3.1 Numbers
Numbers
Computers are designed to perform numerical calculations, but there are some important details about working with numbers that every programmer working with quantitative data should know. Python (and most other programming languages) distinguishes between two different types of numbers:
- Integers are called
int
values in the Python language. They can only represent whole numbers (negative, zero, or positive) that don’t have a fractional component - Real numbers are called
float
values (or floating point values) in the Python language. They can represent whole or fractional numbers but have some limitations.
The type of a number is evident from the way it is displayed: int
values
have no decimal point and float
values always have a decimal point.
# Some int values
2
2
1 + 3
4
-1234567890000000000
-1234567890000000000
# Some float values
1.2
1.2
1.5 + 2
3.5
3 / 1
3.0
-12345678900000000000.0
-1.23456789e+19
When a float
value is combined with an int
value using some arithmetic
operator, then the result is always a float
value. In most cases, two
integers combine to form another integer, but any number (int
or float
)
divided by another will be a float
value. Very large or very small float
values are displayed using scientific notation.
Float Values
Float values are very flexible, but they do have limits.
- A
float
can represent extremely large and extremely small numbers. There are limits, but you will rarely encounter them. - A
float
only represents 15 or 16 significant digits for any number; the remaining precision is lost. This limited precision is enough for the vast majority of applications. - After combining
float
values with arithmetic, the last few digits may be incorrect. Small rounding errors are often confusing when first encountered.
The first limit can be observed in two ways. If the result of a computation is a very large number, then it is represented as infinite. If the result is a very small number, then it is represented as zero.
2e306 * 10
2e+307
2e306 * 100
inf
2e-322 / 10
2e-323
2e-322 / 100
0.0
The second limit can be observed by an expression that involves numbers with more than 15 significant digits. These extra digits are discarded before any arithmetic is carried out.
0.6666666666666666 - 0.6666666666666666123456789
0.0
The third limit can be observed when taking the difference between two
expressions that should be equivalent. For example, the expression 2 ** 0.5
computes the square root of 2, but squaring this value does not exactly
recover 2.
import math
math.sqrt(2)
1.4142135623730951
math.sqrt(2) * math.sqrt(2)
2.0000000000000004
math.sqrt(2) * math.sqrt(2) - 2
4.440892098500626e-16
The final result above is 0.0000000000000004440892098500626
, a number that
is very close to zero. The correct answer to this arithmetic expression is 0,
but a small error in the final significant digit appears very different in
scientific notation. This behavior appears in almost all programming languages
because it is the result of the standard way that arithmetic is carried out on
computers.
Although float
values are not always exact, they are predictable and work
in the same way across all different kinds of computers and programming
languages.