How to split an array into two arrays in C

Question

Say I have an array in C

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

how can i divide this into

 {1,2,3} 

and

 {4,5,6} 

Is this possible with memcpy?

Thanks,

nonono

Answer 1

Of course. The direct solution is to allocate two new arrays using malloc and then using memcpy to copy the data into 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, if the original array exists long enough, you may not need to do this. You can simply “split” the array into two new arrays using the pointers in 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 do not need to separate them. if you have

 int *b = array + 3; 

you have a second array. When you pass an array to a function, it turns into a pointer anyway.

Answer 4

See the magic of how memcpy works, and you don't need to split only arrays. Changes made to the destination array automatically go to the original array and vice 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"); } }