How to do it…

1. Define two arrays of a certain size and assign elements of your choice to both the arrays. Let's assume that we created two arrays called p and q, both of size four elements:

Figure 1.11

2. Define one more array. Let's call it array r, to be used for storing the elements that are common between the two arrays.
3. If an element in array p exists in the array q, it is added to array r. For instance, if the element at the first location in array p, which is at p[0], does not appear in array q, it is discarded, and the next element, at p[1], is picked up for comparison.

4. And if the element at p[0] is found anywhere in array q, it is added to array r, as follows:

Figure 1.12

5. This procedure is repeated with other elements of array q. That is, p[1] is compared with q[0], q[1], q[2], and q[3]. If p[1] is not found in array q, then before inserting it straightaway into array r, it is compared with the existing elements of array r to avoid repetitive elements.
6. Because the element at p[1] appears in array q and is not already present in array r, it is added to array r as follows:

Figure 1.13

The commoninarray.c program for establishing common elements among the two arrays is as follows:

#include<stdio.h>
#define max 100

int ifexists(int z[], int u, int v)
{
    int i;
    if (u==0) return 0;
    for (i=0; i<=u;i++)
        if (z[i]==v) return (1);
    return (0);
}
void main()
{
    int p[max], q[max], r[max];
    int m,n;
    int i,j,k;
    k=0;
    printf("Enter the length of the first array:");
    scanf("%d",&m);
    printf("Enter %d elements of the first array\n",m);
    for(i=0;i<m;i++ )
        scanf("%d",&p[i]);
    printf("\nEnter the length of the second array:");
    scanf("%d",&n);
    printf("Enter %d elements of the second array\n",n);
    for(i=0;i<n;i++ )
        scanf("%d",&q[i]);
    k=0;
    for (i=0;i<m;i++)
    {
        for (j=0;j<n;j++)
        {
           if (p[i]==q[j])
           {
               if(!ifexists(r,k,p[i]))
               {
                   r[k]=p[i];
                   k++;
               }
            }
        }
    }
    if(k>0)
    {
        printf("\nThe common elements in the two arrays are:\n");
        for(i = 0;i<k;i++)
            printf("%d\n",r[i]);
    }
    else
        printf("There are no common elements in the two arrays\n");
}

Now, let's go behind the scenes to understand the code better.