Very simple PHP addition problem

Question

I think I have been looking at this for too long. Why is this code printing 'no', it should be printing 'yes' shouldn't it? I've tried it on PHP 5.3 and PHP 5.2 and both print 'no'.

<?php

$total = 14.05;
$var1 = 0;
$var2 = 0.11;
$var3 = 13.94;

if(($var1 + $var2 + $var3) == $total)
{
    echo 'yes';
}
else
{
    echo 'no';
}

?>

**Never** compare floating point values for direct equality. There are lots of questions here on SO about that. — Jon, Apr 07 '11 at 14:38
Probably relevant: [What Every Computer Scientist Should Know About Floating-Point Arithmetic](http://download.oracle.com/docs/cd/E19957-01/806-3568/ncg_goldberg.html) — eldarerathis, Apr 07 '11 at 14:38
See this: http://stackoverflow.com/questions/3148937/compare-floats-in-php/3148991#3148991 — dusan, Apr 07 '11 at 14:38
Thanks for all the answers, they help a lot - would vote up but can't yet. — RandomCoder, Apr 07 '11 at 18:58

score 6 · Accepted Answer · answered Apr 07 '11 at 14:38

6

See Comparing floating point numbers.

This doesn't work because floating point numbers are not represented exactly. A small rounding error is enough for the equality operator to fail.

answered Apr 07 '11 at 14:38

MaxVT

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score 2 · Answer 2 · answered Apr 07 '11 at 14:41

Like everybody else say, dont compare floats directly. Just do a if (abs($float1-$float2)<0.0000001)

or similair

In your case

<?php

$total = 14.05;
$var1 = 0;
$var2 = 0.11;
$var3 = 13.94;

if (abs(($var1 + $var2 + $var3)-$total)<0.000001)
{
    echo 'yes';
}
else
{
    echo 'no';
}

?>

score 2 · Answer 3 · edited Apr 08 '11 at 22:52

2

As mentioned in other answers, be careful when comparing floats. Try this instead:

echo ((int)(($var1+$var2+$var3)-$total)==0)?"yes":"no";

edited Apr 08 '11 at 22:52

answered Apr 08 '11 at 09:13

xdebug

1,178
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score 1 · Answer 4 · answered Apr 07 '11 at 14:42

1

A very lazy way around this:

$var4 = $var1 + $var2 + $var3;
$var4 = number_format($var4, 2);
$total = number_format($total, 2);

if($var4 == $total)
{
    echo 'yes';
}
else
{
    echo 'no';
}

http://php.net/number_format

answered Apr 07 '11 at 14:42

Treffynnon

21,365
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score 1 · Answer 5 · answered Apr 07 '11 at 14:47

1

If you need to do precise floating point math a lot, it's worth using a library like GMP or BCMath

answered Apr 07 '11 at 14:47

dnagirl

20,196
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score 0 · Answer 6 · answered Apr 07 '11 at 14:38

Because .. thou shalt not compare floats for equality ... it's a rounding issue ... something like 14.05 is actually represented as 14.0499999... all you could do would be comparing with a certain tolarence, e.g. t=0.01 then a=b would be considered true if a+t >= b >= a-t.

score 0 · Answer 7 · answered Apr 07 '11 at 14:41

I suspect that this is due to floating point precision.

Basically floating point numbers can't be stored with full precision and sometimes results are not what you expect. In order to compare floating point numbers you should allow some tolerance. If you are sure that each number has exactly two decimal places you could multiply by 100 to make them integer values and compare the total then.

score 0 · Answer 8 · answered Apr 07 '11 at 14:41

This is caused by floating point precision. Floating point number represent the actual value, but only to a certain precision. Try the following instead:

<?
$total = 14.05;
$var1 = 0;
$var2 = 0.11;
$var3 = 13.94;

if((($var1 + $var2 + $var3) > ($total - 0.0000001)) && (($var1 + $var2 + $var3) < ($total + 0.0000001)))
{
    echo 'yes';
}
else
{
    echo 'no';
}
?>

score 0 · Answer 9 · answered Apr 07 '11 at 14:45

It's a basic floating point maths problem.

Computers work with binary maths. For integer values, there's no problems. But for decimals it uses a technique called floating point maths. The trouble is that while floating points are good, the don't map directly to decimal values; even relatively simple decimal values may not be possible to represent precisely in binary.

If you're always working with two decimal places - eg for currencies - it is often a better idea to deal with your values as integers (eg store cents rather than dollars, or pence rather than pounds), so that all your maths is done using integers and the results will be accurate. You only need it as a decimal for display purposes, which you can do at the point when you need it.

score 0 · Answer 10 · answered Apr 07 '11 at 14:56

Floating point values have a limited precision. Hence a value might not have the same string representation after any processing. That also includes writing a floating point value in your script and directly printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE 754 is, read this:

http://docs.sun.com/source/806-3568/ncg_goldberg.html

you can use number_formattig or string casting before comperation

Very simple PHP addition problem

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