I have been learning the paradigm of functional programming recently. For the most part, things have been going well. The hardest part has been to learn how to use recursion instead of the usual ‘for’ and ‘while’ loops that are common to imperative programming. I wanted something simple to start with, so I decided to write a recursive function that would return an array of numerical values by counting up to some number according to some stepping size. For example, cons(10, 1) would return array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) and cons(10, 2) would return array(2, 4, 6, 8, 10).
About 24 hours ago, I wrote that function and I tested it by calling cons(10, 1), cons(10,2), cons(10,4), etc. and it behaved as expected, so I thought my goal had been accomplished, but it dawned on me a little while ago that the function is wonky when it comes to step sizes of 3 and 6 (and I presume other multiples of 3). I know the function is behaving wrongly, but the code looks correct to me, so I am stumped as to what the problem could be. Here is the code for the function and a few calls that print the resulting list to the screen:
<?php
function cons($number, $step = 1, $list = array())
{
if ($number > 0)
{
$list = cons($number-$step, $step);
$list[] = $number;
}
return $list;
}
$list1 = cons(20, 1); echo '<pre>$list1 == ', print_r($list1, true), '</pre>';
$list2 = cons(20, 2); echo '<pre>$list2 == ', print_r($list2, true), '</pre>';
$list3 = cons(20, 3); echo '<pre>$list3 == ', print_r($list3, true), '</pre>';
$list4 = cons(20, 4); echo '<pre>$list4 == ', print_r($list4, true), '</pre>';
$list5 = cons(20, 5); echo '<pre>$list5 == ', print_r($list5, true), '</pre>';
$list6 = cons(20, 6); echo '<pre>$list6 == ', print_r($list6, true), '</pre>';
?>
Look at the results of $list3 and $list6. The numbers in the array increase according to the stepping size that I passed into the function, but the arrays both begin with the number 2, instead of the 3 and 6 that I had expected. Can someone please enlighten me as to why this is happening and what could be done to fix it?
