How to use use recursion to return an array of substrings

Question

I am trying to break a string down into substrings of size 3, and convert each substring into an array of strings, and return the final array recursively.

So far I have the following:

 private static String[] substrings(String string) {

    // base case
    if (string.length() <= 3) return new String[] { string };


            // this will return 
           return (Stream.concat(Arrays.stream(new String[]{string.substring(0,3)}), Arrays.stream(new String[] {string.substring(3)})).toArray(String[]::new));



        }

How would you call the last function recursively and how I would merge the String substrings recursively.

Any input appreciated.

If you're using recursion, I don't think you'll want to use streams. — NomadMaker
– NomadMaker, Commented Mar 23, 2020 at 20:48

bob tang · Accepted Answer · 2020-03-25 02:22:06Z

1

is this what you are after? ArrayUtils.addAll() is from apache common lang library.

Although i don't think it is very intuitive and efficient. iterative way is preferred.

    String[] substrings(String string){
        //exist condition
        if (string.length() <= 3) return new String[] { string };
        //get the substrings recursively
        String first = string.substring(0,3);
        return ArrayUtils.addAll(new String[] { first }, substrings(string.substring(3)));

    }

edited Mar 25, 2020 at 2:22

answered Mar 23, 2020 at 22:12

bob tang

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Comments

smac89 · Accepted Answer · 2020-03-25 04:38:31Z

I believe this is what you were trying to do:

private static String[] substrings(String string) {

    // base case
    if (string.length() <= 3) {
        return new String[] { string };
    }

    // this will return 
    return Stream.concat(Stream.of(string.substring(0, 3)),
      Arrays.stream(substrings(string.substring(3))).toArray(String::new);
}

This is quite wasteful because it creates a lot of arrays as the recursion unwinds.

You can fix this by having it return Stream instead:

private static Stream<String> substrings(String string) {

    // base case
    if (string.length() <= 3) {
        return Stream.of(string);
    }

    // this will return 
    return Stream.concat(Stream.of(string.substring(0, 3)), substrings(string.substring(3)));
}

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