Modeling with Data

Tip 27: Foreach in C

24 November 11. [link]

level: medium
purpose: borrow a useful scripting language construct

Part of a series of tips on POSIX and C. Start from the tip intro page, or get 21st Century C, the book based on this series.

Last time, you saw that you can use a compound literal anywhere you would put an array or structure.

For example, here is an array of strings declared via a compound literal:

char **strings = (char*[]){"Yarn", "twine"};

The compound literal is automatically allocated, meaning that you need neither malloc nor free to bother with it. At the end of your function it just disappears.

Now let's put that in a for loop. The first element of the loop declares the array of strings, so we can use the line above. Then, we step through until we get to the NULL marker at the end. For additional comprehensibility, I'll typedef a string type, as per Tip 17.

#include <stdio.h>

typedef char* string;

int main(){
    string str = "thread";
    for (string *list = (string[]){"yarn", str, "rope", NULL}; *list; list++)
        printf("%s\n", *list);
}

It's still noisy, so let's hide all the syntactic noise in a macro. Then main is as clean as can be:

#include <stdio.h>
//I'll do it without the typedef this time.

#define foreach_string(iterator, ...)\
    for (char **iterator = (char*[]){__VA_ARGS__, NULL}; *iterator; iterator++)

int main(){
    char *str = "thread";
    foreach_string(i, "yarn", str, "rope"){
        printf("%s\n", *i);
    }
}

To do:
Rewrite the macro to use void pointers rather than strings. If you're the sort of person who never tries even the easy exercises when reading tutorials then (1) I hate you, and (2) just wait until next time, when I'll use the solution to this as part of the next tip.

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Modeling With Data

Tip 27: Foreach in C