To understand how to use Pure Data for electronic music, we have to understand signal flow. The best way to understand is with mathematical expressions.
We use the expr object to find the average of two numbers. expr knows we want two inlets since the expression has $f1 and $f2. If we had more variables, there will be more inlets.
On the right side, we evaluate using + and / objects. Both sides are equivalent; however expr is more compact, especially with more complicated expressions.
Changing the first number will sent a bang as well and the result will be updated in the Number atoms in last row. However changing the second number will not send a bang, and we have to click on the bang to recalculate.