Dynamic array of pointers

Question

I have an array of pointers to strings.

char **array;

I declare it this way and not char *array[N] because this array won't have a static number of elements.

Declaring the array this way, I will probably have to realloc the sizeof it every time I add a new element (pointer to string).

int main(void)
{
    char **array;

    char *word = "lolol";
    char *word2 = "blabla";

    return 0;
}

Could you give me an example on how I should "create space" in the array in order to store pointers to these strings?

"I have an array of pointers to strings. " No, you have a pointer to a pointer to char. — juanchopanza
– juanchopanza, Commented Mar 1, 2015 at 15:59

user3476093 · Accepted Answer · 2015-03-01 19:28:49Z

The best way of doing it is probably by making a struct

This way, you can resize it, and add as many strings as you want without needing to choose a specific size.

Note: setting the string_array's capacity and size to 0 is necessary for it to work.

You could do it by a function like this instead:

void load_array(string_array *array)
     {
     array->size = 0;
     array->capacity = 0;
     }

And call it like this:

load_array(&my_array);

Note, when getting the value from one of these arrays using [], you must call it like this:

my_array.arr[index]

This is because you must refer to the pointer in the array struct, which is as arr (char **)

I have tested the below, and it works perfectly.

# include <stdio.h>

typedef struct string_array
    {
    char **arr;
    unsigned capacity, size;
    } string_array;

void add_to_array(string_array *array, char *str)
    {
    if(array->capacity == 0)
        {
        array->arr = (char **)malloc((array->capacity = 3) * sizeof(char *));
        array->arr[array->size++] = str;
        }
    else if(array->capacity == array->size)
        {
        array->arr = (char **)realloc(array->arr, (array->capacity *= 1.5) * sizeof(char *));
        array->arr[array->size++] = str;
        }
    else
        {
        array->arr[array->size++] = str;
        }
    }

int main(void)
    {
    char *str1 = "Hello World";
    char *str2 = "Hello World2";
    char *str3 = "Hello World3";
    char *str4 = "Hello World4";
    char *str5 = "Hello World5";
    string_array my_array;
    my_array.capacity = 0;
    my_array.size = 0;
    add_to_array(&my_array, str1);
    add_to_array(&my_array, str2);
    add_to_array(&my_array, str3);
    add_to_array(&my_array, str4);
    add_to_array(&my_array, str5);
    // and so on
    for (int i = 0; i < my_array.size; ++i)
        {
        printf(my_array.arr[i]);
        printf("\n");
        }
    free(my_array.arr);
    getchar(); // this means pressing enter closes the console
    return (0);
    }

Vlad from Moscow · Accepted Answer · 2015-03-01 15:51:14Z

4

Here is a demonstrative program

#include <stdio.h>
#include <stdlib.h>

int main(void) 
{
    size_t N = 2;
    char **array = malloc( N * sizeof( char * ) );

    if ( !array ) return 1;

    char *word = "lolol";
    char *word2 = "blabla";

    array[0] = word;
    array[1] = word2;

    char *word3 = "Hello";

    ++N;

    array = realloc( array, N * sizeof( char * ) );

    if ( !array ) return 2;

    array[2] = word3;

    for ( size_t i = 0; i < N; i++ ) puts( array[i] );

    free( array );

    return 0;
}

The program output is

lolol
blabla
Hello

answered Mar 1, 2015 at 15:51

Vlad from Moscow

313k27 gold badges204 silver badges358 bronze badges

Comments

tux3 · Accepted Answer · 2015-03-01 15:40:34Z

0

Simply allocate some room for pointers in your array, and free it when you're done.

char *word = "lolol";
char *word2 = "blabla";

char** array = malloc(2*sizeof(char*));

array[0] = word;
array[1] = word2;

free(array);

You can change that 2*sizeof(char*) to a N*sizeof(char*) if you want more elements.

answered Mar 1, 2015 at 15:40

tux3

7,4316 gold badges41 silver badges51 bronze badges

3 Comments

pirox22 Over a year ago

and if i want to add another word?

tux3 Over a year ago

@Joe He doesn't need to copy if he uses realloc, though.

Joe Over a year ago

@tux sorry, mental typo

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