Python Tutorial 3 – Integers and Floats

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As you saw in the final section of the previous tutorial on numeric expressions in Python, Fractions, the following two expressions give different answers:

[sourcecode language=”Python”]
18.0 / 7.0
# This returns 2.571485714285716

18 / 7
# This returns 2
[/sourcecode]

So what’s going on here? Well, this is an example of the two main number systems in Python, integers and floats.

A float can be distinguished from an integer because it has a fractional part, even if it is just .0, such as the .0 at the end of the 18 and 7 in the above example.

If you choose to leave out the fractional portion of a set of numbers, Python will always return it as an integer, unless you force it to return it as a float which I will explain in a moment.

If you put in the fractional part, even if it is just a 0, Python will always return it as a float, unless you force it to do otherwise.

Now what happens if you use both a float and an int? Such as 18 / 7.0?

Let’s try it and find out.

[sourcecode language=”Python”]
18 / 7.0
# I get the same as 18.0 / 7.0
# 2.571485714285716
[/sourcecode]

Why does it do this?

By using a float with an integer, Python defaults to a float for the result.

Forcing a Number Type in Python

Forcing a number to be either an integer or a float is a pretty simple task. All you have to do is add the following to your expressions:

[sourcecode language=”Python”]
int (18.0 / 7.0)
# This will force Python to return an integer value
# instead of the default float.

float (18 / 7)
# This will force Python to return a float value
# instead of the default integer.
[/sourcecode]

If you need to determine what number type something is, this can be determined very easily with the following function:

[sourcecode language=”Python”]
type (18)
# This will return <type ‘int’>
# In other words, 18 is an integer.

type (18.0)
# This will return <type ‘float’>
# So, 18.0 is a float.
[/sourcecode]

If you convert a floating point number to an integer, the fraction is lost, so if you need to use it, it is a good idea to keep your numbers in the floating point format.

Does 1 = 1.0?

You would expect 1 and 1.0 to be the same, and logically, they are, right?

While the integer 1 is numerically the same as 1.0, their value is not equal. You can test this in Python by doing the following:

[sourcecode language=”Python”]
1 == 1.0
# This asks Python if 1 is exactly equal to 1.0
# It returns True.

1 is 1.0
# This asks Python if 1 and 1.0 hold the same value.
# It returns False.
[/sourcecode]

Floating point numbers are encoded as finite approximations in binary. This can cause some slight rounding inaccuracies to occur that are machine based.

Have a look below at some examples:

[sourcecode language=”Python”]
1.0 / 3.0
# Python cannot encode the infinite fraction, try it and see.

(0.1 * 3.0) / 0.1
# This should return 3.0, I do not get exactly that though.
# Try it and see what you get. It will be 3.something.
[/sourcecode]

If you have any comments thus far, please let me know, the same goes if you have any corrections or input for the tutorials.

Next up I will be looking at strings in Python, so stay tuned.

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