CODEWITHSUNDEEP
carrayspointers
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Reputation: 682

printing values in array pointer in C

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?

Upvotes: 7

Views: 9213

Answers (4)

olegarch
olegarch

Reputation: 3891

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;

Upvotes: 7

tddguru
tddguru

Reputation: 392

The argv[] structure contains a null pointer as the last entry which is why your code works. You can add a null pointer to your data structure. You also need to realize that argv is a parameter (just like a local variable) that points to the internal data structure holding the command line arguments. You need to create a similar local variable that can iterate through the array.

Here is a solution:

#include <stdio.h>
int main(void) {
    char *days[] = {"Sunday", "Monday", "Tuesday", 0 };
    char **v = days;

  while (v) {
    printf("day is %s\n", *v);
    v++;
  }

  return 0;
}

Upvotes: 6

MayurK
MayurK

Reputation: 1947

  1. days is a 2D character array. Though the name of the array points to the first address, it is not a pointer. You can not do pointer arithmetic on an array name.
  2. while ( *days ) will never become false. You should change the logic.

In the below code I have retained most of your code.

 #include <stdio.h>

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

       for(int i=0; i < 3; i++) {
            printf("day is %s\n", (days[i]));
        }

        return 0;

Upvotes: 1

Minnow
Minnow

Reputation: 64

Instead of trying to increment the character pointer, you should probably loop through each element in the character array.

For example:

char *days[] = {
    "Sunday",
    "Monday",
    "Tuesday"
};
int i;    
for (i = 0; i < 3; i++) 
    printf("day is %s\n", days[i]);

Upvotes: 1

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