While loop not counting correctly

Question

I've been learning programming in python for the last two weeks and it's going great so far. But now I'm stuck and can't seem to find an answer. I found a really weird behaviour of a while loop, I just can't wrap my head around.

x=0
step_size=0.2

while x<2:
    print x
    x+=step_size

This code prints:

0
0.2
0.4
...
1.8
2.0

2.0 should not be printed, right? When x becomes 2.0 the statement "x<2" is false, therefore the loop should exit and never print 2.0.

And now for the really weird part: it works with other numbers. Step_size=0.4 prints up to 1.6, step_size=0.1 up to 1.9. Using "x<1" as a statement and step_size=0.2 also works.

What am I missing?

Best regards, Leo

Edit: I'm using python 2.7.5 and the default Idle Editior v2.7.5

Its one of the strange python situations where the value is not 2 but 1.9999999999999998 — Stef van der Zon, Nov 08 '18 at 12:48
You can find some more information about your problem at https://docs.python.org/3/tutorial/floatingpoint.html — TM90, Nov 08 '18 at 12:53

score 3 · Accepted Answer · edited Nov 08 '18 at 12:53

3

It's floating point arythmetic. Output in console for python 3.6

0
0.2
0.4
0.6000000000000001
0.8
1.0
1.2
1.4
1.5999999999999999
1.7999999999999998
1.9999999999999998

edited Nov 08 '18 at 12:53

Fildor

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answered Nov 08 '18 at 12:51

vencaslac

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While loop not counting correctly

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