Implementation of Dynamic Table in C

Question

Consider a dynamic table with the following properties:

• Elements are stored in a dynamic array
• Capacity is the size of the dynamic array
• Size is defined is the number of elements stored in the array

Insert elements into this dynamic table. Double capacity if size is equal to capacity before push_back()

Don't use malloc or calloc functions.

Input: (n, elements)
9
6 7 8 12 4 10 11 1 15
Output:
capacity = 1; size = 1; elements = 6
capacity = 2; size = 2; elements = 6 7
capacity = 4; size = 3; elements = 6 7 8
capacity = 4; size = 4; elements = 6 7 8 12
capacity = 8; size = 5; elements = 6 7 8 12 4
capacity = 8; size = 6; elements = 6 7 8 12 4 10
capacity = 8; size = 7; elements = 6 7 8 12 4 10 11
capacity = 8; size = 8; elements = 6 7 8 12 4 10 11 1
capacity = 16; size = 9; elements = 6 7 8 12 4 10 11 1 15

#include <stdio.h>
int size=0;
int capacity=0;

int * double_capacity(int *a) {
    int l=0;
    if(capacity==0) capacity++;
    capacity=capacity*2;
    int b[capacity];
    for(l;l<size;l++){
        //printf("%d : %d \n",l,a[l]);
        b[l]=a[l];
    }
    return b;
}

int * push_back(int *a,int j) {
    if(size==capacity)
        a=double_capacity(a);
    a[size]=j;
    size++;
    int k=0;
    printf("capacity = %d; size = %d; elements = ",capacity,size);
    for(k=0;k<size;k++) {
        printf(" %d",*(a+k));
    }
    printf("\n");
    return a;
}


main() {
    int *a;
    int n,i,j,k,l;
    scanf("%d",&n);
    int temp[n];
    for(i=0; i<n; i++) {
        scanf("%d",&temp[i]);
    }

    for(i=0; i<n; i++) {
        a=push_back(a,temp[i]);
    }
}

it is showing compile error like
\temp.c: In function ‘double_capacity’:
temp.c:16:2: warning: function returns address of local variable [-Wreturn-local-addr]
return b;
^

even though i run this

when i gave input
3 3 2 1
output
capacity = 1; size = 1; elements = 3
capacity = 2; size = 2; elements = 3 2
capacity = 4; size = 3; elements = 3 2 1

when i gave input
5 5 4 3 2 1
output
capacity = 1; size = 1; elements = 5
capacity = 2; size = 2; elements = 5 4
capacity = 4; size = 3; elements = 5 4 3
capacity = 4; size = 4; elements = 0 0 -2128976676 2
capacity = 8; size = 5; elements = 0 0 -2128976676 32524 1

Answer 1

This won't work the way you want it to:

int * double_capacity(int *a) 
{
  int l=0;

  if(capacity==0) 
    capacity++;

  capacity=capacity*2;
  int b[capacity];
  for(l;l<size;l++)
  {
    //printf("%d : %d \n",l,a[l]);
    b[l]=a[l];
  }
  return b;
}

The array b only exists for the lifetime of the double_capacity function; once the function exits, b no longer exists, and any pointer to it is now invalid.

You can't use VLAs this way.

I notice your instructions said nothing about the realloc function...

EDIT

If you cannot use any of the standard memory management functions (malloc, calloc, or realloc), then the only real alternative I can think of is to create your own memory pool and allocate your table elements from it.

Basically, you'd declare a large array of unsigned char at file scope like so:

#define HEAP_SIZE 8192 // 8 KB heap, use whatever size you need
static unsigned char heap[HEAP_SIZE];

then you'd grab chunks of this array to build your table. You'll need to do some bookkeeping to manage available and allocated blocks, their addresses and sizes within that array, etc. If you know what you're doing, this is a day or so of solid effort depending on how robust you want it to be.

Then again, there may be something a little more straightforward that I'm forgetting about. I just can't think of it at the moment.