Question

I’m a newbie to Haskell, got stuck on a simple question:

aaa ::  [[(Char, Float)]] -> Float ->  [[(Char, Float)]]
aaa [[]] a = error "no indata"
aaa [[(a,b)]] c = [[(a, b/c)]] 
aaa inD c = ??

How to make it work with more than 1 element in Array?

Ex: aaa [[('a',3)],[('b',4)],[('c',5)]] 4

the result: [[('a',0.75)],[('b',1)],[('c',1.25)]]

Any hint pls, thx!

Answer 1

You can define operations on lists as follows (I give you a simpler example that adds 1 to each list item)

f [] = []
f (head:tail) = (head + 1):(f tail)

I.e. head:tail represents a list; to be more specific, it represents the first list item (head) and the remaining list if we take the first item away (tail). Then, you usually apply your stuff to head and make a recursive call using tail.

Completing your example (without testing) this would yield:

aaa ([(a,b)]:tail) c = [(a, b/c)] : (aaa tail c)

One thing: You are dealing with a list and want to modify each element of the list in a specific way (but each element is transformed the same way). For such occasions, Haskell provides its intrinsic map function, which takes:

• the function to transform a list items
• the list of items

as parameters and returns the transformed list.