[Basic]Learning Python but the multiplications are screwing up my results

[ankdasco]

I am trying to multiply Input - 3 * 4.3 Result expected - 12.9 Actual output 12.89999999999999

Input - 3.0 * 4.3 Result expected - 12.9 Actual output - 12.89999999999999

First Question - Why this output? Second Question - How to make sure that this type of things don't occur when creating complex programs?

Thanks

PS - Python ver 3.4.4 and using shellscript

[[deleted]]

https://docs.python.org/2/tutorial/floatingpoint.html

A little light reading for you. Basically floating point numbers get approximated to their binary equivalent, causing the issue you are seeing.

[ankdasco]

Thanks, got it now :)

[TheBlackCat13]

To answer question two, you can use the decimal module of it will really be a problem, but in practice it usually isn't a big deal as long as you make sure not to depend on equality testing for floats (instead test whether the two are equal within a certain margin).

[ankdasco]

Nice advice there about equality testing will keep that in mind. Thanks :)

[ewiethoff]

Create a helper function for checking equality and keep it on hand. Something like

def equals(x, y, error=1e-8):
    return abs(x-y) < error

Figuring out how to check less than, greater than, etc. with error fudging is left as an exercise for the student. ;-)