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CS 140: Introduction
to Computer Programming
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Variables and
Data Types
There are four data types that we will encounter
in our course: integers, strings, floating point
numbers, and lists.
- An integer
is a number with no digits past the decimal
point; in other words, numbers like 17, 41,
0, or –3. (Thus –2.7 and 3.1416 are
not integers.)
- A string
is a sequence of characters typed from the
keyboard. Such a sequence might
contain spaces, lower and upper case
letters, or even digits. Place
characters inside quotes to let Python know
to treat them as a string. Examples of
strings include "fantabulous",
"agent 007", and even "90210".
- A floating point
number (or float for
short) is a real number with digits before
and after the decimal point, such as 3.1416,
–2.7, and even 5.0. Note that Python
will treat 5 as an integer but will treat
5.0 as a float, even though they are the
same number. This will make a
difference when we discuss arithmetic.
- A list
is simply an ordered list of items, placed
between square brackets and separated by
commas. Two examples of lists are ["a",
"b", "c", "d"] and [17,
"twix", 2.7]. Lists
usually contain items of the same type, such
as in the first example, although it is not
required. Later on we will devote an
entire set of class notes to lists later on,
so we won't say more about them here.
A fundamental task in almost any program is to
perform actions or make decisions based on the
values of quantities, either numerical or string
type. Such a quantity can vary in the
course of the program execution, so we call its
name a variable.
Here is a short program that involves two
variables.
name = "Peter"
age = 8
print "Next year", name,
"will be",
age+1, "years old."
If you were to run this program the output would
be
Next year Peter will be 9 years old.
Notice that the program treated age
as a number by performing arithmetic with
it. (We will discuss how the print
statement works in the notes on Input and
Output, and in again in more detail when we
reach Formatting.)
Variable names may consist of any sequence of
lower or upper case letters, digits, or the
underscore character. However, a variable
name cannot start with a digit and cannot
contain any spaces. Thus number,
num3,
and number_one
are all valid variable names, but 7up
and seven up
are not. Be aware that Python
distinguishes between lower and upper case
letters! This means that name,
Name,
and NAME
are all interpreted as different variables.
A variable changes value during program
execution whenever that variable name is set
equal to a new value. For instance, the
program
age = 8
age = age+1
print "Next year Peter will be",
age, "years old."
will have the output
Next year Peter
will be 9 years old.
same as before. The second line
effectively increases the value of the variable
by one. Similarly, we could write
age = 8
age = age*2
print "Twice Peter's age is",
age, "years old."
to obtain
Twice Peter's age
is 16 years old.
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