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

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.