# Am I retarded?

**URL:** <https://www.ruby-forum.com/t/am-i-retarded/81145>\
**Category:** Rails\
**Created:** [January 4, 2007, 7:36pm UTC](https://www.ruby-forum.com/t/am-i-retarded/81145 "2007-01-04T19:36:25Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![benjohnson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/3ec8ea/32.png) [@benjohnson](https://www.ruby-forum.com/u/benjohnson)\
**Post date:** [January 4, 2007, 7:36pm UTC](https://www.ruby-forum.com/t/am-i-retarded/81145/1 "2007-01-04T19:36:25Z")

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Doing a simple test here and I’m not really sure why this isn’t passing,  
which makes me feel retarded. All that the amount method is doing is  
taking 10% from the argument.

“#{@some\_discount.amount(51)} == #{5.1}”  
=\> “5.1 == 5.1”

“#{@some\_discount.amount(51).class.name} == #{(5.1).class.name}”  
=\> “Float == Float”

“#{@some\_discount.amount(51) == 5.1}”  
=\> false

What’s the deal?

Thanks for your help.

---

<div class="post-metadata">

**Author:** ![benjohnson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/3ec8ea/32.png) [@benjohnson](https://www.ruby-forum.com/u/benjohnson)\
**Post date:** [January 4, 2007, 9:23pm UTC](https://www.ruby-forum.com/t/am-i-retarded/81145/2 "2007-01-04T21:23:31Z")

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Ben J. wrote:

> Doing a simple test here and I’m not really sure why this isn’t passing,  
> which makes me feel retarded. All that the amount method is doing is  
> taking 10% from the argument.
> 
> “#{@some\_discount.amount(51)} == #{5.1}”  
> =\> “5.1 == 5.1”
> 
> “#{@some\_discount.amount(51).class.name} == #{(5.1).class.name}”  
> =\> “Float == Float”
> 
> “#{@some\_discount.amount(51) == 5.1}”  
> =\> false
> 
> What’s the deal?
> 
> Thanks for your help.

Floating point arithmetic cannot store some numbers exactly, and 5.1 is  
one of them. Try executing “#{51/10 == 5.1}” and see what you get. :]

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<div class="post-metadata">

**Author:** ![benjohnson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/3ec8ea/32.png) [@benjohnson](https://www.ruby-forum.com/u/benjohnson)\
**Post date:** [January 4, 2007, 9:32pm UTC](https://www.ruby-forum.com/t/am-i-retarded/81145/3 "2007-01-04T21:32:11Z")

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Ben J. wrote:

> What’s the deal?
> 
> Thanks for your help.

Ben -

I’m fairly sure that your problem here is the way ruby (and computers in  
general) deal with floats. Floats are inherently imprecise and when you  
perform mathematical operations with them, this imprecision will become  
apparent.

irb(main):001:0\> a = 5.1  
=\> 5.1  
irb(main):002:0\> b = 5.1  
=\> 5.1  
irb(main):003:0\> a == b  
=\> true  
irb(main):004:0\> x = 51 \* 0.1  
=\> 5.1  
irb(main):005:0\> y = 5.1  
=\> 5.1  
irb(main):006:0\> x == y  
=\> false

Hope this helps.

-Drew

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<div class="post-metadata">

**Author:** ![benjohnson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/3ec8ea/32.png) [@benjohnson](https://www.ruby-forum.com/u/benjohnson)\
**Post date:** [January 6, 2007, 7:30pm UTC](https://www.ruby-forum.com/t/am-i-retarded/81145/4 "2007-01-06T19:30:45Z")

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What you see is not what you get with floating point numbers. Have Ruby  
print out 20+ decimal places and you’ll see that there is in-fact a  
difference. When trying to compare floats, you need to use an epsilon  
and  
check that the two values are close enough to each other. e.g.

x = 5.1  
y = 5.1  
epsilon = 0.001

abs(x - y) \< epsilon

Jason