Expanding a Dynamically Allocated Pointer Array

Question

This has been such a pain. I'm trying to dynamically allocate an array and used realloc, calloc, and malloc but neither of the three has lead me to anything. It seems like I have successfully expanded it, but I have not been able to copy it correctly. Everything within the expand function is okay but after I call the function it becomes useless.

typedef struct ArrayList
{
    // We will store an array of strings (i.e., an array of char arrays)
    char **array;

    // Size of list (i.e., number of elements that have been added to the array)
    int size;

    // Length of the array (i.e., the array's current maximum capacity)
    int capacity;

} ArrayList;

ArrayList *expandArrayList(ArrayList *list, int length){
    struct ArrayList *temp=realloc(list, length);
    if (length<list->capacity){
        return NULL;
        free(temp);
    }
    if (temp)
        list=temp;
    else{
        free(temp);
        return NULL;
    }
    list->capacity=length;
    printf("-> Expanded ArrayList to size %d.\n", length);
    return list;
}

Xi Han · Accepted Answer · 2014-02-03 01:02:06Z

1

I guess you would like to expand the capacity of the array in ArrayList actually. Then the function should be like:

#include <errno.h>
ArrayList *expandArrayList(ArrayList *list, int new_capacity){
    if (list->capacity >= new_capacity) {
        return list; // If a smaller capacity is passed in, do nothing.
    }

    char **tmp = realloc(list->array, new_capacity * sizeof(char*));
    if (tmp) {
        list->capacity = new_capacity;
        list->array = tmp; // On success, set 'array' field of 'list' to tmp.
    } else {
        if (errno == ENOMEM) {
            fprintf(stderr, "Error: not enough memory.\n"); // On failure, report the error and exit.
            exit(1);
        }
    }

    return list;  
}

I have not tested the code, but I hope this would help you.

answered Feb 3, 2014 at 1:02

Xi Han

3232 silver badges7 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Hammad Over a year ago

If you're able, can you tell me what I did wrong? everything works perfectly now.

Xi Han Over a year ago

Some major issues: 1. In your original code, you were trying to reallocate the ArrayList, but the true data structure you wanted to reallocate was the char** array which held the pointers. 2. You could not just pass the capacity to realloc because char* takes 4 bytes in memory and you should multiply capacity with sizeof(char *).

nilo · Accepted Answer · 2014-02-03 01:14:21Z

0

ArrayList has a fixed size: one pointer + two ints. Reallocating memory on a pointer to an ArrayList makes no sense. It's either the memory pointed to by the list pointer, or the memory pointed to by the array pointers, or both, that you want to reallocate. The free() put after a return will never be run, and the whole code makes me wonder what you are trying to achieve in a broader context.

answered Feb 3, 2014 at 1:14

nilo

1,1761 gold badge10 silver badges25 bronze badges

Collectives™ on Stack Overflow

Expanding a Dynamically Allocated Pointer Array

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related