printing values in array pointer in C

Question

I can't seem to wrap my head around a problem. What am I missing?

Consider the following

int main(int argc, char *argv[]) {
    while ( *argv ) {
        printf("argv[] is: %s\n", *argv);
        ++argv;
    }

    return 0;
}

This prints out every value of argv. So a command line such as ./example arg1 arg2 would output the following:

`argv[] is: ./example`
`argv[] is: arg1`
`argv[] is: arg2`

Now consider the following (which I am having trouble with):

int main(void) {
    char *days[] = {
        "Sunday",
        "Monday",
        "Tuesday"
    };
    while ( *days ) {
        printf("day is %s\n", *days);
        *days++;
    }

    return 0;
}

If I try to compile, I get error cannot increment value of type 'char *[3]'

If I change *days++ to (*days)++ it compiles. If I run it, it runs forever and eventually fails with bus error.

However, it does not iterate through every value of days[]. I even tried putting in a Null pointer in the form of '\0' and "\0" in the days array to no effect.

What am I missing?

Answer 1

You have several errors in your code:

1. There is difference between variable argv and constant days. Variable can be changed, constant array label - cannot.

2. In your array, terminator NULL is missing at the end of the array.

3. *days++; is senseless in this case. This is a dummy and returns value of days, and increment days thereafter. It is enough to just use days++.

Thus, your code must be like:

#include <stdio.h>

int main(void) {
    char *days[] = {
        "Sunday",
        "Monday",
        "Tuesday",
        NULL
    };

    // Your style
    char **d = days;
    while ( *d ) {
        printf("day is %s\n", *d);
        d++;
    }

    // Or, more "scholastic" code
    for(const char **d = days; *d; d++)
        printf("day is %s\n", *d);

    return 0;