Question

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

Answer 1

function print_bits($n) {
    for ($i = 7; $i >= 0; $i--) {
        echo ($n >> $i) & 1;
    }
    echo "<br />";
}

function is_only_first_and_last_bit_set($n) {
    return ((($n - 1) & ($n - 2)) == 0);
}

$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)) {
    echo "Yes";
}
else {
    echo "No"; 
}
 
 
 
/*
run:
      
10000001
10000000
01111111
00000000
Yes
     
*/

 

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