval;
intval = c - '0';
while(isdigit(c = getchar())) {
intval = 10 * intval + (c - '0');
}
ungetc(c, stdin); /* push non-digit character back onto input stream */
*next_pointer_arg = intval;
n_vals_converted++;
} else {
/* saw no digit; processing has failed */
ungetc(c, stdin);
return n_vals_converted;
}
You will notice that I have added two calls to ungetc, in both places in the code where, after calling getchar and then isdigit, the code has just discovered that it has read a character that is not a digit.
It might seem strange to read a character and then change your mind, meaning that you have to "unread" it. It might make more sense to peek at at an upcoming character (to determine whether or not it's a digit) without reading it. Or, having read a character and discovered that it's not a digit, if the next piece of code that's going to process that character is right here in scanf, it might make sense to just keep it in the local variable c, rather than calling ungetc to push it back on the input stream, and then later calling getchar to fetch it from the input stream a second time. But, having called out these other two possibilities, I'm just going to say that, for now, I'm going to plough ahead with the example that uses ungetc.
So far I've shown the code that you might have imagined lay behind scanf's processing of %d. But the code I've shown so far is still significantly incomplete, because it does not show the "secret extra power". It starts looking for digit characters right away; it doesn't do any skipping of leading whitespace.
Here, then, is my third and final sample fragment of %d-processing code:
/* skip leading whitespace */
while(isspace(c = getchar())) {
/* discard */
}
if(isdigit(c)) {
int intval;
intval = c - '0';
while(isdigit(c = getchar())) {
intval = 10 * intval + (c - '0');
}
ungetc(c, stdin); /* push non-digit character back onto input stream */
*next_pointer_arg = intval;
n_vals_converted++;
} else {
/* saw no digit; processing has failed */
ungetc(c, stdin);
return n_vals_converted;
}
That initial loop reads and discards characters as long as they're whitespace. Its form is very similar to the later loop that reads and processes characters as long as they're digits. The initial loop will read one more character than it seems like it should: when the isspace call fails, that means that it has just read a non whitespace character. But that's okay, because we were just about to read a character to see if it was the first digit.
[Footnotes: This code is still far from perfect. One pretty significant problem is that it doesn't have any checks for an EOF coming along in the middle of its parsing. Another problem is that it doesn't look for - or + before the digits, so it won't handle negative numbers. Yet another, more obscure problem is that, ironically, obvious-looking calls like isdigit(c) are not always correct — strictly speaking they need to be somewhat cumbersomely rendered as isdigit((unsigned char)c).]
If you're still with me, my point in all this is to illustrate these two points in a concrete way:
The reason %d is able to automatically skip leading whitespace is because (a) the specification says it's supposed to and (b) it has explicit code to do so, as my third example illustrates.
The reason scanf always leaves unprocessed input (that is, input that comes after the input it does read and process) on the input stream is because (a) again, the specification says it's supposed to and (b) its code is typically sprinkled with explicit calls to ungetc, or the equivalent, to make sure that every unprocessed character remains on the input, as my second example illustrates.
There are some problems with you approach:
you use an undocumented, implementation specific member of the FILE object _Placeholder which may or may not be available on different platforms and whose contents are implementation specific anyway.
you use scanf_s(), which is a Microsoft specific so-called secure version of scanf(): this function is optional and may not be available on all platforms. Furthermore, Microsoft's implementation does not conform to the C Standard: for example the size argument passed after &ch is documented in VS with a type of UINT whereas the C Standard specifies it as a size_t, which on 64-bit versions of Windows has a different size.
scanf() is quite tricky to use: even experienced C programmers get bitten by its many quirks and pitfalls. In your code, you test %d and %c, which behave very differently:
for %d, scanf() will first read and discard any white space characters, such as space, TAB and newlines, then read an optional sign + or -, it then expects to read at least one digit and stop when it gets a byte that is not a digit and leave this byte in the input stream, pushing it back with ungetc() or equivalent. If no digits can be read, the conversion fails and the first non digit character is left pending in the input stream, but the previous bytes are not necessarily pushed back.
processing %c is much simpler: a single byte is read and stored into the char object or the conversion fails if the stream is at end of file.
Processing %c after %d is tricky if the input stream is bound to a terminal as the user will enter a newline after the number expected for %d and this newline will be read immediately for the %c. The program can ignore white space before the byte expected for %c by inserting a space before %c in the format string: res = scanf(" %c", &ch);
To better understand the behavior of scanf(), you should output the return value of each call and the stream current position, obtained via ftell(). It is also more reliable to first set the stream to binary mode for the return value of ftell() to be exactly the number of bytes from the beginning of the file.
Here is a modified version:
#include <stdio.h>
#ifdef _MSC_VER
#include <fcntl.h>
#include <io.h>
#endif
int main() {
int x, res;
char ch;
long A, B, C, D;
#ifdef _MSC_VER
_setmode(_fileno(stdin), _O_BINARY);
#endif
A = ftell(stdin);
printf("A : %ld\n", A);
x = 0;
res = scanf_s("%d", &x);
B = ftell(stdin);
printf("B : %ld, res=%d, x=%d\n", B, res, x);
rewind(stdin);
C = ftell(stdin);
printf("C : %ld\n", C);
ch = 0;
res = scanf_s("%c", &ch, 1);
D = ftell(stdin);
printf("D : %ld, res=%d, ch=%d (%c)\n", D, res, ch, ch);
return 0;
}
Here's some code that illustrates the behavior of the %d conversion specifier; it may help understand how that aspect of scanf works. This isn't how it's actually implemented anywhere, but it follows the same rules (Updated to handle leading +/- sign, checks for overflow, etc).
#include <stdio.h>
#include <ctype.h>
#include <errno.h>
#include <limits.h>
/**
* Mimics the behavior of the scanf %d conversion specifier.
* Skips over leading whitespace, then reads and converts
* decimal digits up to the next non-digit character.
*
* Returns EOF if no non-whitespace characters are
* seen before EOF.
*
* Returns 0 if the first non-whitespace character
* is not a digit.
*
* Returns 1 if at least one decimal digit was
* read and converted.
*
* Stops reading on the first non-digit
* character, pushes that character back
* on the input stream.
*
* In the event of a signed integer overflow,
* sets errno to ERANGE.
*/
int scan_to_int( FILE *stream, int *value )
{
int conv = 0;
int tmp = 0;
int c;
int sign = 0;
/**
* Skip over leading whitespace
*/
while( ( c = fgetc( stream ) ) != EOF && isspace( c ) )
; // empty loop
/**
* If we see end of file before any non-whitespace characters,
* return EOF.
*/
if ( c == EOF )
return c;
/**
* Account for a leading sign character.
*/
if ( c == '-' || c == '+' )
{
sign = c;
c = fgetc( stream );
}
/**
* As long as we see decimal digits, read and convert them
* to an integer value. We store the value to a temporary
* variable until we're done converting - we don't want
* to update value unless we know the operation was
* successful
*/
while( c != EOF && isdigit( c ) )
{
/**
* Check for overflow. While setting errno on overflow
* isn't required by the C language definition, I'm adding
* it anyway.
*/
if ( tmp > INT_MAX / 10 - (c - '0') )
errno = ERANGE;
tmp = tmp * 10 + (c - '0');
conv = 1;
c = fgetc( stream );
}
/**
* Push the last character read back onto the input
* stream.
*/
if ( c != EOF )
ungetc( c, stream );
/**
* If we read a sign character (+ or -) but did not have a
* successful conversion, then that character was not part
* of a numeric string and we need to put it back on the
* input stream in case it's part of a non-numeric input.
*/
if ( sign && !conv )
ungetc( sign, stream );
/**
* If there was a successful read and conversion,
* update the output parameter.
*/
if ( conv )
*value = tmp * (sign == '-' ? -1 : 1);
/**
* Return 1 if the read was successful, 0 if there
* were no digits in the input.
*/
return conv;
}
/**
* Simple test program - attempts to read 1 integer from
* standard input and display it. Display any trailing
* characters in the input stream up to and including
* the next newline character.
*/
int main( void )
{
int val;
int r;
errno = 0;
/**
* Read the next item from standard input and
* attempt to convert it to an integer value.
*/
if ( (r = scan_to_int( stdin, &val )) != 1 )
printf( "Failed to read input, r = %d\n", r );
else
printf( "Read %d%s\n", val, errno == ERANGE ? " (overflow)" : "" );
/**
* If we didn't hit EOF, display the remaining
* contents of the input stream.
*/
if ( r != EOF )
{
fputs( "Remainder of input stream: {", stdout );
int c;
do {
c = fgetc( stdin );
switch( c )
{
case '\a': fputs( "\\a", stdout ); break;
case '\b': fputs( "\\b", stdout ); break;
case '\f': fputs( "\\f", stdout ); break;
case '\n': fputs( "\\n", stdout ); break;
case '\r': fputs( "\\r", stdout ); break;
case '\t': fputs( "\\t", stdout ); break;
default: fputc( c, stdout ); break;
}
} while( c != '\n' );
fputs( "}\n", stdout );
}
return 0;
}
Some examples - first, we signal EOF (in my case, by typing Ctrl-D):
$ ./convert
Failed to read input, r = -1
Next, we pass in a non-numeric string:
$ ./convert
abcd
Failed to read input, r = 0
Remainder of input stream: {abcd\n}
Since nothing was converted, the remainder of the input stream contains everything we typed (including the newline from hitting Enter).
Next, a numeric string with non-numeric trailing characters:
$ ./convert
12cd45
Read 12
Remainder of input stream: {cd45\n}
We stopped reading at 'c' - only the leading 12 is read and converted.
Several numeric strings separated by whitespace - only the first string is converted:
$ ./convert
123 456 789
Read 123
Remainder of input stream: {\t456\t789\n}
And a numeric string with leading whitespace:
$ ./convert
12345
Read 12345
Remainder of input stream: {\n}
Handle leading signs:
$ ./convert
-123abd
Read -123
Remainder of input stream: {abd\n}
$ ./convert
+456
Read 456
Remainder of input stream: {\n}
$ ./convert
-abcd
Failed to read input, r = 0
Remainder of input stream: {-abcd\n}
And, finally, we add an overflow check - note that scanf is not required to check for overflow by the C language standard, but I figured it was a useful thing to do:
$ ./convert
123456789012345678990012345667890
Read -701837006 (overflow)
Remainder of input stream: {\n}
%d, %i, %f, %s, etc., all skip over leading whitespace, since whitespace is not meaningful in those cases except to act as a separator between inputs. %c and %[ do not skip over leading whitespace, because it may be meaningful for those particular conversions (there are times when you want to know whether the character you just read is a space, or a tab, or a newline).
As Steve points out, whitespace handling in C stdio routines is and always has been a thorny problem, and no one solution always works the best, especially since different library routines handle it differently.
Related
I have wrote the following code using c programming language (Standard 89):
#include <stdio.h>
#include <stdlib.h>
int main()
{
int cc,dd;
scanf("%d/%d",&cc,&dd);
int ll;
scanf("%d",&ll);
printf("Value of ll is: %d",ll);
return 0;
}
If I submit the following as an input in one line: 4/5h I get the following output: Value of ll is: 67
So I have 2 questions;
1) where that 67 value came from? (I tried to change the input to something like 1/2t but got the same result)
According to what I have read since there is no integers in the buffer the application should wait until one is available (For example to wait for a new input)
2) When I run my code using debug mode I can see that ll value is 65 but not 67!
By typing non-digit characters in entries like "5h" or "2t" for dd, you're fouling up the read for ll in the second scanf call.
%d tells scanf to skip any leading whitespace, then to read decimal digit characters up to the first non-digit character. If you type a string like "5h" or "2t", that leading digit will be successfully converted and assigned to dd, but the trailing non-digit character will be left in the input stream, and that's fouling up the read for ll. No new value is being read into ll, you're getting whatever indeterminate value it had when the program started up.
Always check the result of scanf (and fscanf and sscanf) - if it's less than the number of inputs you expect, then you have a matching failure (you're not handling some input correctly). If it's EOF, then you have a failure on the input stream itself.
For this particular case, you can work around the problem by checking the result of scanf - if it's 0, then there's a bad character in the stream. Throw it away and try again:
int r;
while ( ( r = scanf( "%d", &ll ) ) != 1 && r != EOF )
getchar();
This will call scanf and try to read a value into ll. We expect scanf to return a 1 on a successful input, so we'll loop while the result of scanf isn't 1 (and isn't EOF, either). If the read isn't successful, we assume there's a non-digit character stuck in the input stream, so we read and discard it with the getchar call.
I want to make a loop in C that, when the program asks for an integer and the user types a non-digit character, the program asks again for an integer.
I just found the below code. but I don't understand what this means scanf("%*[^\n]%*c"). What does ^\n mean? What does the * before ^\n and c mean?
/*
This program calculate the mean score of an user 4 individual scores,
and outputs the mean and a final grade
Input: score1, score2,score2, score3
Output: Mean, FinalGrade
*/
#include <stdio.h>
//#include <stdlib.h>
int main(void){
int userScore = 0; //Stores the scores that the user inputs
float meanValue = 0.0f; //Stores the user mean of all the notes
char testChar = 'f'; //Used to avoid that the code crashes
char grade = 'E'; //Stores the final
int i = 0; //Auxiliar used in the for statement
printf("\nWelcome to the program \n Tell me if Im clever enough! \n Designed for humans \n\n\n");
printf("Enter your 4 notes between 0 and 100 to calculate your course grade\n\n");
// Asks the 4 notes.
for ( ; i<=3 ; i++ ){
printf("Please, enter your score number %d: ", i+1);
//If the note is not valid, ask for it again
//This is tests if the user input is a valid integer.
if ( ( scanf("%d%c", &userScore, &testChar)!=2 || testChar!='\n')){
i-=1;
scanf("%*[^\n]%*c");
}else{ //Enter here if the user input is an integer
if ( userScore>=0 && userScore<=100 ){
//Add the value to the mean
meanValue += userScore;
}else{ //Enter here if the user input a non valid integer
i-=1;
//scanf("%*[^\n]%*c");
}
}
}
//Calculates the mean value of the 4 scores
meanValue = meanValue/4;
// Select your final grade according to the final mean
if (meanValue>= 90 && meanValue <=100){
grade = 'A';
} else if(meanValue>= 80 && meanValue <90){
grade = 'B';
} else if (meanValue>= 70 && meanValue <80){
grade = 'C';
} else if(meanValue>= 60 && meanValue <70){
grade = 'D';
}
printf("Your final score is: %2.2f --> %c \n\n" , meanValue, grade);
return 0;
}
Breakdown of scanf("%*[^\n]%*c"):
%*[^\n] scans everything until a \n, but doesn't scan in the \n. The asterisk(*) tells it to discard whatever was scanned.
%*c scans a single character, which will be the \n left over by %*[^\n] in this case. The asterisk instructs scanf to discard the scanned character.
Both %[ and %c are format specifiers. You can see what they do here. The asterisks in both the specifiers tell scanf, not to store the data read by these format specifiers.
As #chux commented below, it will clear a single line of the stdin (Standard Input Stream) up to and including the newline character. In your case, the line with invalid input gets cleared from the stdin.
It is better to use
scanf("%*[^\n]");
scanf("%*c");
to clear the stdin. This is because, in the former case (single scanf), %*[^\n] will fail when the first character to be scanned is the \n character and the rest of the format string of the scanf will be skipped which means that the %*c will not function and thus, the \n from the input will still be in the input stream. In this case, this will not happen as even when the first scanf fails, the second one will execute as they are separate scanf statements.
You can take a string as input in C using scanf(“%s”, s). But, it accepts string only until it finds the first space.
In order to take a line as input, you can use scanf("%[^\n]%*c", s); where s is defined as char s[MAX_LEN] where MAX_LEN is the maximum size of s. Here, [] is the scanset character. ^\n stands for taking input until a newline isn't encountered. Then, with this %*c, it reads the newline character and here, the used * indicates that this newline character is discarded.
Suppose char sen[max_length] where maximum length is maximum size of sen[].
this scanf(“%[^\n]%*c”,&sen[]); will help you to get a entire sentence until the next line isn’t encountered “\n” or enter is pressed which is done with the help of “%[^\n]” here [ ] is the scan set character . the ”%*c” will read the newline character , asterisk ” * ” is used to indicate that the next line character is discarded.
%[^\n]%*c
Which will read everything up to the newline into the string you pass in, then will consume a single character (the newline) without assigning it to anything (that '*' is 'assignment suppression').
Otherwise,the newline is left in the input stream waiting to immediately terminate the the subsequent %[^\n] format directives.
The problem with adding a space character to the format directive (%[^\n]) is that the space will match any white space. So, it will eat the newline from the end of the previous input, but it will also eat any other whitespace (including multiple newlines).
Essentially this is two reads using scanf. The first part "%[^\n]"
means 'read everything that is not a newline' and this is assigned to
the character buffer str.
The second part "%*c" means 'read the next character but don't save it
anywhere'; this gets rid of the newline that we didn't read in the
first part.
https://www.sololearn.com/Discuss/2249429/what-s-the-use-of-scanf-n-c-in-c-programming
In order to take a line as input, you can use scanf("%[^\n]%*c", s); where is defined as char s[MAX_LEN] where is the maximum size of . Here, [] is the scanset character. ^\n stands for taking input until a newline isn't encountered. Then, with this %*c, it reads the newline character and here, the used * indicates that this newline character is discarded.
I'm not sure if I properly understand how flushing works in C. I just can't get it to work as described in multiple manuals and reference books. Here's an example with comments:
#include <stdio.h>
int main(void) {
int x;
char ch;
printf("Prompt: ");
scanf("%d", &x); /* I key in 67 and press Enter. At this very moment,
the input buffer should contain the ASCII codes
for the numbers 6 and 7 as well as the ASCII code
for the newline character, which is 10. The scanf
function is going to find the ASCII codes for 6
and 7, convert them to the integer 67, assign it
to the variable x and remove them from the
buffer. At this point, what still remains in the
buffer is the newline character. */
fflush(stdin); /* I want to do some more input. So, I flush the
buffer to remove anything that still might be
there, but it doesn't really look like fflush is
doing that. */
printf("Prompt: "); /* I'm not going to be able to get my hands on
the following line of code because fflush is
not doing its job properly. The remaining
newline character is going to be read into the
variable ch automatically, thus not allowing
me to enter anything from the keyboard. */
scanf("%c", &ch);
printf("x: %d, ch: %d\n", x, ch);
/*
OUTPUT:
Prompt: 67
Prompt: x: 67, ch: 10
*/
return 0;
}
Don't do fflush(stdin);, it invokes undefined behavior.
Quoting C11,
Ifstream points to an output stream or an update stream in which the most recent
operation was not input, the fflush function causes any unwritten data for that stream
to be delivered to the host environment to be written to the file; otherwise, the behavior is
undefined.
and stdin is not an output stream.
For completion's sake:
Some implementations do define fflush on input streams. Examples are Microsoft's msvcrt and GNU libc. Others, like the BSD libc, may additionally provide a separate function for purging the buffer.
These functions, beside being unportable, have some serious drawback: They are often used with the assumption that the buffer looks some particular way, e.g. has a single newline.
stdin may be connected to a file, and its buffer might contain more than just a newline. These buffered data would be skipped over, if the input stream is purged.
Further, there can be buffers outside the libc's stdio which aren't affected.
Therefore, you should explicitly read and discard the data you don't want.
Aside from fflush() not being defined on input streams as Sourav pointed out...
I key in 67 and press Enter. At this very moment,
the input buffer should contain the ASCII codes
for the numbers 6 and 7 as well as the ASCII code
for the newline character, which is 10. The scanf
function is going to find the ASCII codes for 6
and 7, convert them to the integer 67, assign it
to the variable x and remove them from the
buffer. At this point, what still remains in the
buffer is the newline character.
And if the user did input something that is not a number, other things could have happened.
If there were non-digits before the first digit, x will not be initialized. You wouldn't know, because you did not check the return value
of scanf().
The user might have anticipated the next prompt (for a character), and have entered something like 67 x, expecting the 67 to satisfy your first prompt, and the x your second. (And he won't be happy your program dropped part of his entry.)
Using *scanf() on input you cannot be sure is of the expected format (user input, as opposed to e.g. reading back something you yourself wrote with *printf()) is fragile.
So my generic advice is to not use the *scanf() functions on user input, but instead use fgets() to read user input line-wise, and parse the input in-memory at your leisure.
You have many much more powerful functions at your disposal that way, and you can handle error conditions more fine-grained (including the ability to give the full line entered in any error messages).
The below is only a rough scetch; depending on your applications, you'll want to organize this differently:
const size_t BUFFERSIZE = 1024;
char buffer[ BUFFERSIZE ];
long x;
char ch;
printf( "Prompt: " );
if ( fgets( buffer, BUFFERSIZE, stdin ) == NULL )
{
// Error occurred, use feof() / ferror() as appropriate
}
else
{
size_t len = strlen( buffer );
if ( buffer[ len - 1 ] != '\n' )
{
// The line entered was too long for the buffer,
// there is unread input. Handle accordingly, e.g.
// resizing the buffer and reading the rest.
}
else
{
// You got the whole line; blot out the newline
buffer[ len - 1 ] = '\0';
// Assuming a decimal number first
char * curr = buffer;
errno = 0;
long x = strtol( curr, &curr, 10 );
if ( errno == ERANGE )
{
// Number exceeds "long" range, handle condition
}
else if ( curr == buffer )
{
// What you got was not a number, handle condition
}
// Keep parsing until you hit end-of-string
}
}
how can I enforce following input restriction in c?
First line contains float ,
Second line contains float ,
Third line int,
after pressing enter three times in console, program should be able to read each line and put the contents in respective int,int,float variables.
After three enter key press program should not wait for user input and start validation.
some test cases
line1: 34
line2:4
line3:12
result: ok
line1:
line2:4
line3:12
result: not ok
line1: Hi
line2:4
line3:12
result: not ok
so far I used the basics
scanf("%f",&p);
scanf("%f",&r);
scanf("%d",&t);
it works fine for test case 1 and 3, but fails when I leave an empty line.
You should always check the return value of scanf.
The reason is that the return value is what scanf uses to communicate conversion errors, among other errors. For example, if your program tells scanf to expect a sequence of decimal digits, and scanf encounters something that doesn't match that pattern, the return value will indicate this failure.
The value returned will be the number of items that are successfully assigned to. For example,
char str[128];
int x, y = scanf("%d %127s", &x, str);
If y is 1, then it should be assumed that x is safe to use. If y is 2, then it should be assumed that both x and str are safe to use.
This answers part of your question. The next part is how you can go about ensuring that the input is in the form of lines. scanf doesn't strictly deal with lines; it deals with other units, such as %d being an int encoded as a sequence of decimal digits (and a sign); it'll return once the decimal digit sequence ends... There's no guarantee that the decimal digits will occupy the entirety of the line.
There are actually two problems here: leading and trailing whitespace. All format specifiers, with the exception of [, c, C, and n, will cause leading whitespace to be discarded. If you want to handle leading whitespace differently, you'll need to codify how you expect leading whitespace to be handled.
Consider that discarding user input is almost always (if not always) a bad idea. If you don't care what the remainder of the line contains, you could use something like scanf("%*[^\n]"); getchar(); to discard everything trailing up to and including the '\n' newline character... The first statement would attempt to read as many non-newline characters as possible, and the second would discard the terminating newline character. However, if you want to ensure that the input occupies the entirety of the line, then you need to test the value returned by getchar.
An example using all of these considerations:
/* Test for leading whitespace (including newlines) */
int c = getchar();
if (c != '-' && !isdigit(c)) {
/* Leading whitespace found */
}
ungetc(c);
/* Test for correct data conversion */
int x, y = scanf("%d", &x);
if (y != 1) {
/* Something non-numeric was entered */
}
/* Test for trailing newline */
c = getchar();
if (c != '\n') {
/* Trailing newline found */
}
Armed with this information, perhaps you can come up with an attempt and update your question with some code if you have any problems...
P.S. I noticed in the code you wrote, you seem to have %f and %d confused; %f is for reading into floats, and %d is for reading into ints, not the other way around...
As soon as I read line wise input, I know that fgets + sscanf must be used instead of direct scanf. Of course you can use getc/getchar as a workaround, but you can get corner cases, where I findfgets + sscanf cleaner. Example to get a float alone on a line:
char line[80], dummy[2];
float val;
if (fgets(line, sizeof(line), stdin) == NULL)...
if (sscanf(line, "%f%1s", &val, dummy) != 1)...
// Ok val was alone on the line with optional ignored blanks before and/or after
You could also add a test for loooong lines :
if ((line[0] != 0) && (line[strlen(line)-1 != '\n'))...
I want to make a loop in C that, when the program asks for an integer and the user types a non-digit character, the program asks again for an integer.
I just found the below code. but I don't understand what this means scanf("%*[^\n]%*c"). What does ^\n mean? What does the * before ^\n and c mean?
/*
This program calculate the mean score of an user 4 individual scores,
and outputs the mean and a final grade
Input: score1, score2,score2, score3
Output: Mean, FinalGrade
*/
#include <stdio.h>
//#include <stdlib.h>
int main(void){
int userScore = 0; //Stores the scores that the user inputs
float meanValue = 0.0f; //Stores the user mean of all the notes
char testChar = 'f'; //Used to avoid that the code crashes
char grade = 'E'; //Stores the final
int i = 0; //Auxiliar used in the for statement
printf("\nWelcome to the program \n Tell me if Im clever enough! \n Designed for humans \n\n\n");
printf("Enter your 4 notes between 0 and 100 to calculate your course grade\n\n");
// Asks the 4 notes.
for ( ; i<=3 ; i++ ){
printf("Please, enter your score number %d: ", i+1);
//If the note is not valid, ask for it again
//This is tests if the user input is a valid integer.
if ( ( scanf("%d%c", &userScore, &testChar)!=2 || testChar!='\n')){
i-=1;
scanf("%*[^\n]%*c");
}else{ //Enter here if the user input is an integer
if ( userScore>=0 && userScore<=100 ){
//Add the value to the mean
meanValue += userScore;
}else{ //Enter here if the user input a non valid integer
i-=1;
//scanf("%*[^\n]%*c");
}
}
}
//Calculates the mean value of the 4 scores
meanValue = meanValue/4;
// Select your final grade according to the final mean
if (meanValue>= 90 && meanValue <=100){
grade = 'A';
} else if(meanValue>= 80 && meanValue <90){
grade = 'B';
} else if (meanValue>= 70 && meanValue <80){
grade = 'C';
} else if(meanValue>= 60 && meanValue <70){
grade = 'D';
}
printf("Your final score is: %2.2f --> %c \n\n" , meanValue, grade);
return 0;
}
Breakdown of scanf("%*[^\n]%*c"):
%*[^\n] scans everything until a \n, but doesn't scan in the \n. The asterisk(*) tells it to discard whatever was scanned.
%*c scans a single character, which will be the \n left over by %*[^\n] in this case. The asterisk instructs scanf to discard the scanned character.
Both %[ and %c are format specifiers. You can see what they do here. The asterisks in both the specifiers tell scanf, not to store the data read by these format specifiers.
As #chux commented below, it will clear a single line of the stdin (Standard Input Stream) up to and including the newline character. In your case, the line with invalid input gets cleared from the stdin.
It is better to use
scanf("%*[^\n]");
scanf("%*c");
to clear the stdin. This is because, in the former case (single scanf), %*[^\n] will fail when the first character to be scanned is the \n character and the rest of the format string of the scanf will be skipped which means that the %*c will not function and thus, the \n from the input will still be in the input stream. In this case, this will not happen as even when the first scanf fails, the second one will execute as they are separate scanf statements.
You can take a string as input in C using scanf(“%s”, s). But, it accepts string only until it finds the first space.
In order to take a line as input, you can use scanf("%[^\n]%*c", s); where s is defined as char s[MAX_LEN] where MAX_LEN is the maximum size of s. Here, [] is the scanset character. ^\n stands for taking input until a newline isn't encountered. Then, with this %*c, it reads the newline character and here, the used * indicates that this newline character is discarded.
Suppose char sen[max_length] where maximum length is maximum size of sen[].
this scanf(“%[^\n]%*c”,&sen[]); will help you to get a entire sentence until the next line isn’t encountered “\n” or enter is pressed which is done with the help of “%[^\n]” here [ ] is the scan set character . the ”%*c” will read the newline character , asterisk ” * ” is used to indicate that the next line character is discarded.
%[^\n]%*c
Which will read everything up to the newline into the string you pass in, then will consume a single character (the newline) without assigning it to anything (that '*' is 'assignment suppression').
Otherwise,the newline is left in the input stream waiting to immediately terminate the the subsequent %[^\n] format directives.
The problem with adding a space character to the format directive (%[^\n]) is that the space will match any white space. So, it will eat the newline from the end of the previous input, but it will also eat any other whitespace (including multiple newlines).
Essentially this is two reads using scanf. The first part "%[^\n]"
means 'read everything that is not a newline' and this is assigned to
the character buffer str.
The second part "%*c" means 'read the next character but don't save it
anywhere'; this gets rid of the newline that we didn't read in the
first part.
https://www.sololearn.com/Discuss/2249429/what-s-the-use-of-scanf-n-c-in-c-programming
In order to take a line as input, you can use scanf("%[^\n]%*c", s); where is defined as char s[MAX_LEN] where is the maximum size of . Here, [] is the scanset character. ^\n stands for taking input until a newline isn't encountered. Then, with this %*c, it reads the newline character and here, the used * indicates that this newline character is discarded.