Floating Point Comparisons In PHP and Javascript

By Justin Laing
2008-01-08


Beware Comparing Floating Point Values Can Be Hazardous!

The curse of the .0000000001 and .9999999999!”

Here’s the problem. Say you’re comparing two floating point numbers a (71.00) and b (71.00) to see if they are the same. The problem is if you’ve done any calculations to arrive at these numbers they might actually be stored as 71.00000000001. Now if one of them is stored that way and the other isn’t and you compare the two to see if they are equal you’ll get a FALSE as the response, even though they should be the same.

This isn’t a bug, it’s how floating point comparisons are designed to work. It makes it so people writing complex software that requires floating point arithmetic can do what they need to. Unfortunately for the rest of us that usually are just using floating point numbers to represent things like currency (dollars in my case) it really makes life difficult!

Tutorial Pages:
» Beware Comparing Floating Point Values Can Be Hazardous!
» PHP Floating Point Comparison Made Easy
» Javascript Floating Point Comparison Made Easy
» Resources And Further Reading


Originally posted on Makebeta

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