I'm trying to make a function working out (and then outputting as a String
) the difference between two elements in a list - 1st and 2nd, then 2nd and 3rd, and so on - I think I'm giving it a good go, but I currently keep running into error whack-a-mole, I've put the current error below, but first, obligatory code dump:
type Name = String
type Coordinates = (Int, Int)
type Pop = Int
type TotalPop = [Pop]
type City = (Name, (Coordinates, TotalPop))
testData :: [City]
testData = [("New York City", ((1,1), [5, 4, 3, 2])),
("Washingotn DC", ((3,3), [3, 2, 1, 1])),
("Los Angeles", ((2,2), [7, 7, 7, 5]))]
getPopGrowth :: [City] -> Name -> String
getPopGrowth cs name = concat
[getPercentages z ++ "\n" | (x,z) <- maybeToList (lookup name cs)] where
getPercentages z = unwords (map show z1) ++ "% " where
z1 = percentageIncrease z
percentageIncrease :: [Int] -> [Float]
percentageIncrease (x:xs)
| length (x:xs) > 2 = percentageIncrease (tail xs)
| otherwise = (a / b - 1) * 100.0 where
a = fromIntegral x :: Float
b = fromIntegral (head xs) :: Float
And the error I'm getting at the moment is:
error:
• Couldn't match expected type ‘[Float]’ with actual type ‘Float’
• In the expression: (a / b - 1) * 100.0
In an equation for ‘percentageIncrease’:
percentageIncrease (x : xs)
| length (x : xs) > 2 = percentageIncrease (tail xs)
| otherwise = (a / b - 1) * 100.0
where
a = fromIntegral x :: Float
b = fromIntegral (head xs) :: Float
|
92 | | otherwise = (a / b - 1) * 100.0 where
| ^^^^^^^^^^^^^^^^^^^
I would like to emphasise, I understand the error, but I do not know how to resolve it in such a way that I get the desired outcome of the function.
Just for some clarity around what I'm trying to do.
Input: getPopGrowth testData "New York City"
should Output: 25% 33.333% 50%