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# Count 1 in bits

More than 3 years have passed since last update.

Problem:
Count 1 in 32 bits

00000000000000000000000000001110

# Bruteforce approach

iterate from whole bit and count 1.
if input is n bits, the answer is O(n)

# Better approach

Utilizing bit operation.
x is the input.

x - 1 would find the first set bit from the end, and then set it to 0, and set all the bits following it.

```Which means if x = 10101001010100
^
|
|
|

First set bit from the end
```

Then x - 1 becomes 10101001010(011)

This approach is O(m). m is the number of 1 in the bits

```#include<iostream>
#include<bitset>

using namespace std;

int main(){
// 11111 in bits
unsigned int x = 15;

int total = 0;

while (x != 0){
cout << x << endl;
bitset<32> a1(x);
cout << a1 << endl;
bitset<32> a2(x - 1);
cout << a2 << endl;

x = x & (x - 1);
total++;

bitset<32> a3(x);
cout << a3 << endl;

}

cout << total << endl;

}

```
```15
// x
00000000000000000000000000001111
// x - 1
00000000000000000000000000001110
// x & (x - 1)
00000000000000000000000000001110
14
// x
00000000000000000000000000001110
// x - 1
00000000000000000000000000001101
// x & (x -1)
00000000000000000000000000001100
12
// x
00000000000000000000000000001100
// x - 1
00000000000000000000000000001011
// x & (x - 1)
00000000000000000000000000001000
8
// x
00000000000000000000000000001000
// x - 1
00000000000000000000000000000111
// x & (x - 1)
00000000000000000000000000000000
4

```
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