如何反转php中while循环输出的顺序

Question

My goal is to convert a decimal integer to binary as described in this video http://youtu.be/XdZqk8BXPwg with a php function, i know that php can do that out of the box with the built in decbin(), but i wanted to write one anyway.

<?php

function decToBin($int) {
$roundInt = intval($int) * 2;
    while ($roundInt > 1) {
        $result = intval($roundInt = $roundInt / 2);
        if ($result % 2 == 0) {
            $result = 0;
        } else {
            $result = 1;
        }
        echo $result;
    }
}
decToBin(123);

I tried while loop but i get the result upside down.

Is there a way i can inverse that, so instead of 11011110 i get 01111011, or better without the zero in front.

Thankyou

Answer 1

Instead of echoing the results one bit at a time, build up a string by adding new values on the left:

<?php
function decToBin($int) {
    $roundInt = intval($int) * 2;
    $output = '';
    while ($roundInt > 1) {
        $result = intval($roundInt = $roundInt / 2);
        if ($result % 2 == 0) {
            $result = 0;
        } else {
            $result = 1;
        }
        $output = $result . $output;
    }
    echo $output;
}

Answer 2

Just a few thing you're doing wrong here:

1. You have a rounding error ( use intdiv for integer division instead of what you're doing, which has a compounding effect ).

2. Specify an actual typehint instead of casting (ensures type safety)

3. Return an actual value from the function, don't output (you retain control over its final composition)

Here's what your function should actually look like...

function decToBin(Int $int): String {
    $bin = ""; // Initialize the return value

    $roundInt = $int * 2;

    while ($roundInt > 1) {
        $roundInt = $result = intdiv($roundInt, 2); // Safe integer division
        $result &= 1;
        $bin = $result . $bin; // Compose with byte endianness at LSB first  
    }

    return $bin;
}

var_dump(decToBin(123));

Now you get the actual correct result...

string(7) "1111011"

Answer 3

I have made minimal changes to existing code without changing the method. You can use strrev function which reverse the output. Here data is append to $return_data which returns and store in $returned_data and then used strrev predefined function.

function decToBin($int) {
$roundInt = intval($int) * 2;
$return_data ='';
    while ($roundInt > 1) {
        $result = intval($roundInt = $roundInt / 2);
        if ($result % 2 == 0) {
            $result = 0;
        } else {
            $result = 1;
        }
        $return_data .=$result;  //Data appending
    }
  return $return_data; //returns
}
$returned_data =  decToBin(123);
echo strrev($returned_data); //reverse function