?
Solved

PHP - For Loop Question

Posted on 2016-09-12
3
Medium Priority
?
45 Views
Last Modified: 2016-09-12
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>";
    
    }
?>

Open in new window


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
0
Comment
Question by:Ridgejp
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
3 Comments
 
LVL 6

Expert Comment

by:pritaeas
ID: 41793929
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
1
 
LVL 58

Accepted Solution

by:
Julian Hansen earned 2000 total points
ID: 41793959
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--)

Open in new window


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.
0
 
LVL 111

Expert Comment

by:Ray Paseur
ID: 41794028
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!
0

Featured Post

Secure Your WordPress Site: 5 Essential Approaches

WordPress is the web's most popular CMS, but its dominance also makes it a target for attackers. Our eBook will show you how to:

Prevent costly exploits of core and plugin vulnerabilities
Repel automated attacks
Lock down your dashboard, secure your code, and protect your users

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Foreword (July, 2015) Since I first wrote this article, years ago, a great many more people have begun using the internet.  They are coming online from every part of the globe, learning, reading, shopping and spending money at an ever-increasing ra…
This article discusses how to create an extensible mechanism for linked drop downs.
The viewer will learn how to create and use a small PHP class to apply a watermark to an image. This video shows the viewer the setup for the PHP watermark as well as important coding language. Continue to Part 2 to learn the core code used in creat…
The viewer will learn how to create a basic form using some HTML5 and PHP for later processing. Set up your basic HTML file. Open your form tag and set the method and action attributes.: (CODE) Set up your first few inputs one for the name and …
Suggested Courses

764 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question