CPSC120B
Fundamentals of Computer Science

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Reading Questions

Quiz

String Slice

• The index operator, [], copies a single character from a string

  text = "banana"
  print(text[0])

• But sometimes you want more than one character
• The slice operator lets you copy a sequence of characters
• It looks a lot like the index operator

• Just need to specify the start and end of the sequence

  text = "banana"
  print(text[1:3])

• Note, it the start index is inclusive and the end index is exclusive
• this is just like the range function
• Can also specify a step, just like the range function

• For you convenience, you can exclude the start or end index, which implies all the way to the end or beginning of the string

  text = "banana"
  print(text[1:])
  print(text[:-1])

String Methods

• Methods are functions that are called called on objects

• We’ve seen lots of them with turtle

  a_turtle = turtle.Turtle()
  a_turtle.forward(10)

• This is different than calling a function that we have written

  go_forward(a_turtle, 10)

• A method call has an object (and a dot) before the method name

• Strings have methods

  text = "banana"
  print(text.upper())

Strings are Immutable

• Strings are different than turtles in one very important way
• Strings are immutable, once a string is created it can’t be changed
• This is for efficiency reasons, if a string can’t be changed, the way it is represented in memory is more simple
• Turtles, on the other hand, can change, they change location, orientation, color, etc.

• So text.upper() does not change text

  text = "banana"
  text.upper()
  print(text)

• Instead it creates a new string, and returns that

  text = "banana"
  print(text.upper())
  print(text)

• Note, this does not mean that the string that a variable references can not be changed

  text_1 = "banana"
  text_2 = text_1
  print(text_1, text_2)
  text_1 = "mango"
  print(text_1, text_2)

• This does not change “banana”, it changes what the variable references
• So the other string, is left unchanged
• This is the same behavior we have with ints, floats, and bools

• It’s also important to note what effect immutability has on function scope

  def do_something(text):
      text.upper()
  
  text = "banana"
  do_something(text)
  print(text)

Lab