CODEWITHSUNDEEP
c++arrayspointers
nyaan
nyaan

Reputation: 31

Swapping sections of an array with each other in C++

Is there an easy method to swap sections (chunks) of arrays with each other? That is, I have an array:

array[0] = 1;
array[1] = 2;
array[2] = 3;
array[3] = 4;
array[4] = 5;
array[5] = 6;
array[6] = 7;
array[7] = 8;

and a function called swapSections(startX, endX, startY, endY) which pretty much given these values swaps out the range of values determined by endX - startX with the range of values from StartY to endY, so from my example...

if x range = 2 and startX = 0 and y range = 3 and startY = 5, it would place array[0] and array[1] to where array[5] and array[6] are, and then place array[7] after array[6], pushing everything else down one. I am not sure how to go about this, and I was physically copying the memory across to a temp array, but I think there is a better way to do this. (btw, the end result from my example would be):

array[0] = 6;
array[1] = 7;
array[2] = 8;    
array[3] = 3;
array[4] = 4;
array[5] = 5;
array[6] = 1;
array[7] = 2;

Upvotes: 2

Views: 2437

Answers (4)

Blastfurnace
Blastfurnace

Reputation: 18651

Assume you have (or can create) a function to reverse a range of elements in place, something like reverse(array, start, end). You can then perform this task in four steps:

// (1) reverse the first range
array[0] = 2;
array[1] = 1;

// (2) reverse the elements between the ranges
array[2] = 5;
array[3] = 4;
array[4] = 3;

// (3) reverse the second range
array[5] = 8;
array[6] = 7;
array[7] = 6;

// (4) finally, reverse the entire array
array[0] = 6;
array[1] = 7;
array[2] = 8;
array[3] = 3;
array[4] = 4;
array[5] = 5;
array[6] = 1;
array[7] = 2;

Spoiler: (do your homework first then read this)

If you are allowed to use the Standard Library there is a std::reverse function in <algorithm> that makes this trivial.

Upvotes: 1

Cubbi
Cubbi

Reputation: 47418

The easiest way to swap sections of an array in terms of readability and effort is to use the standard C++ function swap_ranges()

#include <iostream>
#include <algorithm>
int main()
{
    int a[8] = {1,2,3,4,5,6,7,8};

    std::cout << "Beforeswap: ";
    for(int i=0; i<8; ++i)
            std::cout << a[i] << ' ';
    std::cout << '\n';

    std::swap_ranges(a+0, a+2, a+5);

    std::cout << "After swap: ";
    for(int i=0; i<8; ++i)
            std::cout << a[i] << ' ';
    std::cout << '\n';
}

test: https://ideone.com/ZVv2M

...but it will only swap subranges of equal length, not the unequal length as in your test case. Your case is actually a combination of swap_ranges() and rotate().

Upvotes: 3

NG.
NG.

Reputation: 22904

One approach you can take in your example is swapping the minimum range and then "bubbling up" the last part. So:

array[0] = 1;   --> 6
array[1] = 2;   --> 7
array[2] = 3;
array[3] = 4;
array[4] = 5;
array[5] = 6;   --> 1
array[6] = 7;   --> 2
array[7] = 8;

Then you bubble up the 8 by swapping array[7], with array[6], then array[6] with array[5], etc. until you put the 8 in the right place. Hopefully that gets you started.

Upvotes: 1

Kornel Kisielewicz
Kornel Kisielewicz

Reputation: 57535

You might want to look at valarray and it's slices.

Upvotes: 2

Related Questions