Python Mini-Course University of Oklahoma Department of Psychology Day 4 – Lesson 14 Lists 5/02/09...

Python Mini-CourseUniversity of Oklahoma

Department of Psychology

Day 4 – Lesson 14Lists

5/02/09Python Mini-Course: Day 4 – Lesson 141

Lesson objectives

1. Describe the characteristics of the list data structure in Python

2. Perform basic operations with lists including creation, concatenation, repetition, slicing, and traversing

3. Use string methods that require lists (join, split)

4. Use lists in functions


The list data structure

In Python, a list is a mutable sequence of valuesEach value in the list is an element or item

Elements can be any Python data typeLists can mix data typesElements can be nested lists


Creating lists

numbers = [1, 2, 3, 4]print numberscheeses = ['swiss', 'cheddar', 'ricotta', 'gouda']print cheeses


Creating lists

mixed = [1, 'a', 3.45]print mixedsingle = ['z']print single, type(single)empty = []print empty


Repeating a list

Use the * operator:meat = ['spam']*4print meatprint [1, 2, 3]*3


List indexing

Elements within a list are indexed (see Lesson 10)

print cheeses[0]Lists are mutable cheeses[0] = 'Feta' print cheeses


Slicing a list

Like strings and other sequences, lists can be sliced print cheeses[1:4] print cheeses[:2] print cheeses[2:]


Changing a slice

roster = ['Meghan', 'Tricia', 'Juan', 'Alton', 'Darrel', 'Jen']print rosterroster[1:3] = ['Sam', 'Kerri']print rosterroster[3:5] = ['Tayla']print roster


Inserting elements

Slice notation roster[2:2] = ['Dana', 'Ryan'] print roster


Deleting elements

Set slice to empty list roster[3:5] = [] print rosterThe del keyword del roster[2:3] print roster


The insert and append methods

The insert method roster.insert(2,'Jakob') print rosterThe append method roster.append('Tonya') print roster


The extend method

Adds a list to the end of an existing list adds = ['Ian', 'Stacie'] roster.extend(adds) print roster


Extending a list

Can also use += operator roster += ['Anya'] print roster


Using the + operator

a = [1, 2, 3]b = [4, 5, 6]c = a + bprint a, b, c

*The + operator returns a new list that is a concatenation of two lists


Note on list operations

Be careful when using the + operator and append method

Try this: d = c + 7Or this c.append(b) print c


List assignment and aliasing

a = [1, 2, 3, 4]b = ac = a[:]a[2] = 9print a, b, c

*The slice operator returns a copy of a list


Other list methods

roster.sort()print roster

roster.reverse()print roster


Other list methods

print roster.index('Tonya')print roster.index('Tonya', 2, 5)

print roster.count('Sam')

roster.remove('Sam')print roster


The join string method

Concatenates a sequence of strings into a single string with sep inserted between each item.

Syntax: sep.join(list)


The split string method

Returns a list of words from a string using sep as the delimiter string

Syntax: sep.split(list)


Example: join_split.py

t = ['pining', 'for', 'the', 'fjords']delimiter = '_'s = delimiter.join(t)print s

u = s.split(delimiter)print u


Example

print ''.join(t)print ' '.join(t)print '\t'.join(t)


Traversing a list

for index in range(len(roster)):

print roster[index]

for student in roster:

print student

for index, student in enumerate(roster):

print index, student


Traversing a list

What does this do?empty = []for x in empty: print x


Nested lists

nested = [[1,2,3],[4,5,6],[7,8,9]]print nestedprint nested[0]print nested[0][1]


Traversing nested lists

for i in range(len(nested)): for j in range(len(nested[i])): print nested[i][j]


Traversing nested lists

for nest in nested: for item in nest: print item


Using lists: cumulate.py

def cumulate(seq):

c_sum = 0

for item in seq:

c_sum += item

return c_sum

a = [12, 78, 32, 82]

s = cumulate(a)

print s


Returning lists from functions:only_upper.py

def only_upper(t):

res = []

for s in t:

if s.isupper():

res.append(s)

return res

text = 'Bold cOlOrs Make for Easy Reading'

secret = only_upper(text)

print secret


Modifying lists in functions

In Python, arguments are passed by referenceThe parameter in the function is an alias for the argument that was passed in

If a mutable object is changed inside the function, it is also changed outside the function


Example: byref.py

def change(seq):

print 'Passed in: ' + str(seq)

seq.append('new item')

print 'Changed to: ' + str(seq)

original = [1, 2, 3]

print original

change(original)

print original


Example: byref2.py

def change(seq):

print 'Passed in: ' + str(seq)

seq.append('new item')

print 'Changed to: ' + str(seq)

new_seq = ['created','in','function']

print 'New seq: ' + str(new_seq)

original = [1, 2, 3]

new_seq = ['outside','the','function']

print original

change(original)

print original

print new_seq


Suggested exercises

Exercise 10.5 – Solving the "Birthday Paradox" by a Monte Carlo simulation

Exercise 10.6 – Removing duplicates from a list

Exercise 10.8 – Bisection search


Python Mini-Course University of Oklahoma Department of Psychology Day 4 – Lesson 14 Lists 5/02/09...

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