CODEWITHSUNDEEP
carraysunixnullelement
WyldStallyns
WyldStallyns

Reputation: 201

How to set element in array to null in C program

I am writing a C program in Unix and cannot figure out how to set an array element to NULL. I need to be able to do this to remove multiple characters from a string.

Upvotes: 2

Views: 78354

Answers (3)

nKandel
nKandel

Reputation: 2575

You can't assign null to specific char array index as value represented by that index is char instead of pointer. But if you need to remove specific character from given string, you can implement this as follows

void removeChar(char *str, char garbage) {

    char *src, *dst;
    for (src = dst = str; *src != '\0'; src++) {
        *dst = *src;
        if (*dst != garbage) dst++;
    }
    *dst = '\0';
}

Test Program

#include<stdio.h>
int main(void) {
    char* str = malloc(strlen("abcdef")+1);
    strcpy(str, "abcdbbbef");
    removeChar(str, 'b');
    printf("%s", str);
    free(str);
    return 0;
}

output

acdef

Upvotes: 7

netcoder
netcoder

Reputation: 67745

If you have a char[], you can zero-out individual elements using this:

char arr[10] = "foo";
arr[1] = '\0';

Note that this isn't the same as assigning NULL, since arr[1] is a char and not a pointer, you can't assign NULL to it.

That said, that probably won't do what you think it will. The above example will produce the string f, not fo as you seem to expect.

If you want to remove characters from a string, you have to shift the contents of the string to the left (including the null terminator) using memmove and some pointer arithmetic:

Example:

#include <stdio.h>
#include <string.h>

int removechars(char *str, size_t pos, size_t cnt) {
    size_t len = strlen(str);
    if (pos + cnt > len)
        return -1;

    memmove(str + pos, str + pos + cnt, len - pos - cnt + 1);
    return 0;
}

Then use it like so:

char str[12] = "hello world";
if (removechars(str, 5, 4) == 0)  /* remove 4 chars starting at str[5] */
    printf("%s\n", str);          /* hellold */

Upvotes: 2

1&#39;&#39;
1&#39;&#39;

Reputation: 27125

If you're talking about an array of pointers (say char **), you'd just say array[element] = NULL;. But it sounds as though you really want to just truncate a string (char *), in which case you'd actually want to write string[index] = '\0', where \0 is the null byte. But, as far as I know, 0, '\0', and NULL are all equivalent and equal to 0 (please correct me if I'm wrong). Of course, for clarity, you should use 0 for numbers, '\0' for chars and strings, and NULL for pointers.

Upvotes: 0

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