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2. Python's numeric types

Pretty much all programs need to do numeric calculations. Python has several ways of representing numbers, and an assortment of operators to operate on numbers.

2.1. Basic numeric operations

To do numeric calculations in Python, you can write expressions that look more or less like algebraic expressions in many other common languages. The “+” operator is addition; “-” is subtraction; use “*” to multiply; and use “/” to divide. Here are some examples:

>>> 99 + 1
100
>>> 1 - 99
-98
>>> 7 * 5
35
>>> 81 / 9
9

The examples in this document will often use a lot of extra space between the parts of the expression, just to make things easier to read. However, these spaces are not required:

>>> 99+1
100
>>> 1-99
-98

When an expression contains more than one operation, Python defines the usual order of operations, so that higher-precedence operations like multiplication and division are done before addition and subtraction. In this example, even though the multiplication comes after the addition, it is done first.

>>> 2 + 3 * 4
14

If you want to override the usual precedence of Python operators, use parentheses:

>>> (2+3)*4
20

Here's a result you may not expect:

>>> 1 / 5
0

You might expect a result of 0.2, not zero. However, Python has different kinds of numbers. Any number without a decimal point is considered an integer, a whole number. If any of the numbers involved contain a decimal point, the computation is done using floating point type:

>>> 1.0 / 4.0
0.25
>>> 1.0 / 5.0
0.20000000000000001

That second example above may also surprise you. Mathematically, one-fifth is exactly 0.2. However, in Python (as in pretty much all other contemporary programming languages), many real numbers cannot be represented exactly. The representation of 1.0/5.0 has a slight error in the seventeenth decimal place. This behavior may be slightly annoying, but in conversational mode, Python doesn't know how much precision you want, so you get a ridiculous amount of precision, and this shows up the fact that some values are approximations.

You can use Python's print statement to display values without quite so much precision:

>>> print 1.0/5.0
0.2

It's okay to mix integer and floating point numbers in the same expression. Any integer values are coerced to their floating point equivalents.

>>> print 1.0/5
0.2
>>> print 1/5.0
0.2

Later we will learn about Python's format operator, which allows you to specify exactly how much precision to use when displaying numbers. For now, let's move on to some more of the operators on numbers.

The “%” operator between two numbers gives you the modulo. That is, the expression “x % y” returns the remainder when x is divided by y.

>>> 13 % 5
3
>>> 5.72 % 0.5
0.21999999999999975
>>> print 5.72 % 0.5
0.22

Exponentiation is expressed as “x ** y”, meaning x to the y power.

>>> 2 ** 8
256
>>> 2 ** 30
1073741824
>>> 2.0 ** 0.5
1.4142135623730951
>>> 10.0 ** 5.2
158489.31924611141
>>> 2.0 ** 100
1.2676506002282294e+30

That last number, 1.2676506002282294e+30, is an example of exponential or scientific notation. This number is read as “1.26765... times ten to the 30th power”. Similarly, a number like 1.66e-24 is read as “1.66 times ten to the minus 24th power”.

So far we have seen examples of the integer type, which is called int in Python, and the floating-point type, called the float type in Python. Values in int type must be between -2,147,483,648 and 2,147,483,647 (inclusive).

There is another type called long, that can represent much larger integer values. Python automatically switches to this type whenever an expression has values larger than about two billion. You will see letter “L” appear at the end of such values, but they act just like regular integers.

>>> 2 ** 50
1125899906842624L
>>> 2 ** 100
1267650600228229401496703205376L
>>> 2 ** 1000
107150860718626732094842504906000181056140481170553360744375038837035105112
493612249319837881569585812759467291755314682518714528569231404359845775746
985748039345677748242309854210746050623711418779541821530464749835819412673
987675591655439460770629145711964776865421676604298316526243868372056680693
76L