Haskell Get position from first digit in string

Question

What I got is

digitIndex :: String -> Int
digitIndex [] = 1
digitIndex (x:xs) =
 if
 isDigit x == True
 then
 -- Count list
 else
 -- Create list with x(x is not a digit)

What my idea is is to make a list with all the x that he is passing so when he passes a digit he only needs to count the list and that will be the position of the digit(when you count a +1).

Only thing is I don't know how to get the job more done. Can you guys help me out with tips?

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Answer 1

You can use findIndex:

import Data.List
digitIndex :: String -> Int
digitIndex = maybe 0 id . findIndex isDigit

Answer 2

Just the normal recursion:

digitIndex :: String -> Int
digitIndex [] = 0
digitIndex (x:xs) = if isDigit x
 then 1
 else 1 + digitIndex xs

If the first character itself is a digit, then the function returns 1 else it just adds 1 and passes the remaining string (xs) to the function and the recursion goes on.

Also note that the above function doesn't work properly when the String doesn't have a number at all.

And also checking isDigit == True isn't necessary.

• user3434886: That wont make the good output because if I fill in: digitIndex "hel3lo" it will output 1 and not the 4 (the 3 is in the 4th place of the string)
• Sibi: @user3434886 I understood your question wrongly, have updated the solution.
• user3434886: Maybe I needed to be more clear in my explanation but you solved my problem. Thank you!