Can I check a String for characters other than specific ones?

Question

I am writing a Java program that can convert an inputted Roman Numeral into a Short, and I want it to be able to recognize and ask for input again when any letter has been inputted besides a Roman Numeral (I, V, X, L, C, D, and M).

Is there a method similar to .contains(), but with the opposite function?
Or do I have to check each individual letter with some kind of loop?

There is a funky "not" operator, isn't it? `if (! foo.contains(bar)) ...` — user unknown, Mar 21 '18 at 01:05
@userunknown that will not have the desired effect; the OP wants to check for the presence of characters other than certain ones, not the absence of certain characters — O.O.Balance, Mar 21 '18 at 01:11
@O.O.Balance: `boolean roman (String s) { for (char c: s.toCharArray()) if (! "IVXCLDM".contains(""+c)) return false; return true; }` — user unknown, Mar 21 '18 at 01:38
@userunknown Ah, apologies, now I get it. I like this solution best, would you consider turning it into an answer? — O.O.Balance, Mar 21 '18 at 01:42
Well, since the question showed little effort to solve the problem, I didn't want to give more than a hint. And if I had to solve it in my own code, I would go for pattern matching too. Before checking for complete numbers, I would ask for further restrictions, like 'IV, IX, IC, IM...'. — user unknown, Mar 21 '18 at 01:50

score 1 · Answer 1 · answered Mar 21 '18 at 01:08

1

I suggest you use a regular expression to check whether the input is a Roman numeral. You can find a regular expression for this problem here. Use String#matches() to determine whether your input matches the regex.

if(!input.matches("^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$")) { // input is the String the user entered
    // handle invalid input here
}

answered Mar 21 '18 at 01:08

O.O.Balance

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`IIII` is a valid roman numeral. IV is just a short form. And IM could be used as short form for 999 too. But imho, it was only asked for valid letters, not valid numbers, so `matches ("[IVXCLDM]+")` should be sufficient. – user unknown Mar 21 '18 at 01:46
@userunknown Well, there's no one definition of "valid Roman numeral" :-) But you have a point, the regular expression is a bit OTT. The simpler one you suggest is sufficient. – O.O.Balance Mar 21 '18 at 01:52

score 1 · Accepted Answer · answered Mar 21 '18 at 01:08

Well of course, you need some type of filter to test against the input.

One solution could be to use a string that contains all the possible valid characters in the input and then return false if a character wasn't found in the filter.

public class HelloWorld
{
  public static boolean filter(String test, String filter) {
    for(int i = 0; i < test.length(); i++) {
        if (filter.indexOf(test.charAt(i)) == -1) {
            return false;
        }
    }
    return true;
  }
  // arguments are passed using the text field below this editor
  public static void main(String[] args)
  {
    System.out.println(filter("XDQX", "XDQ"));
  }
}

score 0 · Answer 3 · answered Mar 21 '18 at 02:13

Just testing for valid characters, not for a valid sequence, can be done with contains:

boolean roman (String s) 
{
    for (char c: s.toCharArray()) 
        if (! "IVXCLDM".contains(""+c)) 
            return false; 
    return true; 
}

However, I would prefer a regular expression

boolean roman (String s) 
{
    return s.matches ("[IVXCLDM]+");
}

which means any number(+) of characters from that Set, at least one.

Can I check a String for characters other than specific ones?

3 Answers3