tylerperkins

The sum of all natural numbers from 0 to n is

sum(n) = n*(n+1)/2

Proof

• Base case: Is it true for sum(0)?

  sum(0) = 0(0+1)/2 = 0
  

tylerperkins

Using owlet.firebase

Namespace owlet.firebase serves to integrate a Firebase database into our re-frame web application. We need to talk to Firebase without dropping into evil JS interop — so you shouldn't use .set! But more generally, we must preserve the integrity of our application's re-frame data flow.

In the description below, I've tried to be entirely consistent in my use of names; i.e. the scope of any name is this entire document. So when you see your-value or the-db-ref, you can count on those names indicating the same entities when they appear later in the document. Also, "ref" or "reference" refers to an instance of [firebase.database.Reference](https://firebase.google.com/docs/reference/js/firebase.database.Refe

tylerperkins

;;  Let's say you have a map recording the number of siblings for each individual:

(def num-sibs {:jack 0, :jill 2, :jane 0, :spot 7})

;;  Say you want to find the number of only children. We can use ->>

(->> num-sibs
     (map val)
     (filter zero?)
     count)