COMPUTER SCIENCE

FEBRUARY 2011

Lists in Python

Introduction to Lists

Lists (aka arrays): an ordered set of elements A compound data type, like strings

Think of a CD box The box : list CDs : elements Fixed size CDs stored in order

Why Use Lists?

Faster than defining variables for each element

Can perform operations on each element Think of Robot game: had to copy/paste code for each

item

More flexible Easy to add/remove elements

Can pass an entire list to a function

Lists in Python

Most languages: elements must be of same data type

Python: can mix data types Integers, floats (decimals) , string, even other lists.

Creating Lists & Getting Length

Create by specifying elementsint_list=[202,10, 54, 23]

print(int_list)

..or create an empty listempty_list=[]

print(empty_list)

Use len() to return the number of elementsprint len(int_list)

print len(empty_list)

Accessing elements

Access individual elements using bracketsFirst element is 0.

int_list=[202,10, 54, 23]

int_list[1]

int_list[3]

Access from end using negative indicesint_list[-1]

int_list[-3]

Specifying slices

Access a slice (sub-list) using rangesIncludes first element in range, excludes last in

range int_list=[202,10, 54, 23]

int_list[0:1]

int_list[2:3]

int _list[0:0]

Note: this is different from int _list[0]

Omit values to get start or end of listint_list[:2]

int_list[2:]

Using a while loopUsing a while loop Using for value in listUsing for value in list

int_list=[202,10, 54, 23]

index=0

sum_values=0

while (index<len(int_list)):

value = int_list[index]

sum_values+=value

index+=1

for value in int_list:

sum_values+=value

Iterate through lists

Manipulating lists

Lists are mutable (changeable), unlike strings.Replace a single element

int_list=[202,10, 54, 23]

int_list[1]=11

Replace a sliceint_list[1:3]=[12,99]

Delete by replacing a slice with an empty listint_list[1:3]=[]

Add an element by ‘squeezing’ it in int_list[4:4]=100

int_list[ ] = 20

Nested Lists

Lists can contain other listsint_list=[202,10, 54, 23]

mixed_list=[“a”, int_list, 23]

List length is number of ‘top level’ elementsprint len(mixed_list)