Remove the first char to extract an integer (unhandled exception: Subscript)

I'm trying to write a function which extracts only the integer in a string.

All my strings have the format Ci where C is a single character and i is an integer. I would like to be able to remove the C from my string.

I tried something like this :

fun transformKripke x = 
    if size x > 1
    then String.substring (x, 1, size x)
    else x

But unfortunately, I get an error like unhandled exception: Subscript. I assume it's because sometimes my string will be empty and size of empty string is not working. But I don't know how to make it work... :/

Thanks in advance for your help

Best Regards.

Solution

The problem is calling String.substring (x, 1, size x) when x is not long enough.

The following should fix your immediate problem:

fun transformKripke s =
    if size s = 0
    then s
    else String.substring (s, 1, size s)

or slightly prettier:

fun transformKripke s =
    if size s = 0
    then s
    else String.extract (s, 1, NONE)  (* means "until the end" *)

But you may want to consider naming your function something more general so that it can be useful in more senses than performing a Kripke transform (whatever that is). For example, you may want to be able to extract an actual int the first time one occurs anywhere in a string, regardless of how many non-integer characters that precede it:

fun extractInt s =
    let val len = String.size s
        fun helper pos result =
            if pos = len
            then result
            else let val c = String.sub (s, pos)
                     val d = ord c - ord #"0"
                 in case (Char.isDigit c, result) of
                       (true, NONE)     => helper (pos+1) (SOME d)
                     | (true, SOME ds)  => helper (pos+1) (SOME (ds * 10 + d))
                     | (false, NONE)    => helper (pos+1) NONE
                     | (false, SOME ds) => SOME ds
                 end
    in helper 0 NONE
    end