assert_equal 1.1035, 1.0035 + 0.1 is false

rubyonrails-talk
1

some of my tests failed when comparing two floats.

the comparison of:

1.1035 == 1.0035 + 0.1

evaluates to false - i do not really understand why.

must depend on the value range, because

1.103 == 1.003 + 0.1

evaluates to true.

would be great if anyone could help me getting this working and maybe giving an explanation why ruby do not handle the == operator for float as one would expect (coming from java).

tested on:

1.8.6 (2008-03-03 patchlevel 114) [universal-darwin9.0]

regards jan zimmek

2

some of my tests failed when comparing two floats.

the comparison of:

1.1035 == 1.0035 + 0.1

evaluates to false - i do not really understand why.

must depend on the value range, because

1.103 == 1.003 + 0.1

basically because 0.1 cannot be exactly represented in binary.

assert_in_delta is good for this sort of stuff.

Fred

3

Crud, he beat me to it.

You're going to have to either change the logic or change the testing procedure for it.

4

thanks for clarification ... have found some further explanations/ solutions and will change my testing-logic.

5

Quoting jzimmek <jan.zimmek@web.de>:

some of my tests failed when comparing two floats.

the comparison of:

1.1035 == 1.0035 + 0.1

evaluates to false - i do not really understand why.

must depend on the value range, because

1.103 == 1.003 + 0.1

In general, exact comparisons of floating point numbers is a bad idea. A better idea is something along the lines of:

(value1 - value2).abs <= epsilon

epsilon may be a fixed value of a fraction of the values, maybe

epsilon = (value1 + value2) / 100

i.e. 1/50 the average of the two. The best solution depends on the particular problem.

Jeffrey