1.4. INTEGER ARITHMETIC 12
type(7)
Always remember to end with the Enter key. After the Shell responds, you should see something like
>>> type(7)
<class ’int’>
>>>
In the result, int is short for integer. The work class is basically a synonym for type in Python. At the end
you see a further prompt where you can enter your next line....
For the rest of this section, at the >>> prompt in the Python Shell, individually enter each line below
that is set off in typewriter font. So next enter
type(1.25)
Note the name in the last result is float, not real or decimal, coming from the term “floating point”, for
reasons that will be explained later, in Section 1.14.1. Enter
type(’hello’)
In your last result you see another abbreviation: str rather than string. Enter
type([1, 2, 3])
Strings and lists are both sequences of parts (characters or elements). We can find the length of that sequence
with another function with the abbreviated name len. Try both of the following, separately, in the Shell:
len([2, 4, 6])
len(’abcd’)
Some functions have no parameters, so nothing goes between the parentheses. For example, some types
serve as no-parameter functions to create a simple value of their type. Try
list()
You see the way an empty list is displayed.
Functions may also take more than one parameter. Try
max(5, 11, 2)
Above, max is short for maximum.
Some of the names of types serve as conversion functions (where there is an obvious meaning for the
conversion). Try each of the following, one at a time, in the Shell:
str(23)
int(’125’)
An often handy Shell feature: an earlier Shell line may to copied and edited by clicking anywhere in the
previously displayed line and then pressing Enter. For instance you should have entered several lines
starting with len. click on any one, press Enter, and edit the line for a different test.
1.4. Integer Arithmetic
1.4.1. Addition and Subtraction. We start with the integers and integer arithmetic, not because
arithmetic is exciting, but because the symbolism should be mostly familiar. Of course arithmetic is important
in many cases, but Python is probably more often used to manipulate text and other sorts of data, as
in the sample program in Section 1.2.2.
Python understands numbers and standard arithmetic. For the whole section on integer arithmetic,
where you see a set-off line in typewriter font, type individual lines at the >>> prompt in the Python
Shell. Press Enter after each line to get Python to respond:
77
2 + 3
5 - 7
Python should evaluate and print back the value of each expression. Of course the first one does not require
any calculation. It appears the shell just echoes back what you printed. Do note that the line with the value
produced by the shell does not start with >>> and appears at the left margin. Hence you can distinguish
what you type (after the “>>>” prompt) from what the computer responds.