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# EPI 5.6 - String to integer conversion functions

## Problem

Implement the following methods, which convert (signed) integers to a string and vice versa respectively:

```
intToString x :: Int -> [Char]
stringToInt :: [Char] -> Int
```

Additionally invalid strings (when converting to Int) must return an error of some sort.

## Solution

Even though the question was specific to converting to and from base 10, this can be generalised to any base with *very* little changes. The straightforward solution is:

```
intToString :: Int -> Int -> [Char]
intToString base x
| x < 0 = "-" ++ intToString base (-x)
| x < base = [intToDigit x]
| otherwise = (intToString base (div x base)) ++ [intToDigit (mod x base)]
stringToInt :: Int -> [Char] -> Int
stringToInt base (x:xs)
| x == '-' = - (stringIntHelper base xs)
| otherwise = stringIntHelper base (x:xs)
where stringIntHelper base (x:xs)
| dx >= base = error ("Invalid digit: " ++ [x])
| length xs == 0 = digitToInt x
| otherwise = (dx * (floor (fromIntegral base ** fromIntegral digitsLeft))) + (stringIntHelper base xs)
where
digitsLeft = (length xs)
dx = digitToInt x
```

Here is an alternative and more intuitive version of stringToIntHelper above:

```
stringToIntHelper2 base xs =
sum [ digit_to_value digit position |
(position, digit) <- (zip dig_positions xs)]
where
dig_positions = [(length xs) - 1, (length xs) - 2 .. 0]
digit_to_value digit position = (digitToInt digit) *
floor (base ** fromIntegral position)
```

This version does not check if each digit is between 0 and “base” but that is a trivial check to add via a “any” check in the original input (xs).