How to split array into two arrays in C

Question

Say i have an array in C

int array[6] = {1,2,3,4,5,6}

how could I split this into

{1,2,3}

and

{4,5,6}

Would this be possible using memcpy?

Thank You,

nonono

Answer 1

Sure. The straightforward solution is to allocate two new arrays using malloc and then using memcpy to copy the data into the two arrays.

int array[6] = {1,2,3,4,5,6}
int *firstHalf = malloc(3 * sizeof(int));
if (!firstHalf) {
  /* handle error */
}

int *secondHalf = malloc(3 * sizeof(int));
if (!secondHalf) {
  /* handle error */
}

memcpy(firstHalf, array, 3 * sizeof(int));
memcpy(secondHalf, array + 3, 3 * sizeof(int));

However, in case the original array exists long enough, you might not even need to do that. You could just 'split' the array into two new arrays by using pointers into the original array:

int array[6] = {1,2,3,4,5,6}
int *firstHalf = array;
int *secondHalf = array + 3;

Answer 2

// create space for 6 ints and initialize the first 6
int array[6] = {1,2,3,4,5,6};
// reserve space for two lots of 3 contiguous integers
int one[3], two[3]; 
// copy memory of the first 3 ints of array to one
memcpy(one, array, 3 * sizeof(int)); 
// copy 3 ints worth of memory from the 4th item in array onwards
memcpy(two, &array[3], 3 * sizeof(int)); 

Answer 3

You don't have to split them. If you have

int *b = array + 3;

you have the second array. When you pass an array to a function, it's turned into a pointer anyway.

Answer 4

See the magic how memcpy works, no need to exclusively split the arrays. The changes made in destination array are automatically go to source array and vise versa.

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

double *** double3d(long int dim1,long int dim2,long int dim3)
{
    long int i,j,k;
    double ***array;
    array=(double ***)malloc(dim1*sizeof(double **));
    for(i=0;i<dim1;i++)
    {
     array[i]=(double **)malloc(dim2*sizeof(double *));
     for(j=0;j<dim2;j++)
      array[i][j]=(double *)malloc(dim3*sizeof(double ));
    }
    return array;
}// end double3d

void summ(double ***A,double ***B, double ****C)
{
    int i ,j ,k;
    for(i=0;i<10;i++)
    for(j=0;j<5;j++)
    for(k=0;k<5;k++)
    (*C)[i][j][k]=A[i][j][k]+B[i][j][k];
}

void main()
{
   int i,j,k,nx,ny;
   double ***M1, ***M2, ***M3, ***M4,***M5,***M6;
   nx=5;ny=5;
    M1=double3d(10,nx,ny);
    M2=double3d(10,nx,ny);
    M3=double3d(10,nx,ny);
    M4=double3d(5,nx,ny);
    M5=double3d(5,nx,ny);
    M6=(double ***)malloc(10*sizeof(double **));

    for(i=0;i<10;i++)
    {
        for(j=0;j<nx;j++)
           for(k=0;k<ny;k++)
           {
           M1[i][j][k]=i;
           M2[i][j][k]=1;
           }
    }

// Note random values are in M4 and M5 as they are not initalised
    memcpy(M6,     M4, 5 * sizeof(double **));
    memcpy(M6+5,   M5, 5 * sizeof(double **));

    for(i=0;i<5;i++)
    {
       for(j=0;j<nx;j++)
          for(k=0;k<ny;k++)
          {
               M4[i][j][k]=200;
               M5[i][j][k]=700;
          }
    }

printf(" printing M6 Memcpy before addtion\n");
    for(j=0;j<nx;j++)
    {
       for(k=0;k<ny;k++)
           printf("%f ",M6[4][j][k]);
       printf("\n");
       for(k=0;k<ny;k++)
           printf("%f ",M6[9][j][k]);
       printf("\n");
    }
    // calling for non memcpy array
    summ(M1,M2,&M3); printf(" Non memcpy output last value : %f \n",M3[9][nx-1][ny-1]);
    // calling for memcpy
    summ(M1,M2,&M6); printf(" memcpy output last value : %f \n",M6[9][nx-1][ny-1]);
printf(" printing M6 Memcpy for two sets after addtion\n");
    for(j=0;j<nx;j++)
    {
       for(k=0;k<ny;k++)
           printf("%f ",M6[4][j][k]);
       printf("\n");
    }
    for(j=0;j<nx;j++)
    {
       for(k=0;k<ny;k++)
           printf("%f ",M6[9][j][k]);
       printf("\n");
    }

    free(M6);// cleared M6

printf(" printing M4 Memcpy after deleting M6\n");
    for(j=0;j<nx;j++)
    {
       for(k=0;k<ny;k++)
           printf("%.1f ,%.1f ,%.1f ,%.1f ,%.1f ",M4[0][j][k],M4[1][j][k],M4[2][j][k],M4[3][j][k],M4[4][j][k]);
       printf("\n");
    }

printf(" printing M5 Memcpy after deleting M6\n");
    for(j=0;j<nx;j++)
    {
       for(k=0;k<ny;k++)
           printf("%.1f ,%.1f ,%.1f ,%.1f ,%.1f ",M5[0][j][k],M5[1][j][k],M5[2][j][k],M5[3][j][k],M5[4][j][k]);
       printf("\n");
    }
 }