Transcript We need a list

Computer Science 101
Lists in Python
The need for lists

Often we need many different variables that play a
similar role in the program

For example, suppose we have a temperature
reading for each day of the year and need to
manipulate these values several times within a
program.

With simple variables,
• We would need to use 365 variables.
• We would not be able to loop over these variables.
Lists

A list is a numbered collection of variables.

The variables in a list share the same name
and are distinguished from one another by
their numbers or subscripts.

We can loop through the variables of a list
by using a variable for the subscript.

The subscripts are always 0,1,…, size-1
Referencing List Elements
Suppose we have a list, temperature, for the
temperature readings for the year.
 The subscripts would be 0,1,…,364.
 To refer to the reading for a given day, we
use the name of the list with the subscript in
brackets: temperature[4] for the fifth day.

temperature
75 79
82 70
68 65
58 63
67 61
temperature[4]
Python Session
Python Session
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Random numbers

For testing purposes or for simulations, it
is convenient to let the computer generate
random data for us.

In Python there is a library, random, that
has a lot of tools for working with random
numbers.
Randrange

random.randrange
• random.randrange(num)
gives a random number in range 0,…,(num-1)
• random.randrange(num1,num2)
gives random number in range num1,…,(num2-1)
Randrange
Appending to a List

If we have a list, say scores, we can add
a value, say num, to the list by
scores.append(num)
Random Lists

To build a random list of values, we can
• Start with an empty list, []
• Then append random values to the list
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