Question

How to check whether the number has only first and last bits set in C

Answer 1

#include <stdio.h>
#include <stdbool.h>
 
void print_bits(unsigned int n) { 
    for (int i = 7; i >= 0; i--)
       printf("%d", (n >> i) & 1);
    printf("\n");
}
 
bool is_only_first_and_last_bit_set(unsigned int n) { 
    return (((n - 1) & (n - 2)) == 0); 
} 
   
int main() 
{ 
    unsigned int n = 129;
     
    print_bits(n);
    print_bits(n - 1);
    print_bits(n - 2);
    print_bits((n - 1) & (n - 2));
     
    if (is_only_first_and_last_bit_set(n)) 
        printf("Yes\n"); 
    else
        printf("No\n"); 
   
    return 0; 
} 
 
 
/*
run:
 
10000001
10000000
01111111
00000000
Yes
 
*/

 

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