Checking decimal point python

Question

I'm trying to create a script that counts to 3 (step size 0.1) using while, and I'm trying to make it not display .0 for numbers without decimal number (1.0 should be displayed as 1, 2.0 should be 2...) What I tried to do is convert the float to int and then check if they equal. the problem is that it works only with the first number (0) but it doesn't work when it gets to 1.0 and 2.0..

this is my code:

i = 0
while i < 3.1:
    if int(i) == i:
        print int(i)
    else:
        print i
    i = i + 0.1

that's the output I get:

0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0

the output I should get:

0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3

thank you for your time.

Floating point numbers are not exact. There are many good tutorials on this. For example, see What Every Computer Scientist Should Know About Floating-Point Arithmetic. — John1024
– John1024, Commented Sep 2, 2016 at 21:38
Floating point arithmetic is approximate. When you add .1 10 times, you don't get 1.0. — Barmar
– Barmar, Commented Sep 2, 2016 at 21:38

aganders3 · Accepted Answer · 2016-09-02 22:40:27Z

3

Due to lack of precision in floating point numbers, they will not have an exact integral representation. Therefore, you want to make sure the difference is smaller than some small epsilon.

epsilon = 1e-10
i = 0
while i < 3.1:
    if abs(round(i) - i) < epsilon:
        print round(i)
    else:
        print i
    i = i + 0.1

edited Sep 2, 2016 at 22:40

answered Sep 2, 2016 at 21:38

aganders3

5,97429 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

morha13 Over a year ago

Thank you! Can't accept the answer yet.. what does epsilon means? What is that 1e-10

cwahls Over a year ago

Python 3 has an isclose function that solves this problem. It just means that if the difference is really small, it must be an "integer"

aganders3 Over a year ago

epsilon is just kind of a math-y term for a very small number. 1e-10 is equal to 1 * 10 ^ -10 which is 0.0000000001

morha13 Over a year ago

@aganders3 ok thank you! and what happens if I want to count to 10 instead of 3? When I tried using epsilon it worked just untill 4..

aganders3 Over a year ago

@morha13 thanks, this causes issues because sometimes the floating point representation is smaller than the integer by a bit, and int() will simply truncate the decimals. I updated my answer to use round instead.

adabsurdum · Accepted Answer · 2016-09-02 21:52:30Z

2

You can remove trailing zeros with '{0:g}'.format(1.00).

i = 0
while i < 3.1:
    if int(i) == i:
        print int(i)
    else:
        print '{0:g}'.format(i)
    i = i + 0.1

See: https://docs.python.org/3/library/string.html#format-specification-mini-language

Update: Too lazy while copy/pasting. Thanks @aganders3

i = 0
while i < 3.1:
    print '{0:g}'.format(i)
    i = i + 0.1

edited Sep 2, 2016 at 21:52

answered Sep 2, 2016 at 21:43

adabsurdum

918 bronze badges

1 Comment

aganders3 Over a year ago

Good answer, but I think you should then skip the if/else conditions.

Collectives™ on Stack Overflow

Checking decimal point python

2 Answers 2

5 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related