Lambda without Control Flow Statements

Question

we got a task for the next week with Lambdas. We should write some Lambdas without Control Flow Statements.

Example:
We got a string and this string should repeat n-times. Example: (string,3)-> stringstringstring. The common problem is, that we dont know, how to catch e.g. -9 or 0. My code looks like this:

BiFunction<Integer, String, String> nAnhaengen 
    = (n, word) -> {
    Stream.iterate(word, wordAdd-> word + wordAdd).
    limit(n).
    skip(n-1).          
    forEach(element -> System.out.println(element));
    return "";
};

Have you an idea or tipps, how we can catch negative integers?

Thanks and Greetings Daniel

Anything wrong with http://stackoverflow.com/a/24946101/1743880? — Tunaki, Jan 07 '17 at 16:06
@Tunaki we have to use lambdas with different parameters. :/ — DanielFGA, Jan 07 '17 at 16:08
@DanielFGA so `(n, word) -> String.join("", Collections.nCopies(n, word))` wouldn't work? — Salem, Jan 07 '17 at 16:09
Well `nCopies` will throw an exception if `n` is negative. So that should be taken care of before. — Tunaki, Jan 07 '17 at 16:09
The issue is, that if the integer is < 1, than we get an IllegalArgumentException. — DanielFGA, Jan 07 '17 at 16:12
Because we shouldnt use Control Statements like if/else/?/for... — DanielFGA, Jan 07 '17 at 16:14
Using `Math.max(0, n)` as the argument to `nCopies` can take care of it... If you can call that "without control flow statements". And assuming you want to return an empty String if it is negative... — Tunaki, Jan 07 '17 at 16:15

Kishore Bandi · Accepted Answer · 2017-01-07T17:43:39.740

1

You can use IntStream.

int n = -1;
String word = "string";

String string = IntStream.range(0, n).mapToObj(a -> word).reduce(String::concat).orElse("");
System.out.println(string);

OR

Change your code to have limit to 0 if value specified is lesser than 0

string = Stream.iterate(word, a -> a).limit(Math.max(0, n)).reduce(String::concat).orElse("");
System.out.println(string);

Edit:
As @Tunaki suggested Using Foreach and StringBuilder, as String::concat causes a lot of unnecessary immutable strings to be created.

String string = IntStream.range(0, n).mapToObj(a -> word).collect(Collectors.joining())
System.out.println(string);

edited Jan 07 '17 at 17:43

answered Jan 07 '17 at 16:33

Kishore Bandi

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1

`.reduce(String::concat)` will be quite slow, as it is concatenating strings which are immutable, so creating a lot of new instances of string. This specific example is referenced in the docs themselves https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html#MutableReduction – Tunaki Jan 07 '17 at 17:03
1

@Tunaki Thanks for the suggestion :) Edited the answer to make use of StringBuilder. – Kishore Bandi Jan 07 '17 at 17:27
1

There is a proper way to do that without `forEach`, see the linked documentation in the previous comment, and the use of `collect`. It makes sure the stream can be run in parallel without any issues (like you would encounter with `forEach` here). – Tunaki Jan 07 '17 at 17:29
@Tunaki But collect would collect the string as a list or collections in general, so it wouldn't give the concatenated output the question requested right? i.e., `stringstringstring` for `(string,3)` as input. Is there a way to use collect the result in a single string using Streams.collect? – Kishore Bandi Jan 07 '17 at 17:39
2

Yes, if you use [`joining()`](https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#joining--). – Tunaki Jan 07 '17 at 17:40
@Tunaki Ahh, nice :) Didn't know we could use Joining. Will update with your solution. – Kishore Bandi Jan 07 '17 at 17:42

Lambda without Control Flow Statements

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