Reverse an array without using subtract sign ‘-‘ anywhere in the code

3

Given an array, the task is to reverse the array without using subtract sign ‘-‘ anywhere in your code. It is not tough to reverse an array but the main thing is to note use ‘-‘ operator.

Asked in: Moonfrog Interview

Below are different approaches:

Method 1:
1- Store array elements into a vector in C++.
2- Then reverse the vector using predefined functions.
3- Then store reversed elements into the array back.

Method 2:
1- Store array elements into a stack.
2- As the stack follows Last In First Out, so we can store elements from
top of the stack into the array which will be itself in a reverse manner.

Method 3:
1- In this method the idea is to use negative sign but by storing it into a variable.
2- By using this statement x = (INT_MIN/INT_MAX), we get -1 in a variable x.
3- As INT_MIN and INT_MAX have same values just of opposite signs,so on dividing them it will give -1.
4- Then ‘x’ can be used in decrementing the index from last.

// C++ program to reverse an array without
// using "-" sign
#include<bits/stdc++.h>
using namespace std;

// Function to reverse array
void reverseArray(int arr[], int n)
{
    // Trick to assign -1 to a variable
    int x = (INT_MIN/INT_MAX);

    // Reverse array in simple manner
    for (int i = 0; i<n/2 ; i++)

        // Swap ith index value with (n-i-1)th
        // index value
        swap(arr[i], arr[n + (x*i) + x]);
}

//Drivers code
int main()
{
    int arr[] = {5, 3, 7, 2, 1, 6};
    int n = sizeof(arr) / sizeof(arr[0]);

    reverseArray(arr, n);

    // print the reversed array
    for (int i=0; i< n ; i++)
        cout << arr[i] << " ";

    return 0;
}

Output:

6 1 2 7 3 5

Method 4:
In this method 4, the idea is to use bitwise operator to implement subtraction i.e.
A – B = A + ~B + 1
so, i– can be written as i = i +~1 +1

// C++ program to reverse an array without
// using "-" sign
#include<bits/stdc++.h>
using namespace std;

// Function to reverse array
void reverseArray(int arr[], int n)
{
    int x = (INT_MIN/INT_MAX);

    // Reverse array in simple manner
    for (int i = 0; i<n/2 ; i++)

        // Swap ith index value with (n-i-1)th
        // index value
        // Note : A - B = A + ~B + 1
        // So n - i = n + ~i + 1 then
        //    n - i - 1 = (n + ~i + 1) + ~1 + 1
        swap(arr[i], arr[(n + ~i + 1) + ~1 + 1]);
}

// Driver code
int main()
{
    int arr[] = {5, 3, 7, 2, 1, 6};
    int n = sizeof(arr) / sizeof(arr[0]);

    reverseArray(arr, n);

    // print the reversed array
    for (int i=0; i< n ; i++)
        cout << arr[i] << " ";

    return 0;
}

Output:

6 1 2 7 3 5

Asked in: Infosys, Moonfrog Labs

This article is contributed by Sahil Chhabra. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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