Given a numeric string, I would like to apply an operation every nth digit in the string. Is this possible with java streams?

For example, for the string "12345" and applying a sum of 3 on every 2nd character, the result would be "15375".

public static void main(String[] args) {
    "12345".chars()
           .mapToObj(Character::getNumericValue)
           .mapToInt(c -> c   3) //should only happen every 2nd character
           .forEach(System.out::print);
}

The above results in 45678 because the sum is applied to all characters.

CodePudding user response：

I believe this is what you are looking for:

public static void main(String[] args) {
    String string = "12345";
    String result = IntStream.range(0, string.length())
            .mapToObj(index -> incrementCharAtEvenIndex(string, index, 3))
            .collect(Collectors.joining());

    System.out.println(result);
}

static String incrementCharAtEvenIndex(String string, int index, int increment) {
    int number = Character.getNumericValue(string.charAt(index));
    if ((index   1) % 2 == 0) {
        number = number   increment;
    }
    return String.valueOf(number);
}

Unfortunately, Stream API is not built to work with the indices directly, so you have to use IntStream to simulate for-loop and a custom method to increment a numeric character if it appears at the even index (remember it starts at 0).

CodePudding user response：

You can use the range operator to add a index to your stream:

String myString = "12345";
int n = 2;
        
IntStream
  .range(0, myString.length())
  .mapToObj(i -> Map.entry(i, Character.getNumericValue(myString.charAt(i))))
  .mapToInt(pair -> (pair.getKey()   1) % n == 0 ? pair.getValue()   3 : pair.getValue())
  .forEach(System.out::print);

CodePudding user response：

You can do it like this.

String s = "12345";
String result = update(s, 2, 3);
System.out.println(result);

prints

15375

• takes a string, skip value, and increment
• use (i 1)%n to start with nth character.
• use ternary operator (?:) to map to the sum depending on the remainder of the index to the skip value.
• return new String

public static String update(String str, int n, int inc) {
    return IntStream.range(0, str.length())
            .mapToObj(i -> String.valueOf((i   1) % n == 0 ?
                    str.charAt(i) - '0'   inc :
                        str.charAt(i) - '0'))
            .collect(Collectors.joining());
}   

CodePudding user response：

I actually had a solution using indices but I was not sure if there was a way to avoid that. Apparently not.

This also works:

public static void main(String[] args) {
        char[] chars = "12345".toCharArray();
        String collect = IntStream.range(0, chars.length)
                                  .map(index -> {
                                      int value = getNumericValue(chars[index]);
                                      return (index % 2 != 0) ? value   3 : value;
                                  })
                                  .mapToObj(String::valueOf)
                                  .collect(Collectors.joining());

        System.out.println(collect);
    }