Set K Bits

Posted: 7 Dec, 2020
Difficulty: Easy


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Given a number ‘N’. You have to set bits from 0 to Kth position.

Example: If the Given number is 37 (100101) and ‘K’ = 4, then we need to set the 0,1,2,3 bits and thus after the Kth bit set our answer would be 47 (101111).

Input Format:
The first line contains a single integer ‘T’ representing the number of test cases. 

The first line of each test case will contain two integers ‘N’ and ‘K’ where ‘N’ denotes the number whose 0 to ‘K’th bits have to be set and ‘K’ indicates the limit upto where from 0th bit, the bits have to be set.
Output Format:
For each test case, print the number after 0 to Kth bits are set.

Output for every test case will be printed in a separate line.
You don’t need to print anything; It has already been taken care of.
Follow Up:
 Can you solve this using O(1) time and space complexity? 
1 <= T <= 10
-10^9 <= N <= 10^9
1 <= K <= 31

Time limit: 1 sec
Approach 1

The basic idea is to iterate through each bit from 0 till Kth bit and manually set each of the bits irrespective of whether they are set or not. At each index ‘i’ in the iteration manually set each bit and update the number ‘N’ by performing the Bitwise OR of the number ‘N’ with the set bit i.e. (1<<’i’). At last, after iterating, return the number ‘N’.


Here is the algorithm: 

  1. Run a loop for ‘i’ starting from 0 till less than K:
    • Declare an integer ‘temp’ and assign the value (1<<’i’) to it, where ‘temp’ will store the value where only the ‘i’th bit is set.
    • ‘N’ = ‘N’ | ‘temp’.
  2. Finally after iteration, Return ‘N’.
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