Ch6 Discussion 03

Learn: filter and foldl.

I will write foldL and foldR so it is clear that the last character is a ell not a one. There are also functions foldl1 and foldr1.

Examples

 filter (>=0) [5,-1,3,4,-8]
 filter (0>=) [5,-1,3,4,-8]

Find want all numbers x from a list so that x^2 is less than 100.

 filter (\x -> x*x <= 100) [5,90,13,-4]

Exercises 1

1. Find all of the words of length at most 7.

    words1 = ["cat","wombat","caterpillar","platypus","dog"]
   

foldL

Use (++) to produce a list of all of the numbers in any list, in the order they appear, and beginning with a 0 (which does not count as an element from a list).

foldl (++) [0] [[1,2,3],[4],[5,6,7,8]]

Exercises 2

1. Suppose 999 represents an unavailable score. Find the largest score in the list using foldl, ignoring any 999 in the list.

    scores = [4,70,30,999,80,50]
    answer = 80
   

2. Use foldl to find the sum of all odd numbers in a list.

3. (multAll) Multiply all of the numbers in the list, going left to right. Except if you find a 0 reset the product to a 1 and then keep going. Use foldl with a custom-made update function.

     multAll [2,3,4,0,5,6] == 30
     multAll [2,3,0,4,5,0,6,7] == 42