Calculation debugging techniques

Expert

Use the Data Viewer, intermediate Let() variables, and structured decomposition to diagnose and fix misbehaving FileMaker calculations.

What you'll learn

How to use the Data Viewer to inspect calculation expressions live
How to add debug variables to a Let() to expose intermediate values
How to decompose a failing calculation step by step
Common causes of silent calculation failures

Complex calculations fail silently -- they return an empty value, "?", or a wrong number with no error message. Effective debugging requires decomposing the calculation into parts, inspecting each part in isolation, and systematically ruling out causes. The FileMaker Data Viewer is your primary tool, combined with deliberate use of Let() debug variables.

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Data Viewer: live expression evaluation

The Data Viewer (Tools > Data Viewer, or Shift+Cmd+D on Mac) lets you type any FileMaker expression and see its result immediately for the current record. Use it to test sub-expressions before adding them to a field.

FileMaker Script

// In the Data Viewer, test each piece of a complex calculation:

// Step 1 -- does Position() find the delimiter?
Position ( Contacts::Email ; "@" ; 1 ; 1 )
// If this returns 0, the field has no "@" -- that's the bug

// Step 2 -- does Middle() extract the right length?
Middle ( Contacts::Email ; Position(Contacts::Email;"@";1;1)+1 ; 50 )

// Step 3 -- does the full formula work?
Let ( atPos = Position(Contacts::Email;"@";1;1) ;
  If(atPos>0 ; Middle(Contacts::Email;atPos+1;Length(Contacts::Email)) ; "no @")
)

Global field calculations and multi-user concerns Designing calculation field libraries