Copy set bits in a range
Given two numbers x and y, and a range [l, r] where 1 <= l, r <= 32. The task is consider set bits of y in range [l, r] and set these bits in x also. Examples :
Input : x = 10, y = 13, l = 2, r = 3 Output : x = 14 Binary representation of 10 is 1010 and that of y is 1101. There is one set bit in y at 3’rd position (in given range). After we copy this bit to x, x becomes 1110 which is binary representation of 14. Input : x = 8, y = 7, l = 1, r = 2 Output : x = 11
Source : D E Shaw Interview
Method 1 (One by one copy bits)
We can one by one find set bits of y by traversing given range. For every set bit, we OR it to existing bit of x, so that the becomes set in x, if it was not set. Below is C++ implementation.
// C++ program to rearrange array in alternating// C++ program to copy set bits in a given// range [l, r] from y to x.#include <bits/stdc++.h>using namespace std; // Copy set bits in range [l, r] from y to x.// Note that x is passed by reference and modified// by this function.void copySetBits(unsigned &x, unsigned y, unsigned l, unsigned r){ // l and r must be between 1 to 32 // (assuming ints are stored using // 32 bits) if (l < 1 || r > 32) return ; // Travers in given range for (int i=l; i<=r; i++) { // Find a mask (A number whose // only set bit is at i'th position) int mask = 1 << (i-1); // If i'th bit is set in y, set i'th // bit in x also. if (y & mask) x = x | mask; }} // Driver codeint main(){ unsigned x = 10, y = 13, l = 2, r = 3; copySetBits(x, y, l, r); cout << "Modified x is " << x; return 0; } |
Output :
Modified x is 14
Method 2 (Copy all bits using one bit mask)
// C++ program to copy set bits in a given// range [l, r] from y to x.#include <bits/stdc++.h>using namespace std; // Copy set bits in range [l, r] from y to x.// Note that x is passed by reference and modified// by this function.void copySetBits(unsigned &x, unsigned y, unsigned l, unsigned r){ // l and r must be between 1 to 32 if (l < 1 || r > 32) return ; // get the length of the mask int maskLength = (1<<(r-l+1)) - 1; // Shift the mask to the required position // "&" with y to get the set bits at between // l ad r in y int mask = ((maskLength)<<(l-1)) & y ; x = x | mask;} // Driver codeint main(){ unsigned x = 10, y = 13, l = 2, r = 3; copySetBits(x, y, l, r); cout << "Modified x is " << x; return 0;} |
Output :
Modified x is 14
Thanks to Ashish Rathi for suggesting this solution in a comment.
This article is contributed by Rishi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
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