从C ++中的数字和位掩码生成数字数组

Question

I would like to create a function that returns an array (or vector) of numbers from a bit mask, my function header would be:

vector generateArray(int number, int maskSize)

if I call this function this way:

generateArray(4,2)

4 being equal to 100 in binary,

it would return a vector with these numbers: 5 , 6 , 7 which are in binary 101,110 and 111. So the function changed all the 2 last bits of the number 100 to generate all the possibilities.

I have no clue how to do it .. any idea would help a lot !

Thanks in advance

Answer 1

Your mask size can be thought of as a way to generate all numbers that can be represented using maskSize bits. In your example that is 2 or 2 bits, which has 2 ² possible values: 00 01 10 11, or 0, 1, 2, 3, which if you add each of those to number gives you 4, 5, 6, and 7.

So what you do is compute all the numbers that can be reached from 2 ^maskSize and add those to `number' to fill out your array.

Answer 2

This should do the trick -

void generateArray(int n, int m) {
    if (m == 0) {
        printf ("%d\n", n);
        return;
    }
    int mask = 1 << (m-1);
    generateArray(n & ~mask, m-1);
    generateArray(n |  mask, m-1);
}

This c code prints out the numbers, but the principle is the same - just modify it to accumulate them in your vector, and remove the initial parameter.