How do separate out these elements of an equation in a string to a required format?

Question

I have a string input which looks like this:

String equation = "(5.5 + 65) - 33".

How would I go about separating these elements into an array which looked like this:

String array = {"(", "5.5", "+", "65", ")", "-", "33"}

I tried using the string split() method but because of there being no spaces between the parenthesis and the next digit it produces the incorrect format of:

String array = {"(5.5", "+"

Possibly related: [How to evaluate a math expression given in string form?](https://stackoverflow.com/q/3422673), especially this answer https://stackoverflow.com/a/26227947 — Pshemo, Nov 05 '20 at 14:06
@Spectric only for that specific example. Mathematical expressions are not a regular language. — OrangeDog, Nov 05 '20 at 14:25

OrangeDog · Answer 1 · 2020-11-05T14:56:27.267

You can do this with a StreamTokenizer:

StreamTokenizer st = new StreamTokenizer(new StringReader(equation));
st.parseNumbers();

List<String> tokens = new ArrayList<>();
while (st.nextToken() != StreamTokenizer.TT_EOF) {
    switch (st.ttype) {
        case StreamTokenizer.TT_EOL:
            // ignore
            break;
        case StreamTokenizer.TT_WORD:
            tokens.add(st.sval);
            break;
        case StreamTokenizer.TT_NUMBER:
            tokens.add(String.valueOf(st.nval));
            break;
        default:
            tokens.add(String.valueOf((char) st.ttype));
    }
}
String[] array = tokens.toArray(new String[tokens.size()]);

Note that because this parses the numbers as double, they become e.g. 65.0 when converted back to strings. If you don't want that, you'll need to add some number formatting.

I suspect that whatever you're planning to do with them later, you actually want them as numbers though.

How do separate out these elements of an equation in a string to a required format?

1 Answers1