Python Lecture Slides topic strings handling

Announcements For This Lecture
Assignment 1
• Will post it on Sunday
§ Need one more lecture
§ But start reading it
• Due Wed Sep. 22nd
§ Revise until correct
§ Final version Sep 29th
• Do not put off until end!
Getting Help
• Can work in pairs
§ Will set up
§ Submit one for both
• Lots of consultant hours
§ Come early! Beat the rush
§ Also use TA office hours
• One-on-Ones next week
2
9/6/22 Strings

One-on-One Sessions
• Starting Monday: 1/2-hour one-on-one sessions
§ Bring computer to work with instructor, TA or consultant
§ Hands on, dedicated help with Labs 5 & 6 (and related)
§ To prepare for assignment, not for help on assignment
• Limited availability: we cannot get to everyone
§ Students with experience or confidence should hold back
• Sign up online in CMS: first come, first served
§ Choose assignment One-on-One
§ Pick a time that works for you; will add slots as possible
§ Can sign up starting at 5pm TOMORROW
9/6/22 Strings 3

One-on-One Sessions
• Starting Monday: 1/2-hour one-on-one sessions
§ Bring computer to work with instructor, TA or consultant
§ Hands on, dedicated help with Labs 5 & 6 (and related)
§ To prepare for assignment, not for help on assignment
• Limited availability: we cannot get to everyone
§ Students with experience or confidence should hold back
• Sign up online in CMS: first come, first served
§ Choose assignment One-on-One
§ Pick a time that works for you; will add slots as possible
§ Can sign up starting at 5pm TOMORROW
9/6/22 Strings 4
I personally have only 90 slots!
Leave those for students in need.

Even More Announcements
The AI Quiz
• Must finish before A1
§ Will reject assignment
§ Means a 0 by Sep 29th
• Current statistics
§ 50 have not passed
§ 15 have not taken
• E-mails sent tomorrow
Watching Videos?
• Today
§ Lesson 6: Strings
• Next Time
§ Lesson 7: Specifications
§ Lesson 8: Testing
§ Notice that it is a lot
§ Important for A1
9/6/22 Strings 5

Purpose of Today’s Lecture
• Return to the string (str) type
§ Saw it the first day of class
§ Learn all of the things we can do with it
• See more examples of functions
§ Particularly functions with strings
• Learn the difference between…
§ Procedures and fruitful functions
§ print and return statements
9/6/22 Strings 6

String: Text as a Value
• String are quoted characters
§ 'abc d' (Python prefers)
§ "abc d" (most languages)
• How to write quotes in quotes?
§ Delineate with “other quote”
§ Example: "Don't" or '6" tall'
§ What if need both " and ' ?
• Solution: escape characters
§ Format: + letter
§ Special or invisible chars
9/6/22 Strings 7
Char Meaning
' single quote
" double quote
n new line
t tab
backslash
>>> x = 'I said: "Don't"'
>>> print(x)
I said: "Don't"

String are Indexed
• s = 'abc d'
• Access characters with []
§ s[0] is 'a'
§ s[4] is 'd'
§ s[5] causes an error
§ s[0:2] is 'ab' (excludes c)
§ s[2:] is 'c d'
• Called “string slicing”
• s = 'Hello all'
• What is s[3:6]?
9/6/22 Strings 8
a b c d
0 1 2 3 4
H e l l o
0 1 2 3 4 5
a
6
l
7
l
8
A: 'lo a'
B: 'lo'
C: 'lo '
D: 'o '
E: I do not know

String are Indexed
• s = 'abc d'
§ s[0] is 'a'
§ s[4] is 'd'
§ s[2:] is 'c d'
• s = 'Hello all'
• What is s[3:6]?
9/6/22 Strings 9
a b c d
0 1 2 3 4
H e l l o
0 1 2 3 4 5
a
6
l
7
l
8
A: 'lo a'
B: 'lo'
C: 'lo '
D: 'o '
E: I do not know
CORRECT

String are Indexed
• s = 'abc d'
§ s[0] is 'a'
§ s[4] is 'd'
§ s[2:] is 'c d'
• s = 'Hello all'
• What is s[:4]?
9/6/22 Strings 10
a b c d
0 1 2 3 4
H e l l o
0 1 2 3 4 5
a
6
l
7
l
8
A: 'o all'
B: 'Hello'
C: 'Hell'
D: Error!
E: I do not know

String are Indexed
• s = 'abc d'
§ s[0] is 'a'
§ s[4] is 'd'
§ s[2:] is 'c d'
• s = 'Hello all'
• What is s[:4]?
9/6/22 Strings 11
a b c d
0 1 2 3 4
H e l l o
0 1 2 3 4 5
a
6
l
7
l
8
A: 'o all'
B: 'Hello'
C: 'Hell'
D: Error!
E: I do not know
CORRECT

Other Things We Can Do With Strings
• Operation in: s1 in s2
§ Tests if s1 “a part of” s2
§ Say s1 a substring of s2
§ Evaluates to a bool
• Examples:
§ s = 'abracadabra'
§ 'a' in s == True
§ 'cad' in s == True
§ 'foo' in s == False
• Function len: len(s)
§ Value is # of chars in s
§ Evaluates to an int
• Examples:
§ s = 'abracadabra’
§ len(s) == 11
§ len(s[1:5]) == 4
§ s[1:len(s)-1] == 'bracadabr'
9/6/22 Strings 12

Defining a String Function
• Start w/ string variable
§ Holds string to work on
§ Make it the parameter
• Body is all assignments
§ Make variables as needed
§ But last line is a return
• Try to work in reverse
§ Start with the return
§ Figure ops you need
§ Make a variable if unsure
§ Assign on previous line
def middle(text):
"""Returns: middle 3rd of text
Param text: a string"""
# Get length of text
# Start of middle third
# End of middle third
# Get the text
# Return the result
return result
9/6/22 Strings 13

def middle(text):
# Get the text
result = text[start:end]
# Return the result
return result
9/6/22 Strings 14

def middle(text):
end = 2*size//3
# Get the text
# Return the result
return result
9/6/22 Strings 15

def middle(text):
start = size//3
end = 2*size//3
# Get the text
# Return the result
return result
9/6/22 Strings 16

def middle(text):
size = len(text)
start = size//3
end = 2*size//3
# Get the text
# Return the result
return result
9/6/22 Strings 17

>>> middle('abc')
'b'
>>> middle('aabbcc')
'bb'
>>> middle('aaabbbccc')
'bbb'
def middle(text):
size = len(text)
start = size//3
end = 2*size//3
# Get the text
# Return the result
return result
9/6/22 Strings 18

Not All Functions Need a Return
def greet(n):
"""Prints a greeting to the name n
Parameter n: name to greet
Precondition: n is a string"""
print('Hello '+n+'!')
print('How are you?')
9/6/22 Strings 19
Displays these
strings on the screen
No assignments or return
The call frame is EMPTY
Note the difference

Procedures vs. Fruitful Functions
Procedures
• Functions that do something
• Call them as a statement
• Example: greet('Walker')
Fruitful Functions
• Functions that give a value
• Call them in an expression
• Example: x = round(2.56,1)
9/6/22 Strings 20
Historical Aside
• Historically “function” = “fruitful function”
• But now we use “function” to refer to both

Print vs. Return
Print
• Displays a value on screen
§ Used primarily for testing
§ Not useful for calculations
def print_plus(n):
print(n+1)
>>> x = print_plus(2)
3
>>>
Return
• Defines a function’s value
§ Important for calculations
§ But does not display anything
def return_plus(n):
return (n+1)
>>> x = return_plus(2)
>>>
9/6/22 Strings 21

Print vs. Return
Print
• Displays a value on screen
§ Used primarily for testing
§ Not useful for calculations
def print_plus(n):
print(n+1)
>>> x = print_plus(2)
3
>>>
Return
• Defines a function’s value
§ Important for calculations
§ But does not display anything
def return_plus(n):
return (n+1)
>>> x = return_plus(2)
>>>
9/6/22 Strings 22
x 3
x
Nothing here!

Method Calls
• Methods calls are unique (right now) to strings
§ Like a function call with a “string in front”
• Method calls have the form
string.name(x,y,…)
• The string in front is an additional argument
§ Just one that is not inside of the parentheses
§ Why? Will answer this later in course.
method
name
arguments
argument
9/6/22 Strings 23

Example: upper()
• upper(): Return an upper case copy
>>> s = 'Hello World’
>>> s.upper()
'HELLO WORLD'
>>> s[1:5].upper() # Str before need not be a variable
'ELLO'
>>> 'abc'.upper() # Str before could be a literal
'ABC’
• Notice that only argument is string in front
9/6/22 Strings 24

Examples of String Methods
• s1.index(s2)
§ Returns position of the
first instance of s2 in s1
• s1.count(s2)
§ Returns number of times
s2 appears inside of s1
• s.strip()
§ Returns copy of s with no
white-space at ends
>>> s = 'abracadabra'
>>> s.index('a')
0
>>> s.index('rac')
2
>>> s.count('a')
5
>>> s.count('x')
0
>>> ' a b '.strip()
'a b'
9/6/22 Strings 25

Examples of String Methods
• s1.index(s2)
§ Returns position of the
first instance of s2 in s1
• s1.count(s2)
§ Returns number of times
s2 appears inside of s1
• s.strip()
§ Returns copy of s with no
white-space at ends
>>> s = 'abracadabra'
>>> s.index('a')
0
>>> s.index('rac')
2
>>> s.count('a')
5
>>> s.count('x')
0
>>> ' a b '.strip()
'a b'
9/6/22 Strings 26
See Lecture page for more

Working on Assignment 1
• You will be writing a lot of string functions
• You have three main tools at your disposal
§ Searching: The index method
§ Cutting: The slice operation [start:end]
§ Gluing: The + operator
• Can combine these in different ways
§ Cutting to pull out parts of a string
§ Gluing to put back together in new string
9/6/22 Strings 27

String Extraction Example
def firstparens(text):
"""Returns: substring in ()
Uses the first set of parens
Param text: a string with ()"""
# SEARCH for open parens
start = text.index('(')
# CUT before paren
tail = text[start+1:]
# SEARCH for close parens
end = tail.index(')')
# CUT and return the result
return tail[:end]
>>> s = 'Prof (Walker) White'
>>> firstparens(s)
'Walker'
>>> t = '(A) B (C) D'
>>> firstparens(t)
'A'
9/6/22 Strings 28

String Extraction Puzzle
def second(text):
"""Returns: second elt in text
The text is a sequence of words
separated by commas, spaces.
Ex: second('A, B, C’) rets 'B'
Param text: a list of words"""
start = text.index(',') # SEARCH
tail = text[start+1:] # CUT
end = tail.index(',') # SEARCH
result = tail[:end] # CUT
return result
>>> second('cat, dog, mouse, lion')
'dog'
>>> second('apple, pear, banana')
'pear'
9/6/22 Strings 29
1
2
3
4
5

def second(text):
return result
'dog'
'pear'
9/6/22 Strings 30
1
2
3
4
5
Where is the error?
A: Line 1
B: Line 2
C: Line 3
D: Line 4
E: There is no error

def second(text):
return result
'dog'
'pear'
9/6/22 Strings 31
1
2
3
4
5
result = tail[:end].strip()
tail = text[start+2:]
OR

Python Lecture Slides topic strings handling

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