Thread: Dynamic array (or something like that) in C

I don't think you're going to take anyone else's opinions seriously either

I take everyone's opinion seriously. I can't learn if I don't.

since you seem to sneeze at freeing memory

I guess it does seem like that with the straw man you've built up. I'm saying that in my program there was no need to free the memory when the program is 1) a simple example, 2) prints something and ends right away, and 3) my OS frees the memory automatically. The reason I said I don't think you're taking me seriously is you keep acting like I'm pushing a practice of never freeing memory, period.

Anyway, I don't like confrontation, so I'll always free memory in my examples from now on to avoid being attacked. I'll try to conform to the "best practices" you've decided on as I learn what's expected on this board. My deepest apologies for having an independent thought.

Originally Posted by Banana Man

You can do it like this.

Code:

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

typedef struct
{
	char *name;
	char **keys;
} row;

int main()
{
    row temp;
    int i;

    temp.name = malloc(50 * sizeof(char));
    strcpy(temp.name, "Banana Man");

    temp.keys = malloc(5 * sizeof(char*));

    for ( i = 0; i < 5; ++i )
    {
        temp.keys[i] = malloc(3 * sizeof(char));
        memcpy(temp.keys[i], "1234567890" + i, 3);
    }

    puts(temp.name);

    for ( i = 0; i < 5; ++i )
    {
        int j;

        for ( j = 0; j < 3; ++j )
        {
            putchar(temp.keys[i][j]);
        }

        putchar('\n');
    }

    return 0;
}

But what if I wish to add a key after I have already allocated the memory ? Like if I have a function add_key(row *_row, char *key); ? I tried to reallocate but I failed. How would you accomplish that ?

Best wishes, Desmond5

Originally Posted by xuftugulus

The function would need to know the current number of keys stored in the array.

Code:

void add_key(row *_row, char *key,int n_keys)
{
    _row->keys = realloc( (n_keys+1)*sizeof(*_row->keys) );
    if(key!=NULL)
    { 
        _row->keys[n_keys] = malloc(3 * sizeof(char));
        memcpy(_row->keys[n_keys], key, 3);
    } else {
        _row->keys[n_keys] = NULL;  
}

Yeah..but it gives a warning "[Warning] assignment makes pointer from integer without a cast" on this line: _row->keys = realloc( (n_keys+1)*sizeof(*_row->keys) );

Really ? I don't know what else I need to include..

Code:

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

typedef struct
{
	char *name;
	char **keys;
} row;

void add_key(row *_row, char *key, int n_keys, int columns)
{
    _row->keys = realloc( (n_keys+1)*sizeof(*_row->keys) );
    if(key!=NULL)
    {
        _row->keys[n_keys] = malloc(columns * sizeof(char));
        memcpy(_row->keys[n_keys], key, columns);
    }
	else
	{
        _row->keys[n_keys] = NULL;
	}
}

int main()
{
    row temp;
    int i = 0;

    int n_keys = 0;

    int rows = 10;
    int columns = 5;

    temp.keys = malloc(rows * sizeof(char*));

	add_key(&temp, "somekey", n_keys, columns);


    for ( i = 0; i < rows; ++i )
    {
        int j;

        for ( j = 0; j < columns; ++j )
        {
            putchar(temp.keys[i][j]);
        }

        putchar('\n');
    }

	getchar();
    return 0;
}

Originally Posted by matsp

You should use the <stdlib.h> include for the malloc/realloc calls.

--
Mats

And to pick up on those errors, you just include some warning flags.

If you're using gcc or mingw, -Wall and -ansi is enough I believe.

Note that the errors won't say "You forgot header file x", but rather something like "Implicit declaration of function x" when you try and call a function that hasn't been explicitly declared anywhere, but is still being implicitly declared to exist when you call it.

Many thanks

One more thing..I add three elements to the array. How is it that it has 4 ? I even cannot access the fourth element..

Code:

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

typedef struct
{
	char *name;
	char **keys;
} row;

void add_key(row *_row, char *key, int n_keys, int columns)
{
	if (n_keys == 0) _row->keys = malloc(sizeof(char*));
	else _row->keys = realloc(_row->keys, (n_keys+1)*sizeof(*_row->keys) );

    if(key!=NULL)
    {
        _row->keys[n_keys] = malloc(columns * sizeof(char));
        memcpy(_row->keys[n_keys], key, columns);
    }
	else
	{
        _row->keys[n_keys] = NULL;
	}
}

int main()
{
    row temp;
    int i = 0;

    int n_keys = 0;

    int rows = 10;
    int columns = 5;

	add_key(&temp, "key0", 0, columns);
	add_key(&temp, "key1", 1, columns);
	add_key(&temp, "key2", 2, columns);

	double si = sizeof(temp.keys);

    printf("%s\n", temp.keys[0]);
    printf("%s\n", temp.keys[1]);
    printf("%s\n", temp.keys[2]);

       //printf("%s\n", temp.keys[3]); doesnt excist ?


    printf("Size: %lf", si); // PRINTS 4 not 3 as expected

	getchar();
    return 0;
}