What is this while loop doing?

Question

So I've been doing some math trying to use this loop for my project and I cannot understand no matter how hard I try why does this loop run two times instead of one.

$currValue = 40.3;
$maxValue = 40.6;

while ($currValue < $maxValue) {
  $maxValue = $maxValue - 0.3;
  echo $maxValue . '<br>';
}

// Output:
// 40.3
// 40.0

I would be really grateful if somebody provided me with some sort of explanation on this and or a way to work around this. I hope this question isn't a duplicate, I didn't manage to find anything similliar.

@JohnBupit I think you're on the right track, the logic works fine with integers. — BigScar, Aug 04 '15 at 11:10

RiggsFolly · Answer 1 · 2015-08-04T11:26:13.503

1

Run this it should demonstrate what the problems is. Its related to IEEE floating point representations being slightly inaccurate for some numbers.

<?php
$currValue = 40.3;
$maxValue = 40.6;

while ($currValue <= $maxValue) {
  $maxValue = $maxValue - 0.3;
  echo 'maxValue -  ' . number_format($maxValue, 20) . ' currValue = ' . number_format($currValue,20) . '<br>';
}

edited Aug 04 '15 at 11:26

answered Aug 04 '15 at 11:12

RiggsFolly

93,638
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Best to look at something clever that I am https://en.wikipedia.org/wiki/IEEE_floating_point. Its a mine field for sure! – RiggsFolly Aug 04 '15 at 11:22
And this http://stackoverflow.com/tags/floating-point/info and this http://stackoverflow.com/questions/588004/is-floating-point-math-broken – RiggsFolly Aug 04 '15 at 11:29

score 0 · Answer 2 · answered Aug 04 '15 at 11:16

0

 $currValue = 40.3;
 $maxValue = 40.6;



 while (round( $currValue, 1) < round($maxValue,1)) {

  $maxValue = $maxValue - 0.3;
  echo $maxValue . '<br>';
 }

answered Aug 04 '15 at 11:16

Harish Lalwani

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Could you explain your answer further? – moopet Aug 04 '15 at 13:19
while executing the code in question the loop was executing one more time. This was because of representation floating point numbers. Therefore I have rounded of these numbers before comparing them thereby having a range for precision check while comparing. – Harish Lalwani Aug 04 '15 at 14:05

score 0 · Answer 3 · answered Aug 04 '15 at 11:19

0

Use rounded value in the loop like

round($maxValue,1)

instead of

$maxValue

answered Aug 04 '15 at 11:19

sujivasagam

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What is this while loop doing?

3 Answers3