How to iterate through a char ** pointer?

Question

I have the following code

struct my_struct {
    const char **enjoy;
};

const char * enjy[] = {
    "Cricket", "movie", "",
    "Ball", "eat", "",
    };

static const struct my_struct my_struct_table[1] = {
    [0] = {
        .enjoy = enjy
        }
};

Now I want to use that final structure and want to iterate using that. How can I iterate using my_struct_table[0].enjoy

I want to print all the strings in the enjy variable.

Solution

Let T be any type. When working on an array of T of varying size, represented as T*, you need to specify how the end of such array is represented.

In a simpler case: for a string of characters, i.e. T = char, the end of array char* is typically represented by a null character \0. Thus, you can iterate it as:

char* ptr = myString;
for (char c = *ptr; c; c=*++ptr) {
    ...
}

You iterate over all characters, until you reach the one that is \0, making the expression c evaluate to false/0 and break the loop.

An alternative representation for a string is to represent the length of the string as a separate number. This is done, for example, in Pascal strings.

int size = myStringSize;
for (int idx=0; idx<size; ++idx) {
    char c = myString[idx];
}

---

Either of the approaches can also be used when you have an array of strings (i.e. T = char*). Your options are:

• You store a special non-string value in your enjoy array set to NULL at the end of the array
• Or you store the total size of the enjoy array in a separate value.

You can also use both options -- this is the case, for example, with arguments given to int main(int argc, char** argv). The argc stores the number of string values in the argv, and argv[argc] is guaranteed to be NULL.

If you use the first option you would then iterate it as:

char** ptr = enjoy;
for (char* c = *ptr; c; c=*++ptr) {
    ...
}

and if you use the second option:

int size = enjoySize;
for (int idx=0; idx<size; ++idx) {
    char* str = enjoy[idx];
}

Notice the similarity of these snippets iterating over char**, to those used for iterating over a simple char*.

Note that a value NULL stored in the enjoy array is different than storing a pointer to an empty string. The latter should not be used as a marker for the end of the array, because it can lead to hard-to-track bugs when a legitimate empty-string value is added to your enjoy array.