Q5.1 Set Bits

Q:

You are given two 32-bit numbers, N and M, and two bit positions, i and j. Write a method to set all bits between i and j in N equal to M (e.g., M becomes a substring of N located at i and starting at j).

EXAMPLE:

Input: N = 10000000000, M = 10101, i = 2, j = 6

Output: N = 10001010100

A:

先将N的第0为到i位保存到ret，不包括第i位， 接下来将N的第0位到第j位置0， 包括第j位。 最后或上(m<<i) | ret。 

#include <iostream>
#include <vector>
using namespace std;

void print_binary(int n) {
	vector<int> bit;
	int m = 1;
	int len = 8*sizeof(int);
	while(len--) {
		if (n&m) bit.push_back(1);
		else bit.push_back(0);
		m <<= 1;
	}
	while (!bit.empty()) {
		cout<<bit.back();
		bit.pop_back();
	}
	cout<<endl;
}

int update_bits(int n, int m, int i, int j) {
	int ret = (1<<i)-1;
	ret &= n;
	return (n>>(j+1))<<(j+1) | (m<<i | ret);
}

int main(){
    int n = 1<<10, m = 21;
    int ans = update_bits(n, m, 2, 6);
    print_binary(n);
    print_binary(m);
    print_binary(ans);
    return 0;
}