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CSP LabBy Educator Today
AP CSP course

C27 · Topic 3.6 · 40 minutes · AAP-2.G, AAP-2.H

Choosing a branch

By the end you can:

  • Trace IF and IF/ELSE.
  • Distinguish independent conditions from exclusive branches.
  • Predict all displayed outputs in order.

Before this lesson: topics 3.5.

Selection
Choosing which statements execute based on a condition.
Branch
A path through conditional code.

Understand

An IF executes its body only when its condition is true. An IF/ELSE chooses exactly one of its two branches. Two separate IF statements are not automatically exclusive: both may execute, depending on the current state and conditions.

Write down every output in sequence. Reading only the first true condition or assuming an unstated ELSE can miss later effects. If the first branch changes a variable, evaluate the next condition using that updated state rather than assuming all conditions were tested at once.

Worked example

x←7;IF(x>3){DISPLAY("A")};IF(x<10){DISPLAY("B")}.

  1. 7>3 is true, so A displays.
  2. x is unchanged by DISPLAY.
  3. 7<10 is also true, so B displays after A.

Explore each concept

Choose a concept, change its inputs and follow the explanation. Each example models the stated rule or scenario; completing it is practice rather than evidence of mastery.

  • Multiple outputs in order
  • Exclusive branches
  • Inverted equivalent conditions
Open this example on its own

Change it · step through · explain

Change an input, predict what will happen, then advance one step. Read why the state changes, reset and experiment. This fixed teaching model does not save progress or award XP.

Ready · Make a prediction

Ready to explore: Independent IFs versus IF/ELSE
  1. Score (0–100)85Waiting
  2. Structureindependent-IFsWaiting

Ready to explore: Independent IFs versus IF/ELSE

Why: Change an input, predict the first effect, then advance one step.

Try this: Change one input or choose another listed case. Predict the effect, reset and compare the states and governing rule.

Step through and experiment

Change it · step through · explain

Change an input, predict what will happen, then advance one step. Read why the state changes, reset and experiment. This fixed teaching model does not save progress or award XP.

Ready · Make a prediction

Ready to experiment
  1. Input 114Waiting
  2. Input 20Waiting

Ready to experiment

Why: Predict the first change, then advance one step.

Try this: Compare ages 13 and 14 with places 0 and 1.

Check a misconception

Claim: Only the first true IF in a programme can run.

Independent IF statements are evaluated separately; an IF/ELSE pair is a different structure.

Guided practice

Replace the two independent IFs with IF(x>3){A}ELSE{B}. Predict x 7.

Work with a partner. Make a prediction, compare your reasoning, then identify the rule that supports your answer.

Independent transfer

Trace x 4;IF(x>3){x←12};IF(x<10){DISPLAY("B")}.

Exit check

Why should conditional traces show intermediate state?

Write your reasoning before comparing with a classmate. If your teacher assigned this task, submit it through My classes to save your work and receive feedback.

Check yourself

  1. Question 1 of 2Two independent IF conditions are both true and display A then B. What is shown?

    Select one answer.

  2. Question 2 of 2Which TWO statements describe IF/ELSE correctly?

    Select exactly two answers (0 of 2 selected).

Your practice so far

Check an answer to see its rationale.

Teacher guidance and scoring