Chapter 2

Lists

• Can’t mix various types in lists
• An If function should always have an else block
• List can be joined together using ++; but have to noted that when we do that haskell has to traverse the whole list on the left hand side.
• we can use “:” which is the cons operator

5:[1,2,3] -- [5,1,2,3]

• We can use *”!!”* to get the index of a list. (Strings are basically lists!!)

• Lists

  • head takes a list and returns the first element.
  • tail takes a list and returns the all the element except the first one.
  • last takes a list and returns the last element.
  • init takes a list and returns everything except the last element.
  • reverse reverses the list
  • take takes an number and a list and returns that mn=any elemnts from the list. If index doesn’t exist throws an error.
  • drop does what take does but gives all elements minus what the index given

drop 3 [1,2,3,4,5,6] -- [4,5,6]

drop 100 [1,2,3] -- []

• null gives wheather is list is null or not
• minimum, maximum, product and sum work on lists.

• Ranges

  • cycle takes a list and cycles it infinitely:
  • repeat takes an element and produces it infinitely

[1..10] -- [1,2,3,4,5,6,7,8,9,10]

[2,4..10] -- [2,4,6,8,10]

take 5 (cycle[1,2]) -- [1,2,1,2,1]

• List Comprehension

  • The last condition is called the predicate. It is also called filtering

    [x * 2 | x <- [1..10], x * 12] -- [12,14,16,18,20]
    

  • We have created a function where we take odd numbers. If they are greater than 10 print “BANG!” else “BOOM!”

    boomBangs xs = [if x <10 then "BOOM!" else "BANG!!" | x <- xs, odd x]
    

  • We can give two inputs:

    [x*y | x <- [2,5,10], y <- [8,10,11], x*y > 60] -- [55,80,100,110]
    

  • Strings are lists too:

    removeNonUpperCase st = [c | c <- st, c `elem` [`A`..`Z`]]
    

  • In GHCI (Reference) :

    let removeNonUpperCase st = [c | c <- st, c `elem` [`A`..`Z`]]
    

Tuples

• Tuples

  • A tuple can contain different types, unlike a list.

  • But a tuple of fixed size is its own type. Which means, we cannot have:

    [(1,2), (3,4,5), (6,7)] -- Error
    

  • Like lists fst gives first element, snd gives second element. Does not works on triples, 4-tuples and soon

  • zip is a super cool function:

    zip [1,2,3] [4,5,6,7] -- [(1,4), (2,5), (3,6)]
    
    zip [1..] ["apples" "bananas" "cherry"] -- [(1,"apple"),(2,"bananas"),(3,"cherry")]
    

• Prob, which right triangle has integers for all sides to or smaller than 10 and has a perimeter of 24. This problem is solved in steps:

  let triangle = [(a,b,c)| c <- [1..10], b <- [1..10], c <- [1..10]
  
  let rightTriangle = [(a,b,c) | c<-[1..10], b<-[1..c], a <- [1..b], a^2 + b^2 = c^2, a+b+c == 24]