¶ In this lesson you will
- Build a formula from fields, constants and operators
- Use parentheses correctly
- Avoid formulas that fail at submission
¶ Calculate the price
Price From Formula combines number fields with constants and the operators + - * / ( ). You build it by clicking: pick a field or Constant in Select a field, then click an operator, and so on. The engine evaluates it when the applicant submits.
A formula scenario: Number of Visitors × 120,000 when Donation Type is Event
¶ Example: Kigali convention centre
Conferences: 100,000 + (attendees × 500). Workshops: 50,000 + (attendees × 300) + (hours × 5,000). The form has Event Type (dropdown), Expected Number of Attendees (number, 10–1000) and Event Duration (hours) (number, 2–12).
Number fields with limits so the formula always gets realistic values
Scenario 1, conference: 100000 + ( Expected Number of Attendees * 500 )
Scenario 2, workshop
- Select a field: a number field or Constant
- Operators
- The formula, built chip by chip
- Currency
- When Event Type = Workshop
A workshop for 50 attendees over 6 hours: 50,000 + (50 × 300) + (6 × 5,000) = 50,000 + 15,000 + 30,000 = 95,000 RWF.
Same numbers, different result (100 attendees)
SRD: "(base + attendees x 300), then add a 25% weekend surcharge"
Without parentheses: 50000 + 100 * 300 * 1.25
= 50000 + 37500 -> 87,500 RWF (wrong)
With parentheses: (50000 + 100 * 300) * 1.25
= 80000 * 1.25 -> 100,000 RWF (right)
- Multiplication happens before addition, so without parentheses the surcharge only applies to the attendee part. Add parentheses whenever the SRD says "then multiply the total"
Gotcha · missing values count as zero, bad formulas block submission
A field in the formula that is empty or hidden is taken as 0: the formula still returns a price, often a wrong one. A formula that cannot be evaluated (a dangling operator, division by zero) fails with
INVALID_FORMULA; the engine logsInvalid formula:with the expression and the values, which you can search in Kibana (lesson D5).
¶ Check yourself
Weekend bookings cost (20,000 + hours × 8,000 + participants × 500) × 1.25. Which formula is right?
- A. 20000 + hoursBooked * 8000 + participants * 500 * 1.25
- B. ( 20000 + ( hoursBooked * 8000 ) + ( participants * 500 ) ) * 1.25
- C. 20000 * 1.25 + hoursBooked
- D. 25000 + hoursBooked + participants
Show the answer
B. The 1.25 applies to the whole total, so the total must be wrapped in parentheses before multiplying. The second answer only increases the participant fee.
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