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31 lines
979 B
Haskell
31 lines
979 B
Haskell
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module Calculator where
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-- calculator, integers, operators: +-*/
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-- "2 / 2 + 3 * 4 - 13" == 0
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-- "4 + 3 * 4 / 3 - 6 / 3 * 3 + 8" == 10
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-- <grammar>
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-- expr -> factor | expr {+|-} factor
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-- factor -> num | factor {*|/} num
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evaluate :: String -> Double
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evaluate s = expr.factor.getNum.filter (\x->x/=' ') $s
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getNum "" = (0,"")
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getNum s = let n = length.takeWhile (\x->'0' <=x && x<='9') $s
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(num,res) = splitAt n s
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x = read num::Double
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in (x,res)
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factor (x,s) = if s=="" || s!!0 =='+' || s!!0 =='-' then (x,s)
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else let op = head s
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(y,s2) = getNum $tail s
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z = if op=='*' then x*y else x/y
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in factor (z,s2)
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expr (x,s) = if s=="" then x
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else let op = head s
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(y,s2) = factor.getNum.tail $s
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z = if op=='+' then x+y else x-y
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in expr (z,s2)
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