This question already has answers here:
Why is division in Ruby returning an integer instead of decimal value?
(7 answers)
Closed 8 years ago.
Hi I have the following question:
When I put (1+7/100) in ruby it gives 1.
This is very strange because normally this is how I calculate a 7% increase in Excel.
But when I put (1+7.0/100) it gives me 1.07 which is the correct answer I expected.
Why is ruby doing this? And how do you solve this issue in your calculations in ruby?
This has nothing to do with rounding.
Ruby does division differently on float than it does on an integer.
If you divide integers, you will always get an integer result.
If you divide with floats (or a mixture of integer and float), you will always get a float result.
Remember your order of operations, too. Ruby is going to handle the division before it handles the addition.
7/100 = 0 so 1+0 = 1
7.0/100 = 0.07 so 1+0.07 = 1.07
Related
This question already has answers here:
Why does floating-point arithmetic not give exact results when adding decimal fractions?
(31 answers)
Closed 4 years ago.
I was trying to solve a mathematical problem:
37.9 - 6.05
Ruby gives me:
37.9 - 6.05
#=> 31.849999999999998
37.90 - 6.05
#=> 31.849999999999998
37.90 + 6.05
#=> 43.949999999999996
Why am I getting this?
In a nutshell, computers have a problem working with real numbers and use
floating point representation to deal with them. Much in the same way you can only represent 256 numbers with 8 bits for natural numbers, you can only represent a fixed amount of numbers with 64 bits for real numbers. For more details on this, read this http://floating-point-gui.de/ or google for "floating point arithmetic".
How should i deal with that?
Never store currency values in floating point variables. Use BigDecimal or do your calculations in cents using only integer numbers.
use round to round your floats to a user friendly length. Rounding errors will occur, especially when adding up a lot of floats.
In SQL systems use decimal data type, or use integers and divide them by a constant factor in the UI (say you need 3 decimal digits, you could store 1234 as integer and display 1.234.
This question already has answers here:
Is floating point math broken?
(31 answers)
Closed 6 years ago.
The universal advice to avoid floating point errors in ruby is to use BigDecimal. I must be overlooking something, because I think I've found a case where BigDecimal math is returning an error where a Float does not:
using Float gives the correct answer of 2.75:
> 50.0 * 0.6 / 360.0 * 33
=> 2.75
using BigDecimal gives the incorrect answer of 2.74999999:
> BigDecimal("50") * BigDecimal("0.6") / BigDecimal("360") * BigDecimal("33")
=> #<BigDecimal:7efe74824c80,'0.2749999999 999999989E1',27(36)>
Someone please tell me what I'm missing here?
Let's simplify your example, and use this one instead:
BigDecimal(1) / BigDecimal(3) * BigDecimal(3)
# => #<BigDecimal:19289d8,'0.9999999999 99999999E0',18(36)>
How did it get there?
BigDecimal(1) / BigDecimal(3)
# => #<BigDecimal:1921a70,'0.3333333333 33333333E0',18(36)>
BigDecimal does not provide rational numbers, so when you divide 1 by 3, you get 0, following by a lot of 3s. A lot, but not infinitely many. When you then multiply that by 3, you will get 0 followed by equally many 9s.
I believe you misread the BigDecimal's advertisement (although I am not sure it is anywhere advertised as the solution to floating point errors). It just provides arbitrary precision. It is still a floating point number. If you really want exact numbers when dividing numbers, you might take a look at Rational class:
(Rational(50) * Rational(0.6) / Rational(360) * Rational(33)).to_f
# => 2.75
This question already has answers here:
Why is division in Ruby returning an integer instead of decimal value?
(7 answers)
Closed 8 years ago.
I was trying to write a small program in Ruby, and I ran into the following problem: Ruby doesn't seem to be able to group numbers in parentheses.
For example:
puts (2 - 0) / 10
# prints out 0
There is obviously a flaw in the logic here. What should be happening is that (2 - 0) gets evaluated first (according to the order of operations) and then (2 - 0) should get divided by 10.
Does grouping with parentheses in Ruby not work? By the way, I'm using 2.1.2.
You're doing integer division without realizing it. 2 / 10 does equal 0 in integer division.
Try instead running this:
puts (2 - 0) / 10.0
# prints out 0.2
You will probably get an answer more like what you're expecting. The reason is that by changing 10 to 10.0, you coerce the operation into floating point division.
This question already has answers here:
Float precision in ruby
(2 answers)
Closed 8 years ago.
When I add two floats in an irb console, the result is not as expected
10.43 + 4.56 # should be 14.99
But the actual result is
irb(main):001:0> 10.43+4.56
=> 14.989999999999998
What causes this?
This is expected behavior. Floats are not guaranteed precision because, in short, computers are binary systems (as of 2013) and cannot correctly represent fractional values. This is why you get "unexpected" results, when in reality this is how it will work until there is a non-binary machine that can correctly represent fractional values.
This question already has answers here:
Addition error with ruby-1.9.2 [duplicate]
(2 answers)
Closed 4 years ago.
Whats wrong here? (ruby version: 1.9.2p290 (2011-07-09 revision 32553) [x86_64-darwin11.0.0]
x = 523.8
w = 46.9
xm = x + w
assert_equal w, (xm - x) # FAILS with: <46.9> expected but was <46.89999999999998>
From The Floating-Point Guide:
Why don’t my numbers, like 0.1 + 0.2 add up to a nice round 0.3, and
instead I get a weird result like 0.30000000000000004?
Because internally, computers use a format (binary floating-point)
that cannot accurately represent a number like 0.1, 0.2 or 0.3 at all.
When the code is compiled or interpreted, your “0.1” is already
rounded to the nearest number in that format, which results in a small
rounding error even before the calculation happens.
Read the linked-to site for details and ways to get around this.
This is perfectly normal; it is a fact about the lower-level concept of floating point arithmetic rather than Ruby and therefore can occur in any language.
Floating point arithmetic is not exact. Equality should be replaced with closeness along the lines of assert((xm-x).abs < epsilon), where epsilon is some small number like 0.01.
Read this. It describes the way binary representation of floating point numbers work in every language, not just Ruby.
The answer to your question is: No.
(Other answers tell you why, but you didn't ask that. :p)