Check if and where a substring exists

Question

I saw this question but none of the answers were quite what I was looking for. I've tried strstr but it returns a pointer instead of an integer index.

I need to find if string a contains string b and if so, where it's located, kind of like the index returned by strcmp. Is there a function or easy way to do this in C?

For example, if a is "foobar" and b is "bar", then this function/method would return 3 because "bar" is at index 3 of "foobar".

Any help is appreciated!

strstr does just that - if it doesn't work as you expect it to, you should post your code here and explain the problem you have with the behavior of strstr in it. — DUman
– DUman, Commented Nov 4, 2016 at 20:48

dbush · Accepted Answer · 2016-11-04 20:52:25Z

4

You can use strstr for this, along with some pointer arithmetic.

char *result = strstr(a, b);
if (result != NULL) {
    printf("index = %tu\n", result - a);
}

Here, result points a particular number of bytes ahead of a. So if you subtract the two, that's your index.

edited Nov 4, 2016 at 20:52

answered Nov 4, 2016 at 20:50

dbush

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1 Comment

chqrlie Over a year ago

Correct. Note that Microsoft's C library and some others might not support the %lu conversion specifier. On such systems, use printf("index = %llu\n", (unsigned long long)result - a);

melpomene · Accepted Answer · 2016-11-04 20:50:16Z

1

You can easily convert the returned pointer into an index by subtracting the pointer to the beginning of the string, a:

char *p = strstr(a, b);
int i = p ? p - a : -1;

(Also, strcmp doesn't return an index.)

answered Nov 4, 2016 at 20:50

melpomene

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17 Comments

Peter Over a year ago

@MD XF - no it does not. strcmp() can only detect the nature of the difference (is one lexicographically less than, equal to, or greater than the other). It returns no information about where the difference is.

Peter Over a year ago

@melpomene - i would be better with type ptrdiff_t. That is the result of subtracting pointers - and the result is not necessarily able to be stored in an int. Otherwise, your approach is okay - if strstr(a,b), if not NULL, cannot give a result less than a, so the subtraction p-a will not give a negative value.

alvits Over a year ago

@melpomene /tmp/test foobar Foobar - result: The difference between foobar and Foobar is 32. Here's another The difference between foobar and bar is 4. The statement: printf("The difference between %s and %s is %d\n", argv[1], argv[2], strcmp(argv[1], argv[2]));. So what's wrong? Care to explain? When in doubt, test the code.

chqrlie Over a year ago

@alvits: The tutorial from programiz.com is misleading: the value 32 could be any int value greater than 0. Nothing can be assumed about the return value of strcmp(a,b); beyond the fact that it can 0 if the strings are the same, negative if a is lexicographically less than b and positive otherwise.

chqrlie Over a year ago

The Comment in MSVC source code is rather clear: Compare strings pointed by pSrt1, pSrt2. Return 0 if same, < 0 if pStr1 is less then pStr2, > 0 otherwise. Explaining the OP how a particular library computes the different values returned is counterproductive: many casual readers will believe that you are describing what it must do instead of what it happens to do. Such misconceptions are difficult to dislodge.

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Kamyar Souri · Accepted Answer · 2016-11-04 23:01:48Z

0

Here you go:

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

int main()
{
    char str1[]="foobar";
    char str2[]="bar";
    char *ptr;
    if((ptr = strstr(str1, str2)) != NULL) {
        printf("Start offset: %td\n", ptr - str1);
    }

    return 0;
}

edited Nov 4, 2016 at 23:01

answered Nov 4, 2016 at 20:51

Kamyar Souri

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3 Comments

melpomene Over a year ago

Testing will not save you from UB.

Kamyar Souri Over a year ago

My understanding is ptrdiff_t is a signed int which is what %d in printf expects. Is that true?

chqrlie Over a year ago

No: ptrdiff_t is a signed type that can be int or larger. On linux 64-bit, it is long, on Windows 64-bit, it is long long. Either use printf("Start offset: %td\n", ptr - str1); or printf("Start offset: %lld\n", (long long)(ptr - str1));

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