A Boolean Array Puzzle

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

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

Method 2

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

Method 3
This method doesn’t even need complement.

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

Method 4
Thanks to purvi for suggesting this method.

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.

This article is attributed to GeeksforGeeks.org

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