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October 18th, 2012, 07:30 AM  #1 
Joined: Oct 2012 Posts: 2 Thanks: 0  PHP Maths error or am I losing the plot?
Just did the most inoxuous of PHP lines: print 100.82031100.82; Result? 0.00031000000001313 What is going on here? Did I miss this class at school? Can someone else please try this and tell me I'm not the only one!! 
October 18th, 2012, 07:42 AM  #2 
Joined: Oct 2012 Posts: 2 Thanks: 0  Re: PHP Maths error or am I losing the plot?
I worked backwards from the above and still got the same result. i.e. I did: print 100.82031000000001313100.82; and also get 0.00031000000001313 What on earth is going on??? How can 100.82031100.82 and 100.82031000000001313100.82 equal the same thing?? Is this a problem with the PHP Maths or something more sinister 
October 18th, 2012, 09:56 AM  #3 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 15,540 Thanks: 788 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  Re: PHP Maths error or am I losing the plot? 
January 3rd, 2013, 12:19 PM  #4 
Joined: Dec 2012 Posts: 6 Thanks: 0  Re: PHP Maths error or am I losing the plot?
It's a rounding error, kg123. It happens because the machine is using binary to represent the number, and for this particular number there doesn't exist a way to store it exactly using the floating point representation. There's no way to prevent it; you just need to roll with the punches. Instead of checking if two numbers are equal, consider checking if they're "close enough" to each other. 

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