Cédric Soulas
Motion graphics about programming


Episode 20 - Can you fill in the blanks? Monday, 29 Jan.

This episode will show you how to perform each of those operations

Can you fill in the blanks? Try to find solutions that use only one card. If you are stuck, try at least to solve them with a chain of several cards. Watch the solutions below when you are ready.


Predicates everywhere

Here is a function:

It returns ✔ true or ✘ false. This means you can use this function as a predicate on filtering cards. The six operations on this exercise are filtering operations. And all solutions are based on such a predicate.

Part I - Filter

Here is the most common filtering card:

This is how ❚ filter works with a predicate:

Part II - Slice

Slicing is a sub-category of filtering. Cards from this sub-category take or skip everything while/until a certain condition is met. For example:

This is how ❚ takeWhile operates:

❚ skipWhile is the counterpart of ❚ takeWhile. Note that, with ❚ skipWhile, the input stream and the output stream completes at the same time.

Note: ❚ take is another slicing card (it returns a new stream of at most N values). But you can’t use ❚ take(2) instead of ❚ takeWhile(predicate) to solve this exercise: watch my take vs takeWhile video to see the difference.

Part III - Pick

Finally, you can filter an input stream by emitting zero or one event value. For example, an output stream can emit only the first or the last event value of an input stream. To emit only the last value that satisfies a predicate, you can chain ❚ filter(predicate) and ❚ last().

In some reactive stream libraries (like RxJS), you can simply use ❚ first and ❚ last with an optional predicate:

This is how ❚ last operates with one input stream and a predicate:


Cards to learn Reactive Programming

Focus on one new concept – every Monday

Occasional updates, plus:
Cédric Soulas
Motion graphics about programming