C: Working with strcmp

Question

As a beginner I've been playing around with some of the functions of the library string.h and have some issues regarding the function strcmp.

I wrote the program comparing two string. If they are equal it returns YES and NO otherwise.

#include <stdio.h>
#include <string.h>

int main() {

    char a[100];
    char b[15] = "hello";

    fgets(a, 100, stdin);

    int compare;

    compare = strcmp(a, b);

    if(compare == 0) {

        printf("YES");
    }
    else {

        printf("NO");
    }

    return 0;
}

After running it even when I input from the keyboard a hello I get a NO. When I add the line printf("%d", compare) it turns out that for any input I get a 1, that is, the stopping character in a is greater than the one in b.

Where is my mistake?

The problem is not with `strcmp` but with what [`fgets`](http://en.cppreference.com/w/c/io/fgets) does and how it works. — Some programmer dude, Mar 13 '17 at 14:33
It works with gcc on tio. You can [try it here](https://tio.run/nexus/c-gcc#bY@xDoMgEIZ3nuJCF0xMg0Mna7eu7dCpMQ6IqCSABnFqfHYLarRDb@DCf9/d/TefpOFqrARcB1fJ7tze0K9kpWmChqRxoJk0JIIPQuCDt8wCyxNKi/QQyjy5FJABboVSHU5Xtm6EGwiLwdMxhE0m2kphMO90z6zYlO3nh/j9XPehr9zxmuz1DOjuJkTv3bqa4Pf9haPV07S8Qg3iH/h4HtySrHCjNUBTNM3zcsEX). — izlin, Mar 13 '17 at 14:38
@izlin: That would mean tio does not provide a compliant standard library. — too honest for this site, Mar 13 '17 at 14:43
@Olaf - Or that the "input" field in the online compiler doesn't append a newline to the input :) — StoryTeller - Unslander Monica, Mar 13 '17 at 14:46
@StoryTeller: Yes, that would also be a reason. But that would also mean it differs from a compliant environment. (That's one of the reasons I don't like online compilers). — too honest for this site, Mar 13 '17 at 14:51
@Olaf - You can get a compliant environment to behave the same by inputting EOF instead of a newline, can you not? I think online compilers just exemplify that there is still much to consider even when using the stdio facilities. The input stream abstraction isn't perfectly abstract. — StoryTeller - Unslander Monica, Mar 13 '17 at 14:54
@StoryTeller: If I hit the return-key in an input field, I expect it to be handled as expected. Of course, I assume izlin did not compare apples with oranges. Maybe I assumed too much. — too honest for this site, Mar 13 '17 at 15:00
@Olaf - izlin did. Hitting return inserts a newline (just tested). — StoryTeller - Unslander Monica, Mar 13 '17 at 15:01
@StoryTeller: Ok, I retract my comment about that particular platform then. Want an orange? — too honest for this site, Mar 13 '17 at 15:03

StoryTeller - Unslander Monica · Answer 1 · 2017-03-13T14:44:02.153

4

fgets(a, 100, stdin); stores the newline you input into the buffer as well. As such a contains "hello\n". That extra character offsets the comparison.

You can either try to remove the newline by some means¹, or compare with strncmp instead.

For instance:

char *newline = strchr(a, '\n');
if (newline)
  *newline = '\0';

edited Mar 13 '17 at 14:44

answered Mar 13 '17 at 14:33

StoryTeller - Unslander Monica

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score 4 · Answer 2 · edited May 23 '17 at 12:00

4

It's not a mistake in scanning as such, but the problem is, fgets() scans and stores the trailing newline into the supplied buffer. You need to get rid of that in case you're comparing the buffer with a string literal which does not contain the terminating newline as part of it.

For stripping the trailing newline, you can use something like

size_t len = strlen(a);
if (len > 0 && a[len-1] == '\n') {
    a[--len] = '\0';
}

See this answer for more reference

Without striping out the newline, strcmp() will not announce the comparison as success.

Otherwise, you can use strncmp() and supply the size of the string literal to keep the comparison limited to the valid input.

edited May 23 '17 at 12:00

Community

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1

answered Mar 13 '17 at 14:34

Sourav Ghosh

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1

`strncmp()` is no good unless you also check the length of the input string to make sure it is as short as the literal string otherwise (for example) `hello` and `hello goodbye` would compare equal. – JeremyP Mar 13 '17 at 14:49
@JeremyP Yes, of course, that is why I mentioned _"limited to the valid input"_ – Sourav Ghosh Mar 13 '17 at 16:37

score 4 · Accepted Answer · answered Mar 13 '17 at 14:39

fgets appends the new line character corresponding to the Enter key if there is enough space in the source array.

You should remove the character before comparing the string with another string the following way

a[strcspn( a, "\n" )] = '\0';

For example

#include <stdio.h>
#include <string.h>

int main() {

    char a[100];
    char b[15] = "hello";

    fgets(a, 100, stdin);

    a[strcspn( a, "\n" )] = '\0';

    int compare;

    compare = strcmp(a, b);

    if(compare == 0) {

        printf("YES");
    }
    else {

        printf("NO");
    }

    return 0;
}

C: Working with strcmp

3 Answers3