Haskell - function that sorts list with strings by integer value

user_14798096 0
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Below is my current code that sorts a list in acesending order:

qsort::[Int]->[Int]
qsort[]=[]
qsort(x:xs)
 =qsort[y|y<-xs, y<=x]++[x]++qsort[y|y<-xs,y>x]

How would i modify/create a function that sorts a list such as:

[("Bob",22),("Peter",38),("Charlie",19)]

by the int value

quicksort sorting haskell
3 Answers

You can unpack the items in 2-tuples:

qsort::[Int] -> [Int]
qsort [] = []
qsort(x@(_,vx):xs) = qsort [ y | y@(_, vy) <-xs, … ] ++ [x] ++ qsort [ y | y@(_, vy) <- xs, … ]

Here you still need to fill in the … parts. The vx is the value of x, and vy is the value of the elements y. You thus will need to make comparisons between vx and vy.

user_675790

Here's a possible strategy. First, define a custom "less than or equal to" operator

lte :: (String, Int) -> (String, Int) -> Bool
lte (str1, int1) (str2, int2) = ...

Then, adapt your qsort implementation replacing each use of <= with a call to lte, and each use of > with the negation of a call to lte.

user_32349590

for ascending

test = sortBy (compare `on` snd) [("Bob",22),("Peter",38),("Charlie",19)]

for descending

test = sortBy (flip compare `on` snd) [("Bob",22),("Peter",38),("Charlie",19)]

alternatively descending,

import Data.Function (on)
import Data.Ord (Down(Down))

test = sortOn Down [("Bob",22),("Peter",38),("Charlie",19)]
user_4532610
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