What does an irrelevant question evaluate to in CommCare

Great question! You seem like a swell dude.

The main difference between a calculate condition and default value is when the calculation will occur. Specifically, a default value will only be set once immediately upon opening the form. This is used primarily to load static values like case and session properties into the form.

Meanwhile, a calculate condition will fire multiple times during form entry (including at completion) so that its value always reflects the current state of the form accurately. This should be used for most purposes as calculates will be re-calculated based on answers given in the form.

Unfortunately the hairy part about declarative logic is that it can be a bit tough to decide when processing happens!

To answer your questions:

• A hidden value's display condition being false does not prevent the hidden value's calculation from executing. All calculations are performed regardless of whether their display conditions are met.

• Inside of a calculation (or display condition, etc...) if() statements on the other hand do "short circuit logic", meaning that the branch that isn't used won't be executed at all.

  • This is also true of the logic operators and and or

Example:

There is a hidden value with the calculate expression: long_expensive_calc()

Setting the expression instead to

if( #form/time_to_go = 'yes' , '', long_expensive_calc())

will prevent the "expensive" portion from running until the time_to_go is set to yes.