I'm writing code that, when called from the command line, is redirected a file. The lines of the file (which are sent over stdin) are parsed and read. I want to be able to call a function and have it scan an int, but it seems that there are issues with residual data in scanf (I don't actually know if that's the issue, but that's all I can think of). Here is my code:
dataSetAnalysis(data, experiments);
int selection;
while(1){ //always true. The loop only breaks when the user inputs 4.
printf("DATA SET ANALYSIS\n"
"1. Show all the data.\n"
"2. Calculate the average for an experiment.\n
"3. Calculate the average across all experiments.\n
"4. Quit.\n"
"Selection:__");
switch(selection){
case 1:
displayAll(d,e);
break;
case 2:
individualAverage(d,e);
break;
case 3:
allAverage(d);
break;
case 4:
exit(0);
}
scanf("%d", &selection);
}
And this is the second half of the main method.
while(fgets(currentLine, 20, ifp) != NULL){ //while there is still data in stdin to be read
experiments[i] = currentLine; //experiment[i] points to the same value as current line. Each value in experiments[] should contain pointers to different positions in the allocated buffer array.
currentLine += 20; //currentLine points 20 characters forward in the buffer array.
int j = 0; //counter for the inner while loop
while(j<10){ //while j is less than 10. We know that there are 10 data points for each experiment
scanf("%d ", &intBuffer[j]);
data[i][j] = intBuffer[j];
j++;
}
numExperiments++; //each path through this loop represents one experiment. Here we increment its value.
i++;
}
The program loops infinitely when reaching the while loop in dataSetAnalysis() and continues printing "DATA SET ANALYSIS...." without ever stopping to accept more input over stdin. Is the problem with scanning to the selection variable?
The problem is that your stdin is not clear, You have to clear your input buffer by iterating till you find an '\n' or an enter hit.
Try use this
while('\n' != getchar())
{ }
just before you scanf , it will get rid of the infinite loop
something like
while('\n' != getchar())
{}
scanf("%d", selection);
The first and fundamental rule is 'always check the return value from an input function' (such as scanf()). They always return some indication of whether they were successful or not, and if they're unsuccessful, you should not use the values that would have been set had the function call been successful.
With scanf() et al, the correct way to use the function is to test that you got the expected number of values converted:
if (scanf("%d", &intBuffer[j]) != 1)
…handle error…
You also have the format string: "%d " in the code. That doesn't stop until you type a non-white space character after the number. This is confusing, but any white space (blank, tab, newline) in a format string means read optional white space, but the function only knows when it has finished reading white space when a non-white space character is entered.
Related
#include<stdio.h>
#include<conio.h>
int main()
{
int Dividend,divisor;
printf("Checking Divisiblity of 1st number with 2nd number \n\n") ;
printf("Enter Number \n") ;
scanf("%d",&Dividend);
printf("Enter Divisor = ");
scanf("%d",&divisor) ;
if(Dividend % divisor == 0)
{
printf("Number %d is divisible by %d",Dividend,divisor) ;
}
else
{
printf("Number %d is not divisible by %d",Dividend,divisor) ;
}
getch();
return 0;
}
Above is my code that i have written in C ;
on running this program . Only on first execution of scanf function if i give two input space seperated , the second input is going on right variable . and on hitting enter i am getting result . I am not understanding how is this happing .
When space is pressed, scanf doesn't see anything yet. Something happens only after enter is pressed. It then takes everything to the left of the space character and assigns it to the first variable, and everything to the right of the space character and assigns it to the second variable.
If you don't press the spacebar, scanf will interpret everything you type as a single number and will assign it to the first variable.
Instead what you may want to do is use the %c format specifier to read a single character at a time. You can then check if the character is a space character and if it is, you can break out of the loop. Otherwise, you can keep reading characters until you reach a space character.
stdin is line based by default. Your program gets nothing until you press enter. Then your program has the entire line of text available.
Result of this is, that scanf, getchar, fgets, etc calls will not return until you press enter. After enter press, entire line is available and the function starts to process it.
scanf is kinda special, that if you have int parsed_count = scanf("%d%d", &a, &b);, it will read two integers, no matter how many times you press enter, so you can either do
1 2<enter>
Or you can do
<enter>
1<enter>
<enter>
2<enter>
and scanf will still read these two integers (it returns early if there is parse error, which is why you need to check the return value!).
And vice versa, if there is already input available, then scanf may return immediately, so if you have this code
scanf("%d",&Dividend);
printf("Enter Divisor = ");
scanf("%d",&divisor) ;
and enter text
1 2<enter>
Then first scanf will wait for enter press, then consume the first integer, leaving 2<enter> still unread. Then there's print, and then 2nd scanf starts reading, skipping the whitespace and immediately getting 2nd integer. So you see
1 2 <- this is your typing echoed, not print from your program
Enter Divisor = <- printf printed this
If you want to take only one input per enter press, you can simply read characters until newline, because scanf leaves them there. Example loop to read until newline, or exit program at end of file/error:
while(true) {
int ch = getchar();
if (ch == EOF) exit(1);
if (ch == '\n') break;
}
I'm trying to make a program where the user inputs value to an array. What is actually required is that the program should validate against a char character. So if the user inputs a random char such as 'n' the program should tell him "You introduced a char, please input an integer: ".
How is that possible to make that without using a char variable?
for (i = 1; i <= size; i++) {
printf("Introduce the value #%d of the list: ", i);
scanf("%d", &list[i]);
if () { // I'm blocked right in this line of code.
printf("What you tried to introduce is a char, please input an integer: ");
scanf("%d", &list[i]);
}
Thanks in advance.
As #MFisherKDX says, check the return value of scanf. From the scanf man page:
These functions return the number of input items successfully matched
and assigned, which can be fewer than provided for, or even zero in
the event of an early matching failure.
The value EOF is returned if the end of input is reached before either
the first successful conversion or a matching failure occurs. EOF is
also returned if a read error occurs, in which case the error
indicator for the stream (see ferror(3)) is set, and errno is set
indicate the error.
So capturing the return value of scanf in an int variable and then comparing that variable to 1 (in your case, because you are only attempting to read 1 item) should tell you if scanf successfully read an integer value.
However, there is a nasty pitfall when using scanf that you should be aware of. If you do type n at the prompt, scanf will fail and return 0, but it will also not consume the input you typed. Which means that the next time you call scanf, it will read the same input (the n character you typed), and fail again. And it will keep doing so no matter how many times you call scanf. It always amazes me that computer science educators continue to teach scanf to students, given not only this potential pitfall, but several other pitfalls as well. I wish I had a nickel for every hour that some CS student somewhere has spent struggling to get scanf to behave the way their intuition tells them it should. I'd be retired on my own private island by now. But I digress.
One way around this particular pitfall is to check if scanf failed, and if so, to purposely consume and discard all input from stdin up to and including the next newline character or EOF, whichever comes first.
First let's look at some unfixed code that causes an infinite loop if you enter a non-integer as input:
// Typing the letter 'n' and hitting <Enter> here causes an infinite loop:
int num, status;
while (1) {
printf("Enter a number: ");
status = scanf("%d", &num);
if (status == 1)
printf("OK\n");
else
printf("Invalid number\n");
}
The above code will (after you type n and hit <Enter>), will enter an infinite loop, and just start spewing "Invalid number" over and over. Again, this is because the n you entered never gets cleared out of the input buffer.
There are a few possible ways to get around this problem, but the consensus seems to be that the most portable and reliable way to do so is as follows:
// Fixed. No more infinite loop.
int num, status;
while (1) {
printf("Enter a number: ");
status = scanf("%d", &num);
if (status == 1)
printf("OK\n");
else {
printf("Invalid number\n");
// Consume the bad input, so it doesn't keep getting re-read by scanf
int ch;
while ((ch = getchar()) != '\n' && ch != EOF) ;
if (ch == EOF) break;
}
}
The function scanf() will returns the number of elements read, so in this case it will return 1 every time it reads an int and 0 when it reads a char, so you just need to verify that return value.
Keep in mind that after reading a character it will remain in the buffer so if you use the scanf() command again it will read the character again and repeat the error. To avoid that you need to consume the character with while(getchar() != '\n');
With that in mind I modified your code so that it works properly printing an error message if a character is introduced and asking for a new int.
for (int i = 1; i <= size; i++) {
printf("Introduce the value #%d of the list: ", i);
while (!scanf("%d", &list[i])) { //verifies the return of scanf
while(getchar() != '\n'); //consumes the character in case of error
printf("What you tried to introduce is a char\n");
printf("please introduce the value #%d of the list: ", i);
}
}
Boy, this darn thing is stumping me. I want to create a loop that verifies if a user has entered an int (and not some other data type). To do this, I get the first character of the user's input using fgets(), and then I check if that character is a digit (that is the minimum part of my task for the code to break).
char input[2];
int main(){
printf("Please enter the minimum value the random number can be: ");
fgets(input, sizeof(input), stdin);
printf("%c", input[0]);
if(!isdigit(input[0])){
int escape = 0;
while(escape == 0)
printf("Try again: ");
fgets(input, sizeof(input), stdin); //This will now overwrite whatever was in 'input'
if (isdigit(input[0])) //Will keep looping back to the fgets(), asking for new input each time until you enter a number.
escape = 1;
flushInpt();
}
In the above code (assuming all the right libraries are #included), it should ask for input (which it does) then check if the first character of that input is a digit (which it does) and if it is not a digit it should enter a while loop where it prints "Try again: " with a new fgets() that waits for the user to put in new input. It stays inside that while loop until they input a digit as the first char, at which point it breaks out of the loop (this is the part it does not do).
But whenever I input a non-digit the first time, it enters the while loop as expected, but then infinitely prints "Try again: " over and over again without ever stopping at the getsf() statement to wait for new input? My question is why does it loop infinitely?
I have verified as well that the flushInpt() function is not the culprit, as the problem occurs whether that call is in the loop or not. In case you were wondering, flushInpt is just a basic loop function that walks through the input buffer and removes anything that might be there.
char ch;
void flushInpt(){
while ((ch = getchar()) != '\n' && ch != EOF)
continue;
}
You're missing curly braces:
while(escape == 0)
{ //<--
printf("Try again: ");
fgets(input, sizeof(input), stdin); //This will now overwrite whatever was in 'input'
if (isdigit(input[0])) //Will keep looping back to the fgets(), asking for new input each time until you enter a number.
escape = 1;
flushInpt();
} //<--
I guess this block is your while loop.
I've got a program here which contains a do-while loop within a specified void method. I'm trying to exit the loop within the function, so that the do-while loop actually works as it is supposed to. Except after I run the program and one of the cases occurs, the program continues to run despite my while statement stating that it should only work while(userInput != 1).
I cannot use global variables to solve this problem, as my assignment limits me on using such techniques, thus any help would be much appreciated!
Here is a snippet of my code:
void functionTest()
{
int gameOver = 0;
int userInput;
do
{
printf("please enter a number 1-3");
scanf("%d",&userInput);
switch(userInput)
{
case 1:
printf("You entered %d",userInput);
gameOver = 1;
break;
case 2:
printf("You entered %d",userInput);
gameOver = 1;
break;
case 3:
printf("You entered %d",userInput);
gameOver = 1;
break;
}
}
while(gameOver!= 1);
}
}
The problem probably lies when you use scanf(). Something that you're inputting before hitting enter is not 1, 2 or 3. Could you tell us exactly what you type when it asks you to enter a choice?
Sometimes, the standard output needs to be flushed before using a fresh scanf(). Try fflush(stdout) before the scanf line.
See older question 1 and older question 2.
EDIT:
I can reproduce the problem easily enough if I enter anything apart from "1","2" or "3"...
I would suggest, you do the following before executing the switch statement:
Add fflush(stdout) before scanf()
Accept the input as a string (%s) instead of a number. (char [] needed)
Trim the string of trailing and leading white spaces.
Convert to number using a library function
Then switch-case based on that number
The problem is that if other characters (that aren't part of an integer) are present in the input stream before an integer can be read, scanf() fails and unusable data is never cleared out... which leads to an infinite loop (where scanf() repeatedly fails to read the same characters as an integer, over and over).
So you need to read off the invalid characters when scanf() fails, or as part of the format.
A simple fix would be to change your scanf from:
scanf("%d",&userInput);
to:
scanf("%*[^0-9]%d",&userInput);
to read (and discard) any characters in the input stream that aren't digits 0-9 before reading your integer... but that still doesn't check whether scanf fails for any other reason (like a closed input stream).
You could replace it with something like this:
int scanfRes,c;
do {
scanfRes = scanf("%d",&userInput); /* try to read userInput */
/* ..then discard remainder of line */
do {
if ((c = fgetc(stdin)) == EOF)
return; /* ..return on error or EOF */
} while (c != '\n');
} while (scanfRes != 1); /* ..retry until userInput is assigned */
..which will retry scanf() until the field is assigned, discarding the remainder of the line after each attempt, and exiting the function if fgetc() encounters an error or EOF when doing so.
below is my simple code to enter a number and print it. it is inside a while(1) loop so i need to "Enter the number infinite number of time- each time it will print the number and again wait for the input".
#include<stdio.h>
int main()
{
int i;
while(1){
printf("\nenter i \n");
scanf("%d", &i);
if(i==1)
{
printf("%d \n", i);
}
}
return 0;
}
it was working fine. but suddenly i noticed that IF i ENTER a character(eg: "w") instead of number , from there it won't ask for input!!!**
it continuesly prints,
enter i
1
enter i
1
......
when i debug using GDB, i noticed that after i enter "w", that value of character "w" is not stored in &i . before i enter "w" it had 0x00000001 so that "1" is printed through out the process.
Why it doesn't ask for another input? According to my knowledge, when I enter "w" the ascii value of "w" should be stored in &i. But it doesn't happen.
If I put, "int i; " inside while loop it works fine! Why?
Please test my code in following way:
Copy and paste and run it
When "enter i" prompt will come enter 1
Second time enter "w". See what happens...
scanf with %d format specifier will read everything that "looks like a number", i.e. what satisfies the strictly defined format for a decimal representation of an integer: some optional whitespace followed by an optional sign followed by a sequence of digits. Once it encounters a character that cannot possibly be a part of a decimal representation, scanf stops reading and leaves the rest of the input data in the input stream untouched (to wait for the next scanf). If you enter just w, your scanf will find nothing that "looks like a number". It will read nothing. Instead it will report failure through its return value. Meanwhile your w will remain in the input stream, unread. The next time you try your scanf, exactly the same thing will happen again. And again, and again, and again... That w will sit in the input stream forever, causing each of your scanf calls to fail immediately and your loop to run forever (unless your uninitialized variable i by pure chance happens to start its life with the value of 1 in it).
Your assumption that entering w should make scanf to read ASCII code of w is completely incorrect. This sounds close to what %c format specifier would do, but this is not even close to what %d format specifier does. %d does not read arbitrary characters as ASCII codes.
Note also that every time you attempt to call that scanf with w sitting in the input stream, your scanf fails and leaves the value of i unchanged. If you declare your i inside the loop, the value of i will remain uninitialized and unpredictable after each unsuccessful scanf attempt. In that case the behavior of your program is undefined. It might even produce an illusion of "working fine" (whatever you might understand under that in this case).
You need to check the return value of scanf as well, as it will return the number of successfully scanned and parsed values. If it returns zero (or EOF) then you should exit the loop.
What happens when you enter e.g. the character 'w' instead of a number is that the scanf function will fail with the scanning and parsing, and return zero. But the input will not be removed from the input buffer (because it was not read), so in the next loop scanf will again read the non-numeric input and fail, and it will do this infinitely.
You can try this workaround:
int main()
{
int i;
char c;
while (1)
{
printf("enter i: ");
if (scanf("%d",&i) == 0)
scanf("%c",&c); // catch an erroneous input
else
printf("%d\n",i);
}
return 0;
}
BTW, when were you planning to break out of that (currently infinite) loop?
You need to read up on scanf(), since you seem to be basing your program around some assumptions which are wrong.
It won't parse the character since the conversion format specifier %d means "decimal integer".
Also, note that you must check the return value since I/O can fail. When you enter something which doesn't match the conversion specifier, scanf() fails to parse it.
You would probably be better of reading whole lines using fgets(), then using e.g. sscanf() to parse the line. It's much easier to get robust input-reading that way.
scanf return type can be checked and based on that inputs can be consumed using getchar to solve your problem.
Example code
int main()
{
int i;
int ch;
while(1){
printf("\nenter i \n");
if ( scanf("%d", &i) !=1 )
{
/*consume the non-numeric characters*/
for (; (ch = getchar()) != EOF && ch != '\n'; ) { }
}
if(i==1)
{
printf("%d \n", i);
}
}
return 0;
}
Description:
When scanf("%d", &i) encounters the character, it will not read it. The character will still remains in the input stream. So to consume those characters, getchar() can used. Then scanf will wait for the next input in further iteration.