Question

I’m new to C, and I’m working on my own explode like function. I’m trying to count how many times a specified character occurs in a string.

int count_chars(char * string, char * chr)
{
    int count = 0;
    int i;

    for (i = 0; i < sizeof(string); i++)
    {
        if (string[i] == chr)
        {
            count++;
        }
    }

    return count;
}

It just returns 0 every time. Can anyone explain why, please? 🙂

Answer 1

Your code is hopelessly flawed. Here’s how it should look like:

int count_chars(const char* string, char ch)
{
    int count = 0;
    int i;

    // We are computing the length once at this point
    // because it is a relatively lengthy operation,
    // and we don't want to have to compute it anew
    // every time the i < length condition is checked.
    int length = strlen(string);

    for (i = 0; i < length; i++)
    {
        if (string[i] == ch)
        {
            count++;
        }
    }

    return count;
}

See this code run on example input.

Here’s what you are doing wrong:

1. Since you want to find a character, the second parameter should be a character (and not a char*), This has implications later (see #3).
2. sizeof(string) does not give you the length of the string. It gives the size (in bytes) of a pointer in your architecture, which is a constant number (e.g. 4 on 32-bit systems).
3. You are comparing some value which is not a memory address to the memory address chr points to. This is comparing apples and oranges, and will always return false, so the if will never succeed.
4. What you want to do instead is compare a character (string[i]) to the second parameter of the function (this is the reason why that one is also a char).

A “better” version of the above

Commenters below have correctly identified portions of the original answer which are not the usual way to do things in C, can result in slow code, and possibly in bugs under (admittedly extraordinary) circumstances.

Since I believe that “the” correct implementation of count_chars is probably too involved for someone who is making their first steps in C, I ‘ll just append it here and leave the initial answer almost intact.

int count_chars(const char* string, char ch)
{
    int count = 0;
    for(; *string; count += (*string++ == ch)) ;
    return count;
}

Note: I have intentionally written the loop this way to make the point that at some stage you have to draw the line between what is possible and what is preferable.

See this code run on example input.