I've been trying to look for answer myself, but I can't find one.
I want to insert a part of the programming that reads in a string like "Hello" and stores and can display it when I want, so that printf("%s", blah); produces Hello.
Here's the code part that's giving me trouble
char name[64];
scanf_s("%s", name);
printf("Your name is %s", name);
I know that printf isn't the problem; the program crashes after something is input after a prompt. Please help?
From the specification of fscanf_s() in Annex K.3.5.3.2 of the ISO/IEC 9899:2011 standard:
The fscanf_s function is equivalent to fscanf except that the c, s, and [ conversion
specifiers apply to a pair of arguments (unless assignment suppression is indicated by a
*). The first of these arguments is the same as for fscanf. That argument is
immediately followed in the argument list by the second argument, which has type
rsize_t and gives the number of elements in the array pointed to by the first argument
of the pair. If the first argument points to a scalar object, it is considered to be an array of
one element.
and:
The scanf_s function is equivalent to fscanf_s with the argument stdin
interposed before the arguments to scanf_s.
MSDN says similar things (scanf_s() and fscanf_s()).
Your code doesn't provide the length argument, so some other number is used. It isn't determinate what value it finds, so you get eccentric behaviour from the code. You need something more like this, where the newline helps ensure that the output is actually seen.
char name[64];
if (scanf_s("%s", name, sizeof(name)) == 1)
printf("Your name is %s\n", name);
I used this very often in my university classes so this should work fine in Visual Studio (tested in VS2013):
char name[64]; // the null-terminated string to be read
scanf_s("%63s", name, 64);
// 63 = the max number of symbols EXCLUDING '\0'
// 64 = the size of the string; you can also use _countof(name) instead of that number
// calling scanf_s() that way will read up to 63 symbols (even if you write more) from the console and it will automatically set name[63] = '\0'
// if the number of the actually read symbols is < 63 then '\0' will be stored in the next free position in the string
// Please note that unlike gets(), scanf() stops reading when it reaches ' ' (interval, spacebar key) not just newline terminator (the enter key)
// Also consider calling "fflush(stdin);" before the (eventual) next scanf()
Ref: https://msdn.microsoft.com/en-us/library/w40768et.aspx
The scanf_s function is equivalent to scanf except that %c, %s, and %[ conversion specifiers each expect two arguments (the usual pointer and a value of type rsize_t indicating the size of the receiving array, which may be 1 when reading with a %c into a single char)
Your code doesn't provide the size of receiving array, also the variable name is a pointer pointing to the first character of the array, so it contains the address of name[0]. Therefore your first argument name in scanf_s is correct because name is a pointer, also note that, for the second argument you can't insert the size of a pointer like sizeof(name) because it is always same. You need to specify the size of your char array (name[64]), so for the second argument you should insert sizeof(name[64]) or 64*sizeof(char).
You can correct your code as follows:
char name[64];
if (scanf_s("%s", name, sizeof(name[64])) == 1)
printf("Your name is %s\n", name);
Here is a part of code that works for me fine:
char name[64];
scanf_s("%63s", name,(unsigned)_countof(name));
printf("Your name is %s", name);
For more information here is a link:
https://learn.microsoft.com/de-de/cpp/c-runtime-library/reference/scanf-s-scanf-s-l-wscanf-s-wscanf-s-l?view=msvc-170
Best Regards
#include<stdio.h>
int main()
{
char name[64];
printf("Enter your name: ");
scanf("%s", name);
printf("Your name is %s\n", name);
return 0;
}
#include<stdio.h>
int main()
{
char name[64];
printf("Enter your name: ");
gets(name);
printf("Your name is %s\n", name);
return 0;
}
you should do this : scanf ("%63s", name);
Update:
The below code worked for me:
#include <stdio.h>
int main(void) {
char name[64];
scanf ("%63s", name);
printf("Your name is %s", name);
return 0;
}
if you are using visual studio,
go to Project properties -> Configuration Properties -> C/C++-> Preprocessor -> Preprocessor Definitions click on edit and add _CRT_SECURE_NO_WARNINGS click ok, apply the settings and run again.
Note: this is only good if you are doing your homework or something like that and it's not recommended for production.
Related
#include <stdio.h>
#include <stdlib.h>
int main() {
char a;
printf("What? \t");
scanf("%s", &a);
printf("U have to %s", a);
return 0;
}
Whenever I build and run this code and enter a value in %s, I get an error and the debug program stops working and closes. But when I use ampersand sign like this:
#include <stdio.h>
#include <stdlib.h>
int main() {
char a;
printf("What? \t");
scanf("%s", &a);
printf("U have to %s", &a);
return 0;
}
in the printf... it works. Why is that? It also differs between the format specifier, such as one doesn't need to put & (ampersand) sign in printf when one uses %c or %d in the scanf. Why does this happen and is it related to the data types and which format specifiers concludes this result?
(sorry for my bad English. I am not a native English speaker and this is my first time here).
What you have here is a classic example of code that seems to work, but for the wrong reasons.
Let's review a few things about printf and scanf. The format specifier %d is for values of type int. You can read an integer like this:
int i;
scanf("%d", &i);
And you can print it back out like this:
printf("%d\n", i);
Why does one use an & and one does not? Well, C uses what's called "pass by value". If we wrote
scanf("%d", i); /* WRONG */
we would be passing the value of i to scanf. But we don't want to pass the (old) value of i to scanf, we want scanf to read a new value, and store it into i. In other words, we want scanf to, in effect, pass the new value of i back to us. For that to work, we instead pass scanf a pointer to the variable i where we want it to store the just-read integer. That's what the & does -- it generates a pointer to i.
When we call printf, on the other hand, the regular way of passing arguments works just fine. We do want to pass i's value to printf so that it can print it out. If we were to call
printf("%d\n", &i); /* WRONG */
it wouldn't work, because printf expects an int, and here we're wrongly handing it a pointer-to-int.
So now we've learned that for integers with %d, printf wants an int and scanf wants a pointer-to-int.
Let's talk about characters. The format %c is for characters. We can read one character with scanf:
char c;
scanf("%c", &c);
And we can print it with printf:
printf("%c\n", c);
Again, the pattern is exactly the same. scanf needs a pointer, so that it can fill in the value, so we pass &c. But printf just needs the value, so we pass plain c.
Now we get to strings. A string in C is an array of characters. Also strings in C are always terminated by a special null character, '\0', that marks the end of the string. So if we wanted to declare a variable that could contain strings up to 9 characters long, we might write
char s[10];
That gives us room for 9 characters, plus the terminating '\0'.
But arrays are special in C: Whenever you pass an array to a function, or whenever you do anything that would require the "value" of the array, what you get instead (what the compiler automatically generates for you) is a pointer to the array's first element.
What this means is that to read a string with scanf and %s, we can just call:
scanf("%s", s);
"But where is the &?", you ask. "I thought you always needed an & when calling scanf!"
Well, not quite. You always need a pointer when calling scanf. And in fact, when you called scanf("%s", s), it was just as if you had written
scanf("%s", &s[0]);
When you use %s with scanf, it expects a pointer to the first of several characters, that is, a pointer to the beginning of an array of characters, where it should begin writing the string it reads. (How does it know how big the array is? What if the user types a string that's too long to fit in the array? We'll get to those points in a moment.)
You can print strings with %s too, of course, and it looks like this:
printf("%s\n", s);
This is, again, just as if you had written
printf("%s\n", &s[0]);
When you use %s with printf, it expects a pointer to the first of several characters which it should begin printing, until it finds the terminating '\0' character.
So %s is special with printf and scanf, because strings are special (because arrays are special). With %d and %c and just about every other format specifier, you usually need a & when you call scanf, and you usually don't want that & when you call printf. But with %s, you usually don't want the & for either printf or scanf.
(And if we think about it a bit more carefully, the exception is not so much that scanf and %s does not need the &. Remember, the rule is really, scanf always needs pointers. The only reason scanf and %s doesn't need an & is that when you pass an array, you get a pointer to the array's first element automatically. So the exception is really for printf and %s: printf and %s does expect a pointer, and the reason printf and %s is designed to expect a pointer is that there's no way to not give it one: it has to accept a pointer, because for strings, that's what you always end up giving it.)
So the rule with %s is that scanf expects a pointer to the first of several characters, and printf expects a pointer to the first of several characters, too.
So now, with all that background out of the way, we can look at your code. You basically wrote
char c;
scanf("%s", &c);
At first this might seem to be kinda, sorta, almost correct. scanf and %s wants a pointer to a character, and you gave it &c, which is a pointer to a character. But %s really wants a pointer to the first of several characters. But you gave it a pointer to just a single character. So when the user types a string, the first character typed will get stored in c, but the rest of the characters, and the terminating '\0', will get written to unallocated memory somewhere off to the right of variable c. They'll overwrite ("clobber") memory that was, perhaps, used for something else. This is a serious problem, but it might not become evident right away.
Finally, you tried to print things out again with printf. You first tried
printf("%s\n", c); /* WRONG */
but this didn't work at all. The reason is that %s with printf expects a pointer-to-char, but you gave it a plain char. Suppose c contains the letter 'A'. This would end up asking printf to go to address 65 and begin printing characters until it finds the terminating '\0'. Why address 65? Because 65 is the ASCII code for A. But there's probably not a proper, null-terminated string starting at address 65 in memory; in fact there's a good chance your program doesn't have permission to read from address 65 at all.
So then you tried
printf("%s\n", &c); /* ALSO WRONG */
and this seemed to work. It "worked" because, if scanf succeeded in storing a complete string into c and the unallocated memory off to the right of it, and if clobbering that memory somehow didn't cause (too many) other problems, then when you pass the pointer &c to printf, printf can find those characters, making up a string, and print them out.
So it "works", but as I said, for the wrong reasons: in the process it stomps all over memory it doesn't "own", and sooner or later, something else is going to not work as a result.
How should you have scanned and printed a string? One way is like this, as we saw before:
char s[10];
scanf("%s", s);
printf("%s\n", s);
Now when scanf gets a pointer to the first element of the array s, it has 10 characters to play with.
We really do have to worry about the possibility that the user will type more than 9 characters. But there's a fix for that: we can tell scanf how long a string it's allowed to read, how many characters it's allowed to write to the array we handed it:
scanf("%9s", s);
That 9 in there tells scanf that it's not allowed to read more than 9 characters from the user. And since 9 is less than 10, there's still room for the terminating '\0' character.
There's much more that could be said about scanf. As chqrlie noted in a comment, it's important to check its return value, to make sure it succeeded in converting as many values as you wanted it to. It's got some strange rules about whitespace. Unless you know what you're doing, you can't intermix calls to scanf with calls to other input-reading functions like getchar or fgets -- you'll get strange results. And, finally, scanf is so persnickety and (in the end) so lacking in truly useful functionality that it's not really worth using at all. But those are topics for another day, since this answer is tl;dr already.
The %s format specifier requires a pointer to a string. When used with scanf, it must be a char array with enough characters for the word you enter plus the trailing null byte that indicates the end of the string. In printf() it has to be a null-terminated char array.
Using a pointer to a char variable doesn't work, because it doesn't have room for the null byte. You're causing undefined behavior by writing outside the variable.
char word[100];
scanf("%s", word);
printf("%s\n", word);
You can use %c to read and write a single character rather than a string of multiple characters.
char letter;
scanf("%c", &letter);
printf("%c\n", letter);
In statement char a; a is a character variable & to scan a char variable use %c format specifier.
scanf("%s",a);/* %s expects base address of char buffer, not single char */
scanf(" %c",&a);/* this is correct */
If you want to scan using %s then your input should be char buffer like char buf[10]. for e.g
char a[10];
scanf("%s",a);
u don't need to put &(ampersand) sign in printf when u use %c or %d ? no need to provide address & to printf() as printf() job is to print not to scan. for e.g
char input;
scanf("%c",&input);/* here you need &, As scanf() will store input char into
address you provided i.e &input */
printf("%c",input);/*here no need &input, bcz input char already stored,
printf will just print the char*/
Well, if you print the address you can use %p.
printf("%p",a);/*a is char buffer */
Going through some entry level C programming and having trouble with this seemingly very simple logical operator. The first if statement is skipped if you enter last name Paul. Also trying to understand buffer size when using scanf_s.
int main(){
char name[25];
printf("what is your last name? ");
printf("(Please capitalize the first letter!)\n");
scanf_s(" %s", name, 2);
if ((name[0] >= 'P') && (name[0] <= 'S'))
{
printf("You must go to room 232 for your tickets\n ");
}
else
{
printf("You can get your tickets in the lobby\n");
}
return 0;
}
In your scanf_s(" %s", name, 2); statement, the third argument here specifies the size of the character array (name) into which to read the given response. I'm not sure why you have given 2 as that size, when you have declared, in the char name[25]; line, that you have an actual size of 25.
However, with that 2 value, the read will fail if you type in more than one character (try just giving "P" as input, and you will enter the if block).
From the documentation for scanf_s (bolding mine):
The buffer size includes the terminal null. You can use a width
specification field to ensure the token that's read in fits into the
buffer. When a token is too large to fit, nothing is written to the
buffer unless there's a width specification.
Also, it is always a good idea to check the return value from scanf_s (and related functions), to make sure the read was successful! Adding a few check as shown in the snippet below will show you what's going on, with various different inputs:
//...
int n = scanf_s(" %s", name, 2);
printf("Items read = %d; Name given is: %s ...\n", n, name);
//...
With this, when I enter "Paul" as input, I see the following:
what is your last name? (Please capitalize the first letter!)
Paul
Items read = 0; Name given is: ...
You can get your tickets in the lobby
Note: On the debate about whether or not to use scanf_s in place of scanf (as the Microsoft compiler advises), see here: Difference between scanf and scanf_s.
Very simple C printing question!
#include <stdio.h>
#include <conio.h>
int main() {
int Age = 0;
printf("Enter your Age\n");
scanf("%d",&Age);
char Name;
printf("Enter your Full name\n");
scanf("%s",&Name);
printf("My name is %s and I am aged %d" ,&Name,Age);
return 0;
}
When I input "blah" and 1, for some reason this returns:
"My name is Blah and I am aged 1929323232"
I presume I am misunderstanding a data format in either the scanf or the printf functions but can't work it out.
The problem is because of line
char Name;
Name is of type char. That means that it is supposed to store only one character. As a result
1. The scanf() is not able to store the input text properly (this will result in a crash in most cases or other undefined behaviour depending on the system - which judging by the output you provided is what you got)
2. (if the code didn't crash) Treating Name as a string with the %s argument in printf() essentially outputs garbage.
The type that corresponds to strings in C is char * (or char[]). Essentially, changing Name to some statically allocated char-array while performing the necessary changes in the next lines should fix your error:
char Name[256]; //allocated 256 bytes in Name array
printf("Enter your Full name\n");
scanf("%s",Name); // removed & before Name
printf("My name is %s and I am aged %d" ,Name,Age); // same here
You could also opt to go with a dynamically allocated string of type char * but I guess that's a different topic altogether.
As a general suggestion, I think you should look at pointers more closely. Especially in C, almost all string operations involve being aware of pointer mechanisms.
char name[2];
scanf("%c",name);
printf("%c",name);
I am just starting to learn C. I'm curious about the above code, what I got from the printf output, is not the same with the character I typed in. Rather the output was some funny looking symbol. Can someone explain this to me?
For the %c specifier, scanf needs the address of the location into which the character is to be stored, but printf needs the value of the character, not its address. In C, an array decays into a pointer to the first element of the array when referenced. So, the scanf is being passed the address of the first element of the name array, which is where the character will be stored; however, the printf is also being passed the address, which is wrong. The printf should be like this:
printf("%c", name[0]);
Note that the scanf argument is technically ok, it is a little weird to be passing an array, when a pointer to a single character would suffice. It would be better to declare a single character and pass its address explicitly:
char c;
scanf("%c", &c);
printf("%c", c);
On the other hand, if you were trying to read a string instead of a single character, then you should be using %s instead of %c.
Either Read a single char
char name[2];
scanf("%c",name);
printf("%c",name[0]);
Or read a string
char name[2];
scanf("%1s",name);
printf("%s",name);
You need %s since because name contains 2 elements. %c is used for single character so if you want the user to input something for e.g. "as"(without "") and the program to print it out you need %s.
char name[2];
scanf(" %s", name);
printf("%s",name);
if you give your input which contains characters less than or equal to two you will get a correct output just as your input if your input contains characters greater than 3 then it doesn't work
int main()
{
//Define Variables
char studentName;
//Print instructions to fill the data in the screen
printf("Please type in the Students name:\n");
scanf("%s", &studentName);
printf("\n\n%s", &studentName);
return 0;
}
Seeing the above code, I am only printing to screen out the first word when I type in a sentence.
I know it is a basic thing, but I am just starting with plain C.
Read scanf(3) documentation. For %s is says
s Matches a sequence of non-white-space characters; the next
pointer must be a pointer to character array that is long
enough to hold the input sequence and the terminating null
byte ('\0'), which is added automatically. The input string
stops at white space or at the maximum field width, whichever
occurs first.
So your code is wrong, because it should have an array for studentName i.e.
char studentName[32];
scanf("%s", studentName);
which is still dangerous because of possible buffer overflow (e.g. if you type a name of 32 or more letters). Using %32s instead of %s might be safer.
Take also the habit of compiling with all warnings enabled and with debugging information (i.e. if using GCC with gcc -Wall -g). Some compilers might have warned you. Learn to use your debugger (such as gdb).
Also, take the habit of ending -not starting- your printf format string with \n (or else call fflush, see fflush(3)).
Learn about undefined behavior. Your program had some! And it misses a #include <stdio.h> directive (as the first non-comment significant line).
BTW, reading existing free software code in C will also teach you many things.
There are three problems with your code:
You are writing a string into a block of memory allocated for a single character; this is undefined behavior
You are printing a string from a block of memory allocated for a single character - also an undefined behavior
You are using scanf to read a string with spaces; %s stops at the first space or end-of-line character.
One way to fix this would be using fgets, like this:
char studentName[100];
//Print instructions to fill the data in the screen
printf("Please type in the Students name:\n");
fgets(studentName, 100, stdin);
printf("\n\n%s", &studentName);
return 0;
Try scanf("%[^\n]", &studentName); instead of scanf("%s", &studentName);
This is happening because %s stops reading the input as soon as a white space is encountered.
To avoid this what you can do is declare an array of the length required for your string.
Then use this command to input the string:-
scanf("%[^\n]s",arr);
This way scanf will continue to read characters unless a '\n' is encountered, in other words you press the enter key on your keyboard. This gives a new line signal and the input stops.
int main()
{
//Define Variables
char studentName[50];
//Print instructions to fill the data in the screen
printf("Please type in the Students name:\n");
scanf("%[^\n]s", &studentName);
printf("\n\n%s", &studentName);
return 0;
}
Alternatively you can also use the gets() and puts() method. This will really ease your work if you are writing a code for a very basic problem.
[EDIT] : As dasblinkenlight has pointed out...I will also not recommend you to use the gets function since it has been deprecated.
int main()
{
//Define Variables
char studentName[50];
//Print instructions to fill the data in the screen
printf("Please type in the Students name:\n");
gets(studentName); printf("\n\n");
puts(studentName);
return 0;
}
make the changes below and try it. I added [80] after the studentName definition, to tell the compiler that studentName is an array of 80 characters (otherwise the compiler would treat it as only one char). Also, the & symbol before studentName is not necessary, because the name of the array implicitly implies a pointer.
int main()
{
//Define Variables
char studentName[80];
//Print instructions to fill the data in the screen
printf("Please type in the Students name:\n");
scanf("%s", studentName);
printf("\n\n%s", studentName);
return 0;
}
Your problem is here
char studentName;
It is a char, not a string.
Try:
Define it as an array of chars like char studenName[SIZE];.
allocating memory dynamically using malloc:
.
char buffer[MAX_SIZE];
scanf("%s", &buffer);
char * studentName = malloc (sizeof(buffer) + 1);
strcpy (studentName , buffer);