18. Set Multiple Bits in 8-bit Register

#include <stdio.h>
#include <stdint.h>

uint8_t set_range(uint8_t reg, uint8_t start, uint8_t end) {
    // Your code here       0000 1110
    uint8_t mask = ((1 << (end - start + 1)) - 1) << start;
    return reg | mask;
}

int main() {
    uint8_t reg, start, end;//0   1      3
    scanf("%hhu %hhu %hhu", &reg, &start, &end);
    printf("%u", set_range(reg, start, end));
    return 0;
}

Generate a binary mask for the range using left-shifts, then OR it with the original value.