Change rating numbers to 1 - 5

Question

I have an array of rating numbers.

array(34, 35, 33, 17, 38, 29, 31, 23)

I want the numbers to automatically turn into a rating between 1 and 5 (1, 2, 3, 4 or 5).

Example

• "38" (highest number) should have a rating of 5
• "17" (lowest number) should have a rating of 1.
• "31" should probably have a rating of 3.

How do I calculate this with PHP code? The highest number might be higher than 35 and the lowest might be lower than 17.

Answer 1

You are trying to map the numbers 17 and 38 to the numbers 1 and 5, interpolating all values in between. Let us use these numbers in the below example where num represents any number from the range:

(num - 17)                  maps 17 and 38 to 0 and 21
(num - 17) / (21)           maps  0 and 21 to 0 and  1
(num - 17) / (21) * (4)     maps  0 and  1 to 0 and  4
(num - 17) / (21) * (4) + 1 maps  0 and  4 to 1 and  5

PHP Code:

function mapvaluetorange($array, $a, $b) {
    $map = array();
    $min = min($array);
    $max = max($array);
    foreach ($array as $value) {
        $map[] = array(
            "old" => $value,
            "new" => ($value - $min) / ($max - $min) * ($b - $a) + $a
        );
    }
    return $map;
}
$map = mapvaluetorange(array(34, 35, 33, 17, 38, 29, 31, 23), 1, 5);

Output:

int(34) -> float(4.2380952380952)
int(35) -> float(4.4285714285714)
int(33) -> float(4.047619047619)
int(17) -> int(1)
int(38) -> int(5)
int(29) -> float(3.2857142857143)
int(31) -> float(3.6666666666667)
int(23) -> float(2.1428571428571)

Use the round function to round floats to integers.

Answer 2

As an alternate approach, you could use an anonymous function and array_map() to map the values of the array to the range you want like this:

    <?php
    $x = array(34, 35, 33, 17, 38, 29, 31, 23);
    $max = max($x);
    $min = min($x);
    $map = function($value) use($min,$max) {return (int)round((($value-$min)/($max-$min))*4+1);};
    $x = array_map($map, $x);
    var_dump($x);

Gives:

    array(8) {
        [0] =>   int(4)
        [1] =>   int(4)
        [2] =>   int(4)
        [3] =>   int(1)
        [4] =>   int(5)
        [5] =>   int(3)
        [6] =>   int(4)
        [7] =>   int(2)
    }