Let me understand this recursive function of PHP [closed] - php

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I am trying to understand this recursive function. When i echo out with different numbers it returns different result. I can't understand how this is working actually.
<?php
function head_sum($x) {
return ($x == 1) ? $x : $x + head_sum($x - 1);
}
echo head_sum(5);//15
echo head_sum(2);//3
echo head_sum(3);//6

Answer 1

Your function right now says:
if variable x equals 1 then
return variable x
else
return variable x + this function( variable x -1 )
So this will keep running until $x equals 1. Until then it will keep adding itself.
So when you type echo head_sum(5):
head_sum(5); //$x = 5
head_sum(4); //$x = 5+4
head_sum(3); //$x = 5+4+3
head_sum(2); //$x = 5+4+3+2
head_sum(1); //$x = 5+4+3+2+1 = 15
This can also be represented like this (hope i can show this properly without whiteboard)
function headsum(5){
return 5 + head_sum( 5-1 ){
return 4 + head_sum( 4-1 ){
return 3 + head_sum( 3-1 ){
return 2 + head_sum( 2-1 ){
return 1
}
}
}
}
}
Hope This makes some sense :)

Answer 2

Every time you call head_sum it's called multiple times.
First pass-through goes all the way until the inner head_sum calls itself.
It continues to do so until it decreased by 1 down to 1 at which point the conditional exits out.
So for head_sum(5)
5 goes in and i saved to $x.
Inside recursion calls
head_sum( 5 -1 ) // 4
Then 3
Then 2
Then 1
The ( $x == 1 ) check exits the function after adding the total tally which at this point is incremented to:
5 + 4 + 3 + 2 + 1 = 15