Programming in Python

BEES 2023

Agenda

  • Strings:
    • White space characters
    • String operators
    • String functions
    • Slicing
    • Immutability
    • String comparison
    • For loops
    • In operator
  • Lists
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String data type

  • Text is represented in programs by the string data type
  • A string is an immutable sequence of characters enclosed within quotation marks (") or apostrophes (').
  • Example:
    •  ‘This is a sample sentence.’
    • “2x2=4”

whitespace characters

String indexing

greet = "Hello Bob"
print(greet[0])

print(greet[0], greet[2], greet[4])

x = 8
print(greet[x - 2])

print(greet[-1])
print(greet[-3])
print(greet[50]) # Error

print(greet[-(len(greet)+1)]) # This throws an error,  
# it implies a character before the start of the string

String length function

  • The length of a string is given by the "len()" function

s = "A long string"
print(len(s))
# The empty string case
s = "" 
print(len(s))
# A String with whitespace character
s = "\t"
print(len(s))

Character is just another String

s = "A long string"

# Realise that a character is just another string in Python
# In some languages, like C/C++, individual characters 
# are not strings but have a different type, but Python 
# treats them as a single character string

print(type(s[0])) # Prints str
print(len(s[0])) # Prints 1

String slicing

# Beyond indexing, you can slice strings to create substrings

greet = "Hello Bob"

print(greet[0:3]) # The 'prefix' substring of the first 3 characters

print(greet[3:3]) # The interval [3, 3) is empty

print(greet[5:8])

# Negative length strings?
print(greet[6:0]) # If the second index occurs before the first index it won't
# throw an error, just make a zero length (empty) string

print(greet[:5]) # This is the same as greet[0:5]
# greet[:n] is called a prefix of greet, where n is in [0, len(greet))

print(greet[5:]) # This is the same as greet[5:9]
# greet[n:] is called a suffix of greet, where n is in [0, len(greet))

print(greet[:]) # This is just the whole string, allowing you to make
# a copy of the string

String immutability

  • Strings are immutable
    • You cannot edit a string, you can only make new strings by copying them
s = "Strings can't be changed"

# This doesn't work

s[0] = 's'

# To make s lower case you could instead do:

s = 's' + s[1:]

print(s)

string comparison

in operator

for loops on strings

Example: removing vowels

Lists

  • An ordered sequence of information, accessible by index
  • A list is denoted by sequence brackets "[ ]"
  • A list contains elements
    • Usually homogeneous
    • Can contain mixed types
  • List elements can be changed, so a list is mutable

List functions - review

Lists are mutable

  • Lists are mutable
  • Assigning to an element at an index changes the value

 

 

  • L is now [2, 5, 3], note this is the same object L
L = [2, 1, 3]
L[1] = 5

List mutability

Conversion and nesting

aliasing

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  • "hot" is an alias for "warm" - changing one changes the other
  • For example, here "append" has a side effect
a = 1
b = a
print(a)
print(b)

warm = ['red', 'yellow', 'orange']
hot = warm
hot.append('pink')
print(hot)
print(warm)

cloning

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  • Create a new list and copy every element using
cool = ['blue', 'green', 'gray']
chill = cool[:]
chill.append('black')
print(chill)
print(cool)

chill = cool[:]

lists of lists of lists of ...

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  • We can have nested lists
warm = ['yellow', 'orange']
hot = ['red']
brightcolors = []
brightcolors.append(warm)
brightcolors.append(hot)
print(brightcolors)
# [['yellow', 'orange'], ['red']]
hot.append('pink')
print(hot)
# ['red', 'pink']
print(brightcolors)
# [['yellow', 'orange'], ['red', 'pink']]

Questions?