如何更改两个连续位的无符号整数

Question

How to change unsigned integer two consecutive bits by a given value: 00 01 or 10:

For example:

    unsigned int n = 165; // 1010 0101
    void changeTwoConsecutiveBits(unsigned int& n, int twoBitsValue, int positionStart)
    {
    assert(twoBitsValue== 0 || twoBitsValue== 1 || twoBitsValue== 2);
    ...
    }

    changeTwoConsecutiveBits(n, 2, 2);
    //n == 1010 [10]01 // [1 0] is the value of twoBitsValue: 2
    assert(n == 169);

n = 196; // 1100 0100
changeTwoConsecutiveBits(n, 1, 3);
//n == 110[0 1]100 // [0 1] is the value of twoBitsValue: 1
assert(n == 204);

Answer 1

Here is some sample code that replace the two bits in a position given by the indicated bits.

Probably need also to check that positionStart is valid position for value type (in this case unsigned int ).

Code:

#include <assert.h>
#include <iostream>

void changeTwoConsecutiveBits(unsigned int& n, int twoBitsValue,
                              int positionStart) {
    assert(twoBitsValue == 0 || twoBitsValue == 1 || twoBitsValue == 2);

    // 0b11 or 3 if binary literal are not supported in compiler
    const unsigned int inverse_mask = ~(0b11 << positionStart);
    n = (n & inverse_mask) | (twoBitsValue << positionStart);
}

int main() {
    unsigned int n = 165; // 1010 0101
    std::cout << "n1 pre: " << n << std::endl;
    changeTwoConsecutiveBits(n, 2, 2);
    // n == 1010 [10]01 // [1 0] is the value of twoBitsValue: 2
    assert(n == 169);
    std::cout << "n1 post: " << n << std::endl;

    n = 196; // 1100 0100
    std::cout << "n2 pre: " << n << std::endl;
    changeTwoConsecutiveBits(n, 1, 3);
    // n == 110[0 1]100 // [0 1] is the value of twoBitsValue: 1
    assert(n == 204);
    std::cout << "n2 post: " << n << std::endl;
}