Removing elements from an array in C

Question

I just have a simple question about arrays in C

What\'s the best way to remove elements from an array and in the process make the array smaller.

i.e the ar

Answer 1

You don't really want to be reallocing memory every time you remove something. If you know the rough size of your deck then choose an appropriate size for your array and keep a pointer to the current end of the list. This is a stack.

If you don't know the size of your deck, and think it could get really big as well as keeps changing size, then you will have to do something a little more complex and implement a linked-list.

In C, you have two simple ways to declare an array.

1. On the stack, as a static array

   int myArray[16]; // Static array of 16 integers
   

2. On the heap, as a dynamically allocated array

   // Dynamically allocated array of 16 integers
   int* myArray = calloc(16, sizeof(int));
   

Standard C does not allow arrays of either of these types to be resized. You can either create a new array of a specific size, then copy the contents of the old array to the new one, or you can follow one of the suggestions above for a different abstract data type (ie: linked list, stack, queue, etc).

Answer 2

I usually do this and works always.

---

/try this/

for (i = res; i < *size-1; i++) { 

    arrb[i] = arrb[i + 1];
}

*size = *size - 1; /*in some ides size -- could give problems*/

Answer 3

There are really two separate issues. The first is keeping the elements of the array in proper order so that there are no "holes" after removing an element. The second is actually resizing the array itself.

Arrays in C are allocated as a fixed number of contiguous elements. There is no way to actually remove the memory used by an individual element in the array but the elements can be shifted to fill the hole made by removing an element. For example:

void remove_element(array_type *array, int index, int array_length)
{
   int i;
   for(i = index; i < array_length - 1; i++) array[i] = array[i + 1];
}

Statically allocated arrays can not be resized. Dynamically allocated arrays can be resized with realloc(). This will potentially move the entire array to another location in memory, so all pointers to the array or to its elements will have to be updated. For example:

remove_element(array, index, array_length);  /* First shift the elements, then reallocate */
array_type *tmp = realloc(array, (array_length - 1) * sizeof(array_type) );
if (tmp == NULL && array_length > 1) {
   /* No memory available */
   exit(EXIT_FAILURE);
}
array_length = array_length - 1;
array = tmp;

realloc will return a NULL pointer if the requested size is 0, or if there is an error. Otherwise it returns a pointer to the reallocated array. The temporary pointer is used to detect errors when calling realloc because instead of exiting it is also possible to just leave the original array as it was. When realloc fails to reallocate an array it does not alter the original array.

Note that both of these operations will be fairly slow if the array is large or if a lot of elements are removed. There are other data structures like linked lists and hashes that can be used if efficient insertion and deletion is a priority.

Answer 4

Try this simple code:

#include <stdio.h>
#include <conio.h>
void main(void)
{ 
    clrscr();
    int a[4], i, b;
    printf("enter nos ");
    for (i = 1; i <= 5; i++) {
        scanf("%d", &a[i]);
    }
    for(i = 1; i <= 5; i++) {
        printf("\n%d", a[i]);
    }
    printf("\nenter element you want to delete ");
    scanf("%d", &b);
    for (i = 1; i <= 5; i++) {
        if(i == b) {
            a[i] = i++;
        }
        printf("\n%d", a[i]);
    }
    getch();
}

Answer 5

Interestingly array is randomly accessible by the index. And removing randomly an element may impact the indexes of other elements as well.

    int remove_element(int*from, int total, int index) {
            if((total - index - 1) > 0) {
                      memmove(from+i, from+i+1, sizeof(int)*(total-index-1));
            }
            return total-1; // return the new array size
    }

Note that memcpy will not work in this case because of the overlapping memory.

One of the efficient way (better than memory move) to remove one random element is swapping with the last element.

    int remove_element(int*from, int total, int index) {
            if(index != (total-1))
                    from[index] = from[total-1];
            return total; // **DO NOT DECREASE** the total here
    }

But the order is changed after the removal.

Again if the removal is done in loop operation then the reordering may impact processing. Memory move is one expensive alternative to keep the order while removing an array element. Another of the way to keep the order while in a loop is to defer the removal. It can be done by validity array of the same size.

    int remove_element(int*from, int total, int*is_valid, int index) {
            is_valid[index] = 0;
            return total-1; // return the number of elements
    }

It will create a sparse array. Finally, the sparse array can be made compact(that contains no two valid elements that contain invalid element between them) by doing some reordering.

    int sparse_to_compact(int*arr, int total, int*is_valid) {
            int i = 0;
            int last = total - 1;
            // trim the last invalid elements
            for(; last >= 0 && !is_valid[last]; last--); // trim invalid elements from last

            // now we keep swapping the invalid with last valid element
            for(i=0; i < last; i++) {
                    if(is_valid[i])
                            continue;
                    arr[i] = arr[last]; // swap invalid with the last valid
                    last--;
                    for(; last >= 0 && !is_valid[last]; last--); // trim invalid elements
            }
            return last+1; // return the compact length of the array
    }

Answer 6

What solution you need depends on whether you want your array to retain its order, or not.

Generally, you never only have the array pointer, you also have a variable holding its current logical size, as well as a variable holding its allocated size. I'm also assuming that the removeIndex is within the bounds of the array. With that given, the removal is simple:

Order irrelevant

array[removeIndex] = array[--logicalSize];

That's it. You simply copy the last array element over the element that is to be removed, decrementing the logicalSize of the array in the process.

If removeIndex == logicalSize-1, i.e. the last element is to be removed, this degrades into a self-assignment of that last element, but that is not a problem.

Retaining order

memmove(array + removeIndex, array + removeIndex + 1, (--logicalSize - removeIndex)*sizeof(*array));

A bit more complex, because now we need to call memmove() to perform the shifting of elements, but still a one-liner. Again, this also updates the logicalSize of the array in the process.