Question

I have two lists ["a","b","c","d"] and ["b","d","a","c"]

How can I make a function that orders the first list with the same order of the second one?

In this example something like this:

> ord ["a","b","c","d"] ["b","d","a","c"] 
["b","d","a","c"]

all the function I make give me an incomplete list:

ord :: [String] -> [String] -> [String]
ord [] _ = []
ord (h:t) (x:xs) | (h==x) = h:(ord t xs)
                 | otherwise = ord t (x:xs)

This is only an example; I can’t simply present the second list.

Answer 1

Here’s a quick and dirty solution that builds the result by grouping each string in the first list by the order in the second (I also renamed ord to orderThese):

orderThese :: [String] -> [String] -> [String] 
orderThese _ [] = []
orderThese as (b:bs) = filter (\x -> x == b) as ++ orderThese as bs

As an example, orderThese ["a", "c", "a", "b"] ["b", "a", "c"] returns ["b","a","a","c"].