Question

I am working on C++.. am in a need to swap two blocks of elements in an array..

Say, {1,2,3,4,5,6} is my input array.. block {4,5} should be moved to beginning and the output array should be like {4,5,1,2,3,6}.. all i have is the start index and end index of the block {4,5}.. for doing this i am using a temp array, copying the blocks individually to temp array and moving it back to the original array, which is tedious

but i am sure there will be better methods to do this using memcpy or memmove.. any ideas?

Answer 1

There is a standard algorithm designed specifically for this task called std::rotate():

#include <algorithm>
#include <cstdio>

int main()
{
    int inputArray[] = {1, 2, 3, 4, 5, 6};
    ::printf("Before: ");
    for(int i = 0; i < 6; ++i)
    {
        ::printf("%d ", inputArray[i]);
    }
    ::printf("\n");

    int startIndex = 3; // refers to the number 4 in inputArray
    int endIndex = 5;   // refers one-past the number 5 in inputArray
    std::rotate(inputArray, inputArray+startIndex, inputArray+endIndex);

    ::printf("After: ");
    for(int i = 0; i < 6; ++i)
    {
        ::printf("%d ", inputArray[i]);
    }
    ::printf("\n");
}

Expected output:

Before: 1 2 3 4 5 6

After: 4 5 1 2 3 6

std::rotate() performs the rotation in-place via std::swap(), so there’s no temporary array involved.