CPSC120B
Fundamentals of Computer Science

Moneyball

• Moneyball
• MCSP Conversation Series

Reading Questions

Quiz

Turtle in Idle

• We can also run turtle programs in Idle, but there is one important thing to watch out for
• Do not name a file turtle.py
• If you do, then your program will try to import the file you created instead of the turtle library, and your program will not work
• Name it anything else and it should work fine

Variable Reassignment

• Once a variable has been given a value, it’s value can be changed

• That’s why they are called variables

  x = 1
  x = 2
  print(x)

• Recall that the assignment operator, =, does not have the same meaning that it has in mathematics.
• The above two equations would be impossible in mathematics
• In a program they are possible because only line is executed at a time

• When a variable is assigned to antoher variable, the value is copied

  x = 1
  y = x
  x = 2
  print(y)

• So the variable y in the above example does not change it’s value when the value of x is changed

• And don’t forget order matters

  x = 1
  print(y)
  y = x

• The above program produces an error because the variable y does not have a value when the print statement is executed

Variable Updating

• Can also use a variable’s current value in computing a new value for it

  x = 1
  x = x + 2
  print(x)

• This looks weird becasue in mathmatics this would be impossible
• But remember the assignment operator, =, is not equality
• This works because the assignment operator has the lowest precedence
• So on the second line in the above example, x is added to 2, and becasue x already has a value, 3, the assigment changes the value of x

• Beware of updating a variable that has not been defined yet

  y = y + 3
  print(y)

• Will produce an error because y has no value to add to 3
• This can be used to reduce the number of variables a program needs

• Take this version of the paper folding program from lab

  zero_fold_thickness = 0.004
  one_fold_thickness = zero_fold_thickness * 2
  two_fold_thickness = one_fold_thickness * 2
  three_fold_thickness = two_fold_thickness * 2
  four_fold_thickness = three_fold_thickness * 2
  five_fold_thickness = four_fold_thickness * 2
  six_fold_thickness = five_fold_thickness * 2
  seven_fold_thickness = six_fold_thickness * 2
  eight_fold_thickness = seven_fold_thickness * 2
  nine_fold_thickness = eight_fold_thickness * 2
  ten_fold_thickness = nine_fold_thickness * 2
  print("A piece of paper folded in half 10 times would be", ten_fold_thickness, "inches thick.")

• By using variable updating, this can be simplified

  paper_thickness = 0.004
  paper_thickness = paper_thickness * 2
  paper_thickness = paper_thickness * 2
  paper_thickness = paper_thickness * 2
  paper_thickness = paper_thickness * 2
  paper_thickness = paper_thickness * 2
  paper_thickness = paper_thickness * 2
  paper_thickness = paper_thickness * 2
  paper_thickness = paper_thickness * 2
  paper_thickness = paper_thickness * 2
  paper_thickness = paper_thickness * 2
  print("A piece of paper folded in half 10 times would be", paper_thickness, "inches thick.")

• Of course, this could also be much simplified by using an exponent, but not allo programs will have that advantage

Activity