Understanding reactive expressions

One of the central tenets of reactivity is that reactive expressions are executed lazily, and their values are cached.

Lazy: Evaluated only when it is called, typically by a reactive endpoint.
Cached: Evaluated only when the value of one of its underlying dependencies changes.

Consider the Shiny app shown on the right. How many times do you think the following two messages will get printed:

This exercise is part of the course

Building Web Applications with Shiny in R

Turn theory into action with one of our interactive exercises