CODEWITHSUNDEEP
cpointer-arithmetic
devoured elysium
devoured elysium

Reputation: 105197

Problem with strchr

I can't get why the following bit of C code doesn't work:

int obtainStringLength(char* str, char c1, char c2) {
    char* firstOcurrence = strchr(str, c1);
    char* endOcurrence = strchr(str, c2);
    return 2+(endOcurrence - firstOcurrence) / sizeof(char*);
}

The idea is to find how many characters are between c1 and c2:

printf("%d\n", obtainStringLength("abc def ghi", 'a', 'i')); //should yield 11

Unfortunately, this is always printing 1. What is the problem? Shouldn't strchr work like C#'s string.IndexOf()?

Upvotes: 1

Views: 750

Answers (4)

pmg
pmg

Reputation: 108988

Because each character occupies exactly sizeof (char) bytes; not sizeof (char*) bytes.

And sizeof (char) is, by definition 1, so you can omit it:

return 1 + (endOcurrence - firstOcurrence);

Upvotes: 1

Kos
Kos

Reputation: 72299

Division by sizeof(char*)? That's incorrect - the result of subtracting two pointers is a numerical value (ptrdiff_t) corresponding to the number of values, not a pointer or difference of addresses.

There's also the off-by-one error in calculating the length. So that last line should look like:

return 1 + (endOcurrence - firstOcurrence);

Upvotes: 6

Ben Voigt
Ben Voigt

Reputation: 283793

Your return statement has several problems, due to not understanding pointer arithmetic.

Pointer subtraction already divides by the element size, and char* was the wrong type anyway.

And you should be adding 1, not 2.

Upvotes: 2

Jonathan Wood
Jonathan Wood

Reputation: 67295

No, strchr() returns a pointer (the address of) the character being sought, or NULL if the character was not found.

That's very different from IndexOf().

Upvotes: 0

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