PHP Face Test (calculates the number of ' 1 ' after a decimal number goes into binary)



This is yesterday to interview, met a face question.

At first glance it's pretty simple, but it's not in the details.

After home Baidu, find an answer.

As follows:

To calculate the number of ' 1 ' in a decimal number converted to a binary number//For example, decimal 11 = binary 1011, the result is 3 1     //Solving ideas: Use N & (n-1) To change the last 1 to 0   //xxxx1000 & (xxxx1  000-1) = xxxx1000 & xxxx0111 = xxxx0000   //1011 & (1011-1) = 1011 & 1010 = 1010   //Until the last 1 is associated with 0, results are obtained function Count1 ($n) {        $r = 0;        while ($n! = 0) {             $r + +;             $n &= ($n-1);         }         return $r;    }      

After looking at it is not very easy to understand (my bit of computational science is not good ...) ）

Think for a moment, have the following solution:

function Count1 ($n) {   $r = 0; while ($n!=0) {if (($n%2)!=0) {$r + +;}         $n = $n/2;    } return $r;}    Echo Count1 (8);

This should be easier to understand.

The above introduces the PHP surface of the test together (calculate the number of a decimal number to binary after the ' 1 '), including the aspects of the content, I hope that the PHP tutorial interested in a friend helpful.

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