PHP - For Loop Question

Ridgejp
Ridgejp used Ask the Experts™
on
Hi'

Just researching how 'For loops' behave and I wrote the following ...
<?php

$t = 0;
    
    for($i=1, $j=1; $i<=3, $j>=-3; $i++, $j--)
        
    {
        
        $t = $i * $j;
        echo "$i x $j = $t<br>";
    
    }
?>

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I thought I'd get the following results: -

1 * 1 = 1
2 * 0 = 0
3 * -1 = -3

And yet the following happens: -

1 * 1 = 1
2 * 0 = 0
3 * -1 = -3
4 * -2 = -8
5 * -3 = -15

Why doesn't the for loop stop when the first statement for $i hits 3? Equally, if I change $i to <=12 the loop is affected by $j being a negative int and doesn't go below -3.

Hope that makes sense?
J
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pritaeasSoftware Engineer

Commented:
According to the manual:

http://php.net/manual/en/control-structures.for.php

"Each of the expressions can be empty or contain multiple expressions separated by commas. In expr2, all expressions separated by a comma are evaluated but the result is taken from the last part."

So basically that would mean your code will run while $j>=-3
Most Valuable Expert 2017
Distinguished Expert 2018
Commented:
The comma separation of expressions means each one will be run sequentially after the other - as pritaeas points out - only the last will be evaluated.

This is because there is nothing telling the compiler that you want the evaluation to work on all expressions. To do that you need to use boolean operators

i.e.
for($i=1, $j=1; $i<=3 && $j>=-3; $i++, $j--)

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This is inherited from PHP's C roots where you could almost write an entire application inside the parts of a for loop - not always good practice but possible.

By including the && boolean operator you are changing the execution from running a sequential set of instructions and evaluating each on its own (the last man wins) to a Boolean expression that considers the output value of all the sub-expressions.
Most Valuable Expert 2011
Top Expert 2016

Commented:
Footnote: I'm a fairly experienced programmer, and I've never seen anyone do anything quite like what you've posted with this question.  If it's purely academic, I understand.  But if it's going to be part of a deployed application, you might want to post a new question here, and ask about best practices to solve the problem!

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