Is there a function in c that will return the index of a char in a char array?

Question

For example something like:

char values[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
char find = 'E';

int index = findIndexOf( values, find );

possible duplicate of [String.indexOf function in C](http://stackoverflow.com/questions/4824/string-indexof-function-in-c) — Ciro Santilli OurBigBook.com, Jul 29 '15 at 13:52
[`strchr()`](http://www.opengroup.org/onlinepubs/009695399/functions/strchr.html) — chaos, Sep 25 '09 at 20:21

score 57 · Accepted Answer · answered Sep 25 '09 at 20:22

57

strchr returns the pointer to the first occurrence, so to find the index, just take the offset with the starting pointer. For example:

char values[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
char find = 'E';

const char *ptr = strchr(values, find);
if(ptr) {
   int index = ptr - values;
   // do something
}

answered Sep 25 '09 at 20:22

Jesse Beder

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my question, how do I find *indexes* in a char. If I had 2 `E` s in the string? – Ahmed Can Unbay Oct 16 '17 at 23:07
2

You did not tell me what to `#include` – ctrl-alt-delor Feb 28 '20 at 23:39
@turmuka then it will only return the first occurance – Ratul Hasan Oct 09 '20 at 12:17

score 9 · Answer 2 · answered Sep 25 '09 at 20:29

9

There's also size_t strcspn(const char *str, const char *set); it returns the index of the first occurence of the character in s that is included in set:

size_t index = strcspn(values, "E");

answered Sep 25 '09 at 20:29

John Bode

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score 5 · Answer 3 · answered Sep 25 '09 at 20:23

5

int index = strchr(values,find)-values;

Note, that if there's no find found, then strchr returns NULL, so index will be negative.

answered Sep 25 '09 at 20:23

Michael Krelin - hacker

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Pointer arithmetic on `NULL` is undefined behavior. It doesn't have to return any particular value. – Konrad Borowski Dec 20 '13 at 08:20
That's true, it doesn't have to, but it does usually :). My point was that one should expect NULL pointer here. – Michael Krelin - hacker Dec 20 '13 at 14:05
Just because it appears to work for you, it doesn't mean that's right. Your code may randomly fail when pointer is high enough (I actually checked that, see https://gist.github.com/GlitchMr/8056175 (should be an infinite loop, if this would work correctly)). – Konrad Borowski Dec 20 '13 at 15:21
I didn't say it's right. And yes, that's very correct what you point out, but it is only because the behaviour is de facto defined you could do that ;) – Michael Krelin - hacker Dec 20 '13 at 18:48

score 3 · Answer 4 · answered Dec 20 '13 at 15:36

Safe index_of() function that works even when it finds nothing (returns -1 in such case).

#include <stddef.h>
#include <string.h>
ptrdiff_t index_of(const char *string, char search) {
    const char *moved_string = strchr(string, search);
    /* If not null, return the difference. */
    if (moved_string) {
        return moved_string - string;
    }
    /* Character not found. */
    return -1;
}

score 1 · Answer 5 · answered Sep 25 '09 at 20:22

1

What about strpos?

#include <string.h>

int index;
...
index = strpos(values, find);

Note that strpos expects a zero-terminated string, which means you should add a '\0' at the end. If you can't do that, you're left with a manual loop and search.

answered Sep 25 '09 at 20:22

Lasse V. Karlsen

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I don't think that exists in the C standard library, though I could be wrong. – Ed S. Sep 25 '09 at 20:27
Personally I have more confidence in the strchr answers, I'd really like one of those be marked as the answer instead. I only found one really old piece of source code using that function, and I don't know how standard it is when I consider it. – Lasse V. Karlsen Sep 25 '09 at 20:29
strpos() is not a standard C (or even C++) function. Quite common in PHP, etc... – jesup Sep 25 '09 at 20:29
Odd, but I do have a piece of C code that uses it. But as I said, I don't know how standard it is. – Lasse V. Karlsen Sep 25 '09 at 20:30
1

strpos isn't part of the C std lib - but it is probably present on most systems. – Martin Beckett Sep 25 '09 at 20:30
after looking through some other code I have found some c that uses strpos() – Josh Curren Sep 25 '09 at 20:32
+1 for reminding that we can do the manual loop. Cuz in my case i need something like lfind not the rfind which strchr will return. – Jumabek Alikhanov May 16 '17 at 01:27

score 1 · Answer 6 · answered Jun 30 '21 at 06:45

You can use strcspn() to get the index of a char in a string, or use my lousy implementation:

// Returns the index of the first occurrence of a char
int string_indexof(char ch, char *everything) {
    int everythingLength = strlen(everything);
    for (int i = 0; i < everythingLength; i++) {
        if (ch == everything[i]) {
            return i;
        }
    }

    return -1;
}

score 0 · Answer 7 · answered Sep 25 '09 at 20:25

0

You can use strchr to get a pointer to the first occurrence and the subtract that (if not null) from the original char* to get the position.

answered Sep 25 '09 at 20:25

Ed S.

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Is there a function in c that will return the index of a char in a char array?

7 Answers7

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