A Boolean Array Puzzle

Difficulty Level : Medium
Last Updated : 28 May, 2021

Input: A array arr[] of two elements having value 0 and 1
Output: Make both elements 0.

Specifications: Following are the specifications to follow.

1) It is guaranteed that one element is 0 but we do not know its position.
2) We can’t say about another element it can be 0 or 1.
3) We can only complement array elements, no other operation like and, or, multi, division, …. etc.
4) We can’t use if, else and loop constructs.
5) Obviously, we can’t directly assign 0 to array elements.

There are several ways we can do it as we are sure that always one Zero is there. Thanks to devendraiiit for suggesting following 3 methods.

Method 1

C++

#include <bits/stdc++.h>
using namespace std;
void changeToZero(int a[2])
{
    a[ a[1] ] = a[ !a[1] ];
}
 
// Driver code
int main()
{
    int a[] = {1, 0};
    changeToZero(a);
     
    cout<<"arr[0] = "<<a[0]<<endl;
    cout<<" arr[1] = "<<a[1];
    return 0;
}
 
// This code is contributed by rathbhupendra

C

void changeToZero(int a[2])
{
a[ a[1] ] = a[ !a[1] ];
}
 
int main()
{
int a[] = {1, 0};
changeToZero(a);
 
printf(" arr[0] = %d \n", a[0]);
printf(" arr[1] = %d ", a[1]);
getchar();
return 0;
}

Java

import java.io.*;
 
class GFG{
 
public static void changeToZero(int a[])
{
    a[a[1]] = a[1 - a[1]];
}
 
// Driver code
public static void main(String args[])
{
    int[] arr;
    arr = new int[2];
    arr[0] = 1;
    arr[1] = 0;
    changeToZero(arr);
     
    System.out.println("arr[0]= " + arr[0]);
    System.out.println("arr[1]= " + arr[1]);
}
}
 
// This code is contributed by rohitsingh07052

Python3

def changeToZero(a):
    a[ a[1] ] = a[ not a[1] ]
    return a
 
# Driver code
if __name__=='__main__':
    a = [1, 0]
    a = changeToZero(a);
     
    print(" arr[0] = " + str(a[0]))
    print(" arr[1] = " + str(a[1]))
 
# This code is contributed by Yash_R

Javascript

<script>
 
function changeToZero(a)
{
    a[a[1]] = a[1 - a[1]];
}
     
// Driver code
let arr;
arr = [];
arr[0] = 1;
arr[1] = 0;
changeToZero(arr);
 
document.write("arr[0] = " + arr[0] + "<br/>");
document.write("arr[1] = " + arr[1]);
 
// This code is contributed by avijitmondal1998
 
</script>

Method 2

C

void changeToZero(int a[2])
{
    a[ !a[0] ] = a[ !a[1] ]
}

Method 3
This method doesn’t even need complement.

C

void changeToZero(int a[2])
{
    a[ a[1] ] = a[ a[0] ]
}

Method 4
Thanks to purvi for suggesting this method.

C

void changeToZero(int a[2])
{
  a[0] = a[a[0]];
  a[1] = a[0];
}

There may be many more methods.
Please write comments if you find the above codes incorrect, or find other ways to solve the same problem.

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