C Scanf suddenly stopped reading in values

Question

I'm trying to run a simple C program on my Mac. It worked fine for a while but then suddenly scanf stopped working. I basically want to read in an integer value and output what was entered. No matter the integer I enter the program keeps outputting 0. I've tried the suggestions here but nothing works. I've tried running the program in both terminal and xcode but still nothing. Any ideas?

#include <stdio.h>

int main(){
  int numberOfElements = 0;
  scanf("Number of elements: %d",&numberOfElements);
  printf("%d\n",numberOfElements); //keeps returning 0 no matter the number I enter
  return 0;
}

Remember to check the return value from `scanf()` (or any of that family of functions). It would be returning 0 unless you type `Number of elements: 23456` or something similar. In fact, you need to check (practically) every input operation to make sure it worked correctly. Assuming it worked will lead to problems, sooner or later. — Jonathan Leffler, Jun 08 '13 at 00:31

jxh · Answer 1 · 2013-06-08T00:39:19.173

4

You have to print the prompt, and only scan the input:

printf("Number of elements: ");
fflush(stdout);
scanf("%d", &numberOfElements);

Generally, it is better to avoid using scanf() directly. Instead, you can obtain a line of input and then use sscanf() on the retrieved string for better control of errors.

char line[MAX_LINE];
if (fgets(line, sizeof(line), stdin) != NULL) {
    if (sscanf(line, "%d", &numberOfElements) != 1) {
        printf("You didn't input a number: %s", line);
        /* ... */
    }
}

edited Jun 08 '13 at 00:39

answered Jun 08 '13 at 00:25

jxh

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You do not require `fflush` just prior to `scanf`. The output buffer is flushed by `scanf`. – unxnut Jun 08 '13 at 00:28
@unxnut: I don't see any documentation describing that flush behavior for `scanf()`. Is it a POSIX thing? – jxh Jun 08 '13 at 00:32

score 3 · Answer 2 · answered Jun 08 '13 at 00:26

3

You should not have Number of elements: in the scanf statement. You want to change the scanf statement to:

printf ( "Number of elements: " );
scanf ("%d", &numberOfElements );

answered Jun 08 '13 at 00:26

unxnut

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Not keen on the amount of space around the parentheses...it is a style issue, but it is not conventional. Look at K&R or the C standard. – Jonathan Leffler Jun 08 '13 at 00:33

AnT stands with Russia · Accepted Answer · 2013-06-08T00:46:24.860

When you include any non-format characters into scanf/fscanf/sscanf format string, it means that you require these characters to be present in the input stream (or input string, in case of sscanf). If these characters are not present in the input, scanf fails. Space character has special meaning though, causing scanf to skip all whitespace, but not requiring any whitespace to be present.

So, in order for this scanf to succeed

scanf("Number of elements: %d", &numberOfElements);

you have to enter, literally

Number of elements: 10

or

Numberof    elements:10

or

Numberofelements:    10

or something like that. I.e. you have to type in the words Number, of, elements and the : as well. If you just enter 10, the scanf will fail, since a mere 10 does not match the requested format.

I'm sure this was not your intent. What you really wanted to do, apparently, is to print the prompt using printf and then do a simple

scanf("%d", &numberOfElements);

The aforementioned feature of scanf could be useful, but probably only in some rare cases. This means that when you see any non-format text in scanf format string, there's a good possibility that something is wrong.

Thanks everyone for the quick responses, all the answers are correct. AudreyT just gave a bit more insight into the issue. — SNV7, Jun 08 '13 at 00:39

C Scanf suddenly stopped reading in values

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