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# Same Number Of Set Bits As N

• Difficulty Level : Medium
• Last Updated : 24 May, 2021

Given a positive integer N, find out how many positive integers strictly less than N have the same number of set bits as N.

Examples:

```Input : 8
Output :3
Explanation: Binary representation of
8 : 1000, so number of set bits in 8 is 1.
So the integers less than 8 with same number
of set bits are : 4, 2, 1

Input :1
Output :0

Input :4
Output :2 ```

Approach:

```1. Using __builtin_popcount() inbuilt function, count set bits in N and store into a
temp variable
2. Iterate from n-1 to 1 and also count set bits in i using __builtin_popcount()
function
3. Now, compare temp with __builtin_popcount(i)
4. If both are equal then increment counter variable
5. Return counter```

Below is the implementation of the above approach.

## C++

 `// CPP program to find numbers less than N``// that have same Number Of Set Bits As N``#include ``using` `namespace` `std;` `int` `smallerNumsWithSameSetBits(``int` `n)``{       ``    ``// __builtin_popcount function that count``    ``// set bits in n``    ``int` `temp = __builtin_popcount(n);``    ` `    ``// Iterate from n-1 to 1``    ``int` `count = 0;``    ``for` `(``int` `i = n - 1; i > 0; i--) {``        ` `        ``// check if the number of set bits``        ``// equals to temp increment count       ``        ``if` `(temp == __builtin_popcount(i))``            ``count++;``    ``}``    ``return` `count;``}` `// Driver Code``int` `main()``{``    ``int` `n = 4;``    ``cout << smallerNumsWithSameSetBits(n);``    ``return` `0;``}`

## Java

 `// Java program to find numbers less than N``// that have same Number Of Set Bits As N``class` `GFG {``    ` `    ``// returns number of set bits in a number``    ``static` `int` `__builtin_popcount(``int` `n)``    ``{``        ``int` `d, t = ``0``;``        ` `        ``while``(n > ``0``)``        ``{``            ``d = n % ``2``;``            ``n = n / ``2``;``            ``if``(d == ``1``)``                ``t++;``        ``}``        ``return` `t;``    ``}``    ` `    ``static` `int` `smallerNumsWithSameSetBits(``int` `n)``    ``{    ``        ``// __builtin_popcount function that count``        ``// set bits in n``        ``int` `temp = __builtin_popcount(n);``        ` `        ``// Iterate from n-1 to 1``        ``int` `count = ``0``;``        ``for` `(``int` `i = n - ``1``; i > ``0``; i--) {``            ` `            ``// check if the number of set bits``            ``// equals to temp increment count    ``            ``if` `(temp == __builtin_popcount(i))``                ``count++;``        ``}``        ``return` `count;``    ``}``    ` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``int` `n = ``4``;``        ``System.out.println(``            ``smallerNumsWithSameSetBits(n));``    ``}``}` `// This code is contributed by Arnab Kundu.`

## Python3

 `# Python3 program to find numbers``# less than N that have same``# Number Of Set Bits As N``def` `__builtin_popcount(n) :``    ``t ``=` `0`   `    ``while``(n > ``0``) :``        ``d ``=` `n ``%` `2``        ``n ``=` `int``(n ``/` `2``)``        ``if``(d ``=``=` `1``) :``            ``t ``=` `t ``+` `1``    ``return` `t` `def` `smallerNumsWithSameSetBits(n) :   ``    ` `    ``# __builtin_popcount function``    ``# that count set bits in n``    ``temp ``=` `__builtin_popcount(n)``    ` `    ``# Iterate from n-1 to 1``    ``count ``=` `0``    ``for` `i ``in` `range``(n``-``1``,``0``,``-``1``) :    ``        ` `        ``# check if the number of``        ``# set bits equals to temp``        ``# increment count    ``        ``if` `(temp ``=``=` `__builtin_popcount(i)) :``            ``count ``=` `count ``+` `1``    ``return` `count` `# Driver Code``n ``=` `4``print` `(smallerNumsWithSameSetBits(n))` `# This code is contributed by``# Manish Shaw(manishshaw1)`

## C#

 `// C# program to find numbers less than N``// that have same Number Of Set Bits As N``using` `System;` `class` `GFG {``    ` `    ``// returns number of set bits in a number``    ``static` `int` `__builtin_popcount(``int` `n)``    ``{``        ``int` `d, t = 0;``         ` `        ``while``(n > 0)``        ``{``            ``d = n % 2;``            ``n = n / 2;``            ``if``(d == 1)``                ``t++;``        ``}``        ``return` `t;``    ``}``    ` `    ``static` `int` `smallerNumsWithSameSetBits(``int` `n)``    ``{    ``        ``// __builtin_popcount function that count``        ``// set bits in n``        ``int` `temp = __builtin_popcount(n);``        ` `        ``// Iterate from n-1 to 1``        ``int` `count = 0;``        ``for` `(``int` `i = n - 1; i > 0; i--) {``            ` `            ``// check if the number of set bits``            ``// equals to temp increment count    ``            ``if` `(temp == __builtin_popcount(i))``                ``count++;``        ``}``        ``return` `count;``    ``}``    ` `    ``// Driver Code``    ``static` `public` `void` `Main(String []args)``    ``{``        ``int` `n = 4;``        ``Console.WriteLine(``            ``smallerNumsWithSameSetBits(n));``    ``}``}` `// This code is contributed by Arnab Kundu.`

## PHP

 ` 0)``    ``{``        ``\$d` `= ``\$n` `% 2;``        ``\$n` `= ``intval``(``\$n` `/ 2);``        ``if``(``\$d` `== 1)``            ``\$t``++;``    ``}``    ``return` `\$t``;``}``function` `smallerNumsWithSameSetBits(``\$n``)``{    ``    ``// __builtin_popcount function``    ``// that count set bits in n``    ``\$temp` `= __builtin_popcount(``\$n``);``    ` `    ``// Iterate from n-1 to 1``    ``\$count` `= 0;``    ``for` `(``\$i` `= ``\$n` `- 1; ``\$i` `> 0; ``\$i``--)``    ``{``        ` `        ``// check if the number of``        ``// set bits equals to temp``        ``// increment count    ``        ``if` `(``\$temp` `== __builtin_popcount(``\$i``))``            ``\$count``++;``    ``}``    ``return` `\$count``;``}` `// Driver Code``\$n` `= 4;``echo` `(smallerNumsWithSameSetBits(``\$n``));` `// This code is contributed by``// Manish Shaw(manishshaw1)``?>`

## Javascript

 ``
Output:
`2`

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