I have defined the following function in haskell:
step :: [Int] -> [Char] -> [Int]
step stack str
| str == "*" = remaining ++ [x*y]
| str == "+" = remaining ++ [x+y]
| str == "-" = remaining ++ [x-y]
| str == "/" = remaining ++ [x `div` y]
| otherwise = stack ++ [read str :: Int]
where x = (last . init) stack
y = (last stack)
remaining = (init . init) stack
This functions takes and integer array [10, 4, 3] and a string operator * and applies the operator to the last two items in the array and returns the following array [10, 7].
This is makes up part of an intermediary function, the end result is a reverse polish notation evaluator function.
How can I utilise the step function I've defined and foldl to do the following:
Take the examples string: "10 4 3 + 2 * -".
Add each element onto the string until the first operator is encountered as so:
10, 4, 3 Then apply the operator to the two elements onto top the stack and place the result on the stack:
10, 7.
Continue as so until the final answer is evaluated (-4)
Answer:
For the sake of completeness this was the function I arrived at with the help of @talex
rpn :: String -> Int
rpn input = head result
where arr = words input
result = foldl step [] arr
foldl step [] ["10", "4", "3", "+", "2", "*", "-"]
[] here is initial stack.
If you rewrite your step in the following way it will work faster:
step :: [Int] -> [Char] -> [Int]
step stack str
| str == "*" = (x*y):remaining
| str == "+" = (x+y):remaining
| str == "-" = (x-y):remaining
| str == "/" = (x `div` y):remaining
| otherwise = (read str :: Int):stack
where x = head $ tail stack
y = head stack
remaining = tail $ tail stack