Algorithm to generate string range (1-10, 13, 16, 17-25.. etc) from array of numbers

Question

I'm trying to work out the page range of numbers that exist in an array. I did attempt to search google for it but I only found unrelated posts/pages.

The code I have been working on is:

$numbers = array(1,3,2,4,5,6,7,8,9,11,10,15,14,13,12,16,17,18,19,20,22);

function get_number_ranges($numbers)
{
    $last = null;
    foreach ($numbers as $number) {
        if (is_null($last)) {
            $string = $number;
            $last = $number;
        } elseif ($last + 1 != $number) {
            $string .= '-' . $last . ', ' . $number;
            $last = $number;
        } else {
            $last = $number;
        }
    }

    if ($last == $number) {
        $string .= '-' . $number;
    }

    return $string;
}

and that produces

1-1, 3-3, 2-2, 4-9, 11-11, 10-10, 15-15, 14-14, 13-13, 12-12, 16-20, 22-22

I don't want it to display the duplicate number, so i want it to look like this:

1, 3, 2, 4-9, 11, 10, 15, 14, 13, 12, 16-20, 22

This is so i can represent page ranges when storing history about a document.

Im sure this will be a nice easy one for some1, any help will be greatly received!

Answer 1

I think you looking for something like this

<?php
$numbers = array(1,3,2,4,5,6,7,8,9,11,10,15,14,13,12,16,17,18,19,20,22);

function get_number_ranges($numbers){
    $result = '';
    if( is_array($numbers) ){
        $start = $end = array_shift($numbers);
        for($i=0,$c=count($numbers);$i<$c;$i++){
            if($end == $numbers[$i]-1){
                $end = $numbers[$i];
            }else{
                $result .= ($start == $end) ? $start: $start.'-'.$end;
                $result .= ',';
                $start = $end = $numbers[$i];
            }
        }
        $result .= ($start == $end) ? $start: $start.'-'.$end;
    }

    return $result;
}

echo get_number_ranges($numbers );

See codepad result http://codepad.org/l1JUSVJO

Answer 2

Add a variable to save the first number of a range. If last number equals the first number then it is not a range and doesn't need a '-XX'.

$numbers = array(1,3,2,4,5,6,7,8,9,11,10,15,14,13,12,16,17,18,19,20,22);

function get_number_ranges($numbers)
{
    $last = null;
    $firstFromRange = null;
    foreach ($numbers as $number) {
        if (is_null($last)) {
            $string = $number;
            $firstFromRange = $number; // new range begins, save the number
        } elseif ($last + 1 != $number) {
            if ($firstFromRange == $last) { // if the range is only one number, don't add '-' . $last
                $string .= ', ' . $number;
            } else {
                $string .= '-' . $last . ', ' . $number;
            }
            $firstFromRange = $number; // new range begins, save the number
        }

        $last = $number;
    }

    if ($last == $number && $last != $firstFromRange) {
        $string .= '-' . $number;
    }

    return $string;
}

Answer 3

Basically you need to remember the first number of each range, and when you come to append the hyphen and the last number, only do so if the last number is different. Something like (untested):

function get_number_ranges($numbers)
{
    $last = null;
    foreach ($numbers as $number) {
        if (is_null($last)) {
            $string = $number;
            $last = $number;
            $first = $number ;                          //Remember first
        } elseif ($last + 1 != $number) {
            if ($first != $last )                       //Only append if different
                $string .= '-' . $last ;
            $string .= ', ' . $number;
            $last = $number;
            $first = $number ;                          //Remember first
        } else {
            $last = $number;
        }
    }

    if ($last == $number && $last != $first) {          //Only if different
        $string .= '-' . $number;
    }

    return $string;
}