Day 14 - Variables and loops

Skills: 3

Pre-reading: DRAFT:9.1.8.5 (except 9.1.8.5.4)

Intro (15 mins)

• To define our own list processing functions, we need a way to do computations for each element in the list. Multiple ways to do that, today we will show iteration with for each().
• This requires being able to modify variables, which in Pyret means they must be declared with var, and must be updated with the special operator :=.
• Pattern is:

  var my-var = initial-value
  for each(my-elt from my-list):
    code
    to
    handle
    my-elt
    and update
    my-var
  end
  my-var # final result
• Example: while sum is built in, we could have defined it ourselves as:

  fun my-sum(num-list :: List<Number>) -> Number block:
    var total = 0
    for each(n from num-list):
      total := total + n
    end
    total
  where:
    my-sum([list: 0, 1, 2, 3]) is 6
  end

Class Exercises (40 mins)

• Define your own product function that takes a list of numbers and returns their product (multiply all of them together).
• Define a function sum-even-numbers that takes a list of integers and adds up only the even numbers -- the rest should be ignored (num-modulo may be helpful)!
• Define a function my-length that takes a list of any value and returns the number of elements in the list.

TODO / FIXME ADD MORE

Wrap-up (5 mins)

• Mutable variables can be declared with var and updated with :=.
• for each can run code for each element of a list; combined with mutable variables, this allows writing custom list operations.