Method for adding commas to digit string in Java

Question

The assignment is to add comes after every three digits looking from right to left. So number 1000000 should be 1,000,000 etc.

I have relatively good idea how to tackle this problem, but I have no idea why am I getting no output. Maybe I am making some mistake that I am not aware of or something...

I think that I understand concept that Strings are immutable, so they cannot be changed in place and when you want to change something to string you have to make new string object. But I dont get how is this possible then:

`result = ch + result;`

and this

result = "," + result;

What am I getting wrong here ?

   import acm.program.*;
   import acm.util.*;

    public class AddCommasToNumericString extends ConsoleProgram{

        public void run(){
            while(true){
                String digits = readLine("Enter a numeric string: ");
                if (digits.length() == 0) break;
                println(addCommasToNumericString(digits));
            }
        }

        public String addCommasToNumericString(String digits){
            String result = "";
            int counter = 0;
            for (int i = digits.length()-1; i <= 0 ; i--){
                char ch = digits.charAt(i);
                result = ch + result;
                counter++;
                if (counter % 3 == 0){
                    result = "," + result;
                }
            }
            return result;
        }
    }

`NumberFormat.getNumberInstance(Locale.US).format(digits)` will do the job. — bhantol, Jan 03 '17 at 23:29
@bhantol Yes of course, but I suspect his assignment is to do this manually to learn how to work with strings. — Tim Biegeleisen, Jan 03 '17 at 23:30
Yup thats why I asked so that I don;t post this easy solution. — bhantol, Jan 03 '17 at 23:34
In case you haven't already found the answer to why you can do `result = "," + result` when strings are immutable, it's because all variables are _references_ to objects and are not objects themselves. So when you say `result = something...` you are only setting the variable `result` to refer to a new string; you are not actually changing the string itself. — Klitos Kyriacou, Feb 02 '17 at 19:35

Tim Biegeleisen · Accepted Answer · 2019-08-03T06:11:55.113

3

I suggest eliminating the counter and use only the loop variable by making a small change:

public String addCommasToNumericString(String digits) {
    String result = "";
    for (int i=1; i <= digits.length(); ++i) {
        char ch = digits.charAt(digits.length() - i);
        if (i % 3 == 1 && i > 1) {
            result = "," + result;
        }
        result = ch + result;
    }

    return result;
}

addCommasToNumericString("123");     // 123
addCommasToNumericString("12345");   // 12,345
addCommasToNumericString("1234567"); // 1,234,567

edited Aug 03 '19 at 06:11

answered Jan 03 '17 at 23:30

Tim Biegeleisen

502,043
27
286
360

1

Every time is like this, I complete some assignment and always make these goddamn typing mistakes! I Tnx Tim. – Jan 03 '17 at 23:33

score 3 · Answer 2 · edited Nov 14 '18 at 21:20

3

Convert the read line into an Integer, then format it as a String

String digits = readLine("Enter a numeric string: ");
Integer myInt = new Integer(digits);
String output = NumberFormat.getNumberInstance(Locale.US).format(myInt.value());

edited Nov 14 '18 at 21:20

Raul Cacacho

267
1
4
15

answered Jan 03 '17 at 23:45

older coder

614
1
4
12

Method for adding commas to digit string in Java

2 Answers2