Php subtraction and adding sum

Question

I have to create a sum on php that goes like this

1+2-3+4-5+6-7+8-8+10

For far I got this:

<?php
$start = 1;
$n=10;
$sum = 0;
for($i=$start; $i <=$n; $i++){
$sum += $i;
}
echo "sum from " . $start . " to " . $n . " = " . $sum;
?>

I understand the php code is adding but I'm unsure how to switch between adding and subtracting as the sum continues. Thank you for answering my query.

Hello welcome to stackoverflow? Please visit How to ask and Asking .... why not break it into only additions and only subtractions ? This is definitely possible with what you know !! Have you searched on web for how to branch in php? — รยקคгรђשค
You have received three usable answers. If you do not get into the habit of accepting and up-voting answers, people are going to be less likely to help you in the future. — Patrick Q

Patrick Q Patrick Q · Accepted Answer · 2017-11-01T12:44:26

The logic should be, after adding 1, you add every even number and subtract every odd number. To do this, you make use of the modulo operator.

$start = 1;
$n=10;
$sum = 0;
for($i=$start; $i <=$n; $i++){
    // for 1 or any even number (use modulo operator to check remainder when dividing by 2), add to sum
    if($i == 1 || $i%2 == 0)
    {
        $sum += $i;
    }
    // for any other number (any non-1 odd number), subtract from sum
    else
    {
        $sum -= $i;
    }
}
echo "sum from " . $start . " to " . $n . " = " . $sum;

DEMO

Php subtraction and adding sum

1 Answers