Add Episode 23 (count)

_data/course.yml

@@ -146,4 +146,11 @@ lessons:
     date: Monday, 12 Feb.
     videos:
       -
-      - 255356781
+      - 255356781
+
+  - name: Four ways to count
+    card: count
+    date: Monday, 19 Feb.
+    videos:
+      - 
+      - 256387349

count.md

@@ -0,0 +1,47 @@
+---
+id: count
+lesson: 23
+title: Reactive Programming - Count
+layout: default
+class: post
+preview_image: count/content_preview.jpg
+preview_image_alt:
+---
+
+Last week, you learned about [`❚ scan`](/scan) and [`❚ reduce`](/reduce). Here is how you can use them to count the number of values emitted on an input stream:
+
+{% include card_player.html video=256387874 episode=true %}
+
+In [Episode 21](/scan), the **accumulator** function accumulated _values_. In the example above, it only increments the <code class="two">acc</code> (accumulation) and the values are ignored. The <code class="two">acc</code> is emitted as a count value.
+
+On the one hand, [`❚ scan`](/scan) progressively emits on the output stream the current number of values emitted on the input stream. On the other hand, [`❚ reduce`](/reduce) emits the count only when the input stream completes. If the input stream never completes, the output stream will never emit the count.
+
+In RxJS, you actually have a `❚ count` operator. It is equivalent to the `❚ reduce` card seen above:
+
+{% include card_player.html video=256388299 episode=true %}
+
+This is how `❚ count` operates with one input stream:
+
+- **When the input stream completes**, the output stream:
+  - emits the number of values emitted on the input stream
+  - and immediately completes
+
+## Count values that satisfy a predicate
+
+Can you update the cards above to count the number of `✼`?
+
+You can slightly modify the **accumulator** function to increment the <code class="two">acc</code> only when the input value is `✼`:
+> ![](/img/count/accumulator2.png){:.w400}
+
+![](/img/count/scan_predicate-reduce_predicate.png)
+
+In RxJS, you can simply use `❚ count` with a [predicate](filter#predicate):
+
+![](/img/count/count_predicate.png)
+
+This is how `❚ count` operates with one input stream and a **predicate**:
+
+- When the input stream completes, the output stream:
+  - emits the number of values emitted on the input stream that satisfied the **predicate**
+  - and immediately completes
+