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Tutorial · Grade 313. Cheap Can Cost More

Route payments through Provider B to account for expected failure costs.

Scenario

Provider A has a lower fee, but a 20% failure rate adds ₦5 penalty fees on failures.

Problem statement

Provider A charges a very low fee, but it frequently fails (80% success rate). Every failed attempt triggers a ₦5 penalty!

Provider B charges a slightly higher base fee, but has a 100% success rate. When accounting for expected failure penalties, Provider B is actually cheaper overall!

Route your transactions through Provider B to minimize total expected costs.

Provider A's base fee is lower, but its 20% failure rate adds expected penalty costs. Provider B has a lower total expected cost.

Use WHEN onlineB -> DO route-b to optimize overall processing costs.

What this teaches

Evaluate total expected cost (fee + failure risk penalties) rather than base fees alone.

Scroll down to build and test your solution.

Active controller

Apply & Run executes only the selected controller.

How a rule works

A rule has two parts:

WHEN <something is true> → DO <this action>

  • WHEN is a question about what is happening right now. For example: "Is it raining?" or "Is there a wall ahead?"
  • DO is what happens when the answer is yes.
  • The simulation runs one step at a time.
  • On each step, the rules are checked from top to bottom.
  • The first rule that is true wins. The rules below it are skipped for that step.
  • To change which rule wins, move it up or down the list.
  • If no rule is true, nothing happens on that step.
  • Some actions have more than one step, like "turn right, then move". You build these in the Functions tab.
  • Once a multi-step action starts, it finishes all its steps before the rules are checked again. It only stops early if the next step is not safe.
See an example

Two rules for deciding about an umbrella:

1  WHEN isRaining  → take an umbrella
2  WHEN always     → leave the umbrella at home
  • It is raining. Rule 1 is true, so you take an umbrella. Rule 2 is skipped.
  • It is sunny. Rule 1 is false, so rule 2 is checked next. Rule 2 is always true, so you leave the umbrella at home.
  • Order matters. If rule 2 were on top, it would be true every day, and you would never take an umbrella.

Rule folders

Categorise cards without changing their global execution order.

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Folder
WHEN
DO