Select certain elements in array to create new array

Question

How could I select a certain number of elements from an array by giving a start and ending index number to create a new array?

For example, if my original array was {1,2,3,4,5,6}, and I say x=0 and y=2 for their index values, I would have a new array that is {1,2,3}.

Thank you.

In C, you can just set a pointer to point to the first element and then the pointer will act like an array with those elements in it. What do you want to do with the array after that? Do you need to make a new copy, completely separate, so one can be changed or deallocated while the other remains unchanged (or is changed separately)? You need to add details to your question about what the goal is. — Eric Postpischil, Mar 29 '20 at 00:03
stackoverflow is not a free coding service. Please post what you have tried and how it differs from what you want. — user3629249, Mar 29 '20 at 23:05

Vlad from Moscow · Accepted Answer · 2020-03-29T00:10:36.397

If your compiler supports variable length arrays then you can do this the following way

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

int main(void) 
{
    int a[] = { 1, 2, 3, 4, 5, 6 };
    size_t n1 = 0, n2 = 2;
    int b[n2 - n1 + 1];

    memcpy( b, a + n1, ( n2 - n1 + 1 ) * sizeof( int ) );

    size_t n = sizeof( b ) / sizeof( *b );

    for ( size_t i = 0; i < n; i++ )
    {
        printf( "%d ", b[i] );
    }

    putchar( '\n' );

    return 0;
}

The program output is

1 2 3

Otherwise the new array should be allocated dynamically as for example

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

int main(void) 
{
    int a[] = { 1, 2, 3, 4, 5, 6 };
    size_t n1 = 0, n2 = 2;

    size_t n = n2 - n1 + 1;
    int *b = malloc( n * sizeof( *b ) );

    memcpy( b, a + n1, n * sizeof( int ) );

    for ( size_t i = 0; i < n; i++ )
    {
        printf( "%d ", b[i] );
    }

    putchar( '\n' );

    free( b );

    return 0;
}

score 1 · Answer 2 · answered Mar 29 '20 at 00:06

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

void print_arr(int* arr, int len) {
    for (int i = 0; i < len; i ++) {
        printf("%d", arr[i]);
        if (i != len - 1) {
            printf(" ");
        } else {
            printf("\n");
        }
    }
}

int main() {
    int arr1[] = {1, 2, 3, 4, 5, 6};
    int start, end;
    printf("input the beginning and the end: ");
    scanf("%d %d", &start, &end);
    int len = end - start + 1;
    // we use malloc to dynamically allocate an array of the correct size
    int* arr2 = (int*)malloc(len * sizeof(int));
    // we use memcpy to copy the values
    memcpy(arr2, arr1 + start, len * sizeof(int));
    print_arr(arr1, 6);
    print_arr(arr2, len);
    // we have to free the memory
    free(arr2);
    return 0;
}

Select certain elements in array to create new array

2 Answers2