5 ms·
Ask HN: Why Ruby doesn’t know how to do maths?
I'm not an expert in maths but the following operation gives a different result in ruby than in any other language or calculator I've tried:
Ruby:
(289 / 30 * 30) - (149 / 30 * 30)
150
Rest of the world:
(289 / 30 * 30) - (149 / 30 * 30)
140
An explanation is greatly appreciated
- oregontechninja 7y agoThis is a stack overflow question buddy
- mtmail 7y agoIt defaults to integer arithmetic. 5/3 will return 1. 5.to_f/3 will return 1.6666666666666667
- Dajve_Bloke 7y ago289/30 = 9 as it's truncating the non-integer part of the operation. So the calculation evaluates to 270 - 120 = 150
- ThrowawayR2 7y ago289/30 is exactly 9 and 149/30 is exactly 4 because they are integer expressions. You need to specify the constants as floating point values if you want floating point behavior. Ruby does know how to do math and is doing precisely what you told it to.
- ajimix 7y agothanks for the clear explanation. It's strange to me as a newbie in ruby that this can happen and I think that can lead to errors if you don't take that into account when doing calculations with numbers... Good to know
- ThrowawayR2 7y agoThe distinction between integers and floating point variables is the same in most mainstream languages and is not specific to Ruby.
- Dajve_Bloke 7y agoNB - you get the same result in MS-SQL (and for exactly the same reason). Ruby behaves like SQL in that appending a decimal to the values results in non-integer maths being employed. MS-SQL itself is slightly out (I assume due to floating point inaccuracy) SELECT (289.0 / 30.0 * 30.0) - (149.0 / 30.0 * 30.0) 140.0000100