Question

I want to merge 2 arrays into 1. For example:

• A1= 1,1
• A2= 2,2
• then A3 = 1,2,1,2

For example:

• A1= 1
• A2= 2,2,2,2
• then A3 = 1,2,2,2,2

For example:

• A1= 1,1,1,1
• A2= 2,2
• then A3 = 1,2,1,2,1,1

In last example, when I ran my code, I got 1,2,1,2,1,20.

In the second last, I got 1,2,32767,2,2.

So I guess I have a wrong code. Right after the I finished taking the element of the shorter array and fill up all the rest of the A3 with whoever is longer. But I couldn’t figure out why — can you help me?

code:

int *p3=arr3;   //arr3 is A3 for example, arr1 = A1..etc, all sizes are defined
int index;
int index1=0;
int index2=0;

for(index = 0; index< sizeofArr3 ; index++)
{
    if(index%2==0)
    {
        if(index1<=sizeofArr1)
            *(p3++) = arr1[index1++];
        else
            *(p3++) = arr2[index2++];
    }
    else 
    {
        if(index2<=sizeofArr2)
            *(p3++) = arr2[index2++];
        else
            *(p3++) = arr1[index1++];
    }
}

Answer 1

It’s this line:

if (index1 <= sizeofArr1)

and the equivalent one for index2 and sizeofArr2. You should be using < rather than <=.

The reason has to do with C’s zero-based arrays. For an array of size N, the element indexes are 0 through N-1 inclusive. Because you’re allowing it to access element N (the N+1th element), you’re actually invoking undefined behaviour.

Theoretically, the implementation can do anything in that case, up to and including destruction of the universe. I guess you’re lucky that it just decided to give you results that were slightly awry 🙂