Java string, insert a dash after every 8 characters, starting from the right

Question

What's the best way to insert a - (dash/minus character) after every 8 characters in a Java String, starting from the right?

Examples:

1111 -> 1111
111111111 -> 1-11111111
1111111111111111 -> 11111111-11111111
100001111111111111111 -> 10000-11111111-11111111

My attempt, to show that I have tried doing it myself (a comment below asks: "is this homework?":

import junit.framework.TestCase;
public class InsertCharacterAfterEveryNCharacters extends TestCase {
  public static String insertSpacerAfterNCharactersFromTheRight(char spacer,
      int spacing, String string) {
    final int length = string.length();
    final int newStringCapacity = length + (int) Math.ceil(length / (double) spacing);
    StringBuilder stringBuilder = new StringBuilder(newStringCapacity);
    for (int i = length - 1; i >= 0; i--) {
      stringBuilder.append(string.charAt(i));
      if (i % spacing == 0 && i > 0) {
        stringBuilder.append(spacer);
      }
    }
    return stringBuilder.toString();
  }
  public static void testInsertSpacerAfterNCharactersFromTheRight() {
    assertEquals("", insertSpacerAfterNCharactersFromTheRight('-', 8, ""));
    assertEquals("1", insertSpacerAfterNCharactersFromTheRight('-', 8, "1"));
    assertEquals("11", insertSpacerAfterNCharactersFromTheRight('-', 8, "11"));
    assertEquals("11111111",
        insertSpacerAfterNCharactersFromTheRight('-', 8, "11111111"));
    assertEquals("1-11111111",
        insertSpacerAfterNCharactersFromTheRight('-', 8, "111111111"));
    assertEquals("11111111-11111111",
        insertSpacerAfterNCharactersFromTheRight('-', 8, "1111111111111111"));
  }
}

Answer 1

All answers seem a bit lot of code for what needs to be done. You could use a regular expression to do that.

Let's say you have a method that returns your formatted string.

A simple, clear example:

String myString = "00000000000111111111111100000000001111111000011000000";
String newString = myString.replaceAll("(.{8})(?!$)", "$1-");
return newString;

Above is equal to the following shorter notation:

return myString.replaceAll("(.{8})(?!$)", "$1-");

Another similar, short notation (in case of a fixed, hard-coded string):

return "00000000000111111111111100000000001111111000011000000".replaceAll("(.{8})(?!$)", "$1-");

Each piece of code resturns the following string:

00000000-00011111-11111111-00000000-00111111-10000110-00000

For more info on regular expressions, see for example http://www.regular-expressions.info/java.html

Hope this helps anyone in the future.

Edit note: The last group doesn;t have 8 characters. However, if it would, it won't add another dash.

Answer 2

Build up a char[] from the original String:

String str = "100001111111111111111";

// add enough space for an additional "-" for every 8 chars:
char[] chars = new char[str.length() + (str.length() / 8)];

// this offset will give us the first "-" position from the LEFT:
int offset = str.length() % 8;
int idx = 0, strIdx = 0;

for (; strIdx < str.length(); idx++, strIdx++)
{
    if (((strIdx % 8) == offset) && (strIdx != 0))
        chars[idx++] = '-';
    chars[idx] = str.charAt(strIdx);
}

String str2 = new String(chars);

System.out.println(str2);

Or you could use a StringBuilder, which could involve (length / 8) array copy operations:

StringBuilder str = new StringBuilder("100001111111111111111");
int idx = str.length() - 8;

while (idx > 0)
{
    str.insert(idx, "-");
    idx = idx - 8;
}

System.out.println(str.toString());

Output in either case:

10000-11111111-11111111

Note that this won't insert a hyphen at index 0, you'll need to adjust the code if that's necessary.

Answer 3

The above solutions are good but are very complicated and long, I wrote this and it works for your inputs.

public static String insertCharacterForEveryNDistanceFromRight(int distance, String original, char c){
    StringBuilder sb = new StringBuilder();
    char[] charArrayOfOriginal = original.toCharArray();
    for(int ch = charArrayOfOriginal.length ; ch > 0 ; ch--){
        if(ch % distance == 0 && ch != charArrayOfOriginal.length)
            sb.append(c).append(charArrayOfOriginal[charArrayOfOriginal.length - ch]);
        else
            sb.append(charArrayOfOriginal[charArrayOfOriginal.length - ch]);
    }
    return sb.toString();
}

And call it like this...

String rightResult = InsertSpaces.insertCharacterForEveryNDistanceFromRight(8, "100001111111111111111", '-');
System.out.println(rightResult);

Answer 4

You cannot literally insert a - into an existing Java String because String are immutable. However, you can create a new String instance that meets your output format requirement.

Loop from the end index of your input string down to 0. For each iteration:

• Insert the value at the current index to the beginning of a StringBuilder instance.
• Use the modulo operator to see if you have inserted a multiple of 8 characters. If so, insert a dash.

If needed, use the "toString()" method of your StringBuilder instance to get a string.

Answer 5

Something like this perhaps?

for(int i = s.length() - 1; i > 0; i -= 8)
{
    s = new StringBuffer(s).insert(i, "-").toString();
    i--;
}