2.81 Generate Bits

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Problem:

Write C expressions to generate the bit patterns that follow, where a^k represents k repetitions of symbol a. Assume a w-bit data type. Your code may contain references to parameters j and k, representing the values of j and k, but not a parameter representing w.

A. 1^{w-k}0{k}

-1 << k

B. 0^{w-k-j} 1^k 0^j

~(-1 << k) << j

Code:

#include <stdio.h>
#include <assert.h>

/* 0 <= k < w */
int A(int k) {
    return -1 << k;
}

/* 0 <= k,j < w */
int B(int k, int j) {
    return ~(-1 << k) <<j;
}

int main() {
    assert(A(3) == 0xFFFFFFF8);
    assert(B(3, 4) == 0x00000070);
    return 0;
}