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For teachersTopic 2.3 · Lesson kit

if Statements

  • 2 class periods
  • 45 minutes each
  • Objective 2.3.A
  • 6 downloads with keys

Downloads

At a glance

Pacing
2 class periods of 45 minutes
Learning objectives
2.3.A
Essential knowledge
2.3.A.12.3.A.22.3.A.32.3.A.4
Materials
Projector and the Fork in the Code slide deckWhiteboard and markersMini whiteboards or scrap paperIDE on every computerPrinted Fork in the Code puzzle worksheet (answer key for you) and exit ticket

45 minutes a day

Agenda

Day 145 minutes

  • Warm-up
  • Direct instruction
  • Guided practice
  • Closure
  1. 0:00–0:055 minWarm-upevery line, every time? (slides 2–3)
  2. 0:05–0:1712 minDirect instructionthe one-way if (slides 4–7)
  3. 0:17–0:3013 minDirect instructionthe condition and the body (slides 8–10)
  4. 0:30–0:4010 minGuided practicestep challenge (slide 11)
  5. 0:40–0:455 minClosurewhich lines run?

Day 245 minutes

  • Warm-up
  • Direct instruction
  • Guided practice
  • Activity
  1. 0:00–0:055 minWarm-upfork in the road
  2. 0:05–0:1712 minDirect instructionif-else (slides 12–14)
  3. 0:17–0:236 minGuided practiceparking garage (slide 15)
  4. 0:23–0:3310 minDirect instructionone-way or two-way? (slides 16–18)
  5. 0:33–0:396 minGuided practicedoes the shop open? (slide 19)
  6. 0:39–0:456 minActivityDay 2 closure and exit ticket (slides 20–21)

Overview

Day 1 is the one-way if statement: how it changes the top-to-bottom order statements run in (the flow of control), how to trace whether its body runs, and how to write the condition and body, with braces around a body of several statements (2.3.A.1, 2.3.A.2). Day 2 is the two-way if-else statement, where exactly one of two bodies runs (2.3.A.3), and choosing between one-way and two-way selection, including how separate if statements can affect each other (2.3.A.4).

Students know assignment and compound operators (Topics 1.4 and 1.6), equals (Topic 1.15), selection as an algorithm idea (Topic 2.1), and Boolean expressions (Topic 2.2). Stay inside the topic boundary. Conditions are single Boolean expressions; do not use else if, nested if statements (Topic 2.4), or &&, ||, and ! (Topic 2.5 and beyond). If a student asks about checking more than two cases, tell them that is coming in Topic 2.4.

Teacher note

The slides are the Fork in the Code deck (21 slides, with speaker notes); slide numbers below refer to it. Day 1 uses slides 1–11 and Day 2 uses slides 12–21. The puzzle worksheet (10 questions, about 35 minutes) is split: questions 1, 2, 7, and 8 (rows 1, 2, and 5) after Day 1; questions 3, 4, 5, 6, 9, and 10 and the rest of question 8 after Day 2.

Files ending in .snippet.java hold only the statements that go inside main; paste them into a scratch class to run them. Examples are in the topic’s examples/ folder.

  • Day 1 demonstrations: NoSelection, FreeShipping, PaidShipping, JacketCold, JacketWarm, JacketFifty, SpeedCheck, StraySemicolon, NoParentheses, AssignInCondition, CountCompare, MemberEqualsTrue, MemberDiscount, LivesBug, MissingBracesFixed.
  • Day 2 demonstrations: PassFail, PassFailSeventy, PassBoundaryBug, TicketPrice, ParityCheck, DayCheck, ElseWithCondition, ElseWithoutIf, TwoIfsOverlap, TwoIfsOtherScores, Restock, ThanksBoth, ThanksAfter, UnassignedAfterIf, UnassignedFixedInit, AssignedBothBranches.
  • Exit ticket: reuses TwoIfsBalance, MissingBraces, SegmentI, and SegmentII. Keep these out of class so the questions are new to students.

Day 1 · Warm-up: Every line, every time?

Show slide 2: shipping should be free for orders of $50 or more, but every statement runs once, top to bottom, so the program always prints Shipping: 7.99. Partners decide which line should happen only sometimes and how Java could know when. Run NoSelection as a second example if you like: both lines print for a temperature of 68, even though the jacket line should not. Show the goals (slide 3).

Day 1 · Direct instruction: The one-way if

  1. Run the body only when the condition is true (2.3.A.1, slide 5). A selection statement changes the flow of control, the order statements run in, based on a Boolean expression. Break if (total >= 50) { shipping = 0.0; } into three parts: the condition in parentheses, the body in braces, and the rest of the program after the closing brace, which runs either way.
  2. A true condition. Run FreeShipping: Free shipping!, then Total: 62.5. The body ran and changed shipping.
  3. A false condition (slide 6). Run PaidShipping: the body is skipped, and the output is Total: 47.5. The flowchart has one arrow that skips the body and one that goes through it, and both rejoin.
  4. Boundaries. JacketCold (45), JacketWarm (68), and JacketFifty (50) show that temperature < 50 is false at exactly 50. Students predict each before the run.
  5. A semicolon after the condition (slide 7). StraySemicolon prints Wear a jacket. for 68. The ; ends the if statement, so the braces after it are an ordinary block that always runs. The code compiles, so only the wrong output shows the bug.

Misconception: A semicolon after the condition

Most Java lines end with ;, so students add one after if (...). It becomes an empty body, and the braces that follow always run. Give students a rule: no semicolon after the closing parenthesis of an if.

Misconception: The body always runs once

Students expect a statement to run once, as in Unit 1. With an if, the body runs once or not at all. Ask “Does the body run for this value?” before every trace.

Day 1 · Direct instruction: The condition and the body

  1. The condition must be a boolean (slide 9). Three kinds of condition: a boolean variable (if (raining)), a method call that returns a boolean (day.equals("Sat")), and a comparison (score == 100). Run MemberDiscount for two separate conditions, one a boolean variable and one equals.
  2. Parentheses are required. NoParentheses does not compile ('(' expected).
  3. One = is a mistake here too. AssignInCondition and CountCompare do not compile: incompatible types: int cannot be converted to boolean. Remind students of Topic 2.2: = stores, == asks. MemberEqualsTrue compiles but isMember == true is the same as isMember.
  4. Braces for several statements (slide 10). With no braces, the body is only the next statement; indentation means nothing to Java. Show the slide’s lives example. Then show LivesBug: Ouch! prints even though hitByEnemy is false, because only lives--; was the body. MissingBracesFixed shows the fix.

Misconception: Indenting a line puts it in the body

Java ignores indentation. Without braces the body is exactly one statement. Teach the habit: always write braces, even for one statement.

Misconception: Using = instead of == in the condition

if (score = 100) does not compile because score = 100 is an int, not a boolean. Read == aloud as “is equal to”.

Misconception: Comparing Strings with ==

Use day.equals("Sat"), not day == "Sat" (Topic 2.2). equals is case-sensitive: "sat" is not "Sat".

Day 1 · Guided practice: Step challenge

Slide 11. Students trace the points code for steps of 12000, 10000, and 9999 and write what prints: 55, 55, 5. The boundary case 10000 is the one to discuss: steps >= 10000 is true. Then run SpeedCheck with speed = 30 and limit = 35 (prints only Speed: 30); students change speed to 36 and predict first.

Teacher note

If you have no mini whiteboards, students write answers on scrap paper and show a partner. Keep each trace to about a minute, and have students say “true, so the body runs” or “false, so the body is skipped” aloud.

Day 1 · Independent practice: Independent practice

Puzzle worksheet questions 1, 2, and 7 (be the JVM with a one-way if, a table of fees for five ages, and a one-token fix to a stray semicolon) are homework. Question 2 previews Day 2 by putting two if statements one after the other. Question 1’s Think First (can two separate if statements skip both bodies?) is answered by question 2: yes, for age 13 to 64.

Day 1 · Closure: Closure

Ask: “In if (total >= 50) { shipping = 0.0; } which part is the condition, which is the body, and what runs after?” Students answer on a sticky note.

Day 2 · Warm-up: Fork in the road

Write on the board: “If you have a ticket, go in. Otherwise, buy one.” Ask how many of the two actions happen every time (exactly one). Introduce today’s idea: an if can have an else, so the program picks one path.

Day 2 · Direct instruction: If-else

  1. Exactly one body runs (2.3.A.3, slide 13). if (guess == secret) { ... } else { ... }. The condition is checked once. If it is true the first body runs; otherwise the else body runs. Both paths rejoin afterward, so Thanks for playing always prints.
  2. Pass or fail. Run PassFail (64: Try again next time., Score: 64), then PassFailSeventy (70) and PassBoundaryBug (score > 70 for 70 is a boundary bug that gives Try again next time.).
  3. A value from either path. TicketPrice sets price in both branches. ParityCheck and DayCheck ("sat" is not "Sat", so Weekday) are short alternatives.
  4. else has no condition (slide 14). ElseWithCondition does not compile (not a statement): the else body runs exactly when the if condition is false, so the question has already been asked. ElseWithoutIf, the stray semicolon again, gives 'else' without 'if'.

Misconception: else needs a condition

Students write else (age < 16). else runs when the if condition is false and has no condition of its own (ElseWithCondition).

Misconception: A semicolon after the condition, now with an else

The ; ends the if, so the else has nothing to attach to: 'else' without 'if' (ElseWithoutIf). Delete the semicolon.

Misconception: Both bodies of an if-else can run

Exactly one runs, never both and never neither. Ask students to name which body ran before they trace anything after it.

Day 2 · Guided practice: Parking garage

Slide 15. Students write the value of cost for 1, 2, and 5 hours: 0, 0, 15. Discuss the boundary (2 hours is free) and why cost has a value on both paths. Then show ThanksBoth and ThanksAfter: both print the same thing for total = 18.0, but the second writes Thanks for shopping! only once, after the if-else. Ask: “Which version is easier to change?”

Day 2 · Direct instruction: One-way or two-way?

  1. Separate ifs are independent (2.3.A.4, slide 17). Run TwoIfsOverlap (93): Excellent work! and You passed., both bodies run because both conditions are true. TwoIfsOtherScores shows 80 (only You passed.) and 50 (nothing). The same logic as an if-else runs exactly one body, so 95 prints Honor roll only.
  2. One if can change what the next one sees. Run Restock: the first if raises stock, so the second one’s condition is now true: Restocked., In stock., Stock: 14.
  3. A value on only one path (slide 18). UnassignedAfterIf does not compile (variable message might not have been initialized); the compiler checks the path where the body is skipped. Two fixes: give the variable a starting value (UnassignedFixedInit) or assign it in both branches (AssignedBothBranches).

Misconception: Two ifs in a row are the same as an if-else

Separate if statements can both run, or neither. An if-else always runs exactly one body. The shape of the problem decides: “choose one of two” is an if-else; “check this, and also check that” is two if statements.

Misconception: The second if only sees the original values

Each condition is checked when its statement runs, using the current values. Restock shows the first if changing what the second one sees.

Misconception: A variable assigned in one branch is always initialized

Java checks every path. If a path skips the assignment, using the variable afterward is a compile-time error.

Day 2 · Guided practice: Does the shop open?

Slide 19. Students write the output of version A (two separate if statements) and version B (if-else) and explain why they differ. Version A: coins becomes 13, the second if is now true, so Shop open and 13. Version B prints only 13: coins < 10 was true, so the first body ran and the else body was skipped, so Shop open never prints. Close by asking students to describe, in one sentence, what an if-else guarantees.

Day 2 · Independent practice: Independent practice

Puzzle worksheet questions 3, 4, 5, 6, 8 (the remaining rows), 9, and 10 are homework: rebuild a ticket booth with code tiles (question 3), fill the snippet pool (question 4), match four variants to their output (question 5), two truths and a lie about if-else (question 6), and the reflection on when to use two separate if statements (question 10).

Day 2 · Closure: Closure and exit ticket

Show slide 21; partners finish the sentence starter. Give the exit ticket (about five minutes). Question 1 checks tracing two separate if statements (2.3.A.1, 2.3.A.4), question 2 checks the body of an if without braces (2.3.A.2), and question 3 checks if versus if-else (2.3.A.3, 2.3.A.4). Re-teach the weakest one at the start of Topic 2.4. Slide 20 (vocabulary) is a reference for students.

Exit ticket

Question 1 · Tracing

Write the exact output.

int balance = 12;
if (balance >= 10)
{
    balance -= 10;
    System.out.println("Bought a ticket.");
}
if (balance < 10)
{
    System.out.println("Balance is low.");
}
System.out.println("Balance: " + balance);
Open in IDE
Check your reasoning
Bought a ticket.
Balance is low.
Balance: 2

balance >= 10 is true (12), so the first body runs: balance becomes 2 and Bought a ticket. prints. The second if is a separate statement, checked after that, with balance now 2: 2 < 10 is true, so Balance is low. prints. Last, Balance: 2.

A complete answer has all three lines in order. Leaving out line 2 means the second condition was checked against the original 12.

Question 2 · Multiple choice

What does this code print?

int temperature = 68;
if (temperature < 50)
    System.out.println("Wear a jacket.");
    System.out.println("Bring gloves.");
System.out.println("Have a good day.");
Open in IDE
  1. Nothing
  2. Have a good day.
  3. Bring gloves. and Have a good day.
  4. Wear a jacket., Bring gloves., and Have a good day.
Check your reasoning

C. There are no braces, so the body of the if is only the next statement, the Wear a jacket. line. 68 < 50 is false, so that line is skipped. Indentation does not matter to Java, so Bring gloves. is not in the body and always runs, followed by Have a good day.. A assumes both indented lines were skipped. B assumes the body included Bring gloves.. D assumes the condition was true.

Question 3 · Short answer

Both code segments start with x holding 7. What does each print? Explain why they are different.

Segment I
int x = 7;
if (x > 5)
{
    x -= 3;
}
if (x <= 5)
{
    System.out.println("small");
}
Open in IDE
Segment II
int x = 7;
if (x > 5)
{
    x -= 3;
}
else
{
    System.out.println("small");
}
Open in IDE
Check your reasoning

Model answer: Segment I prints small. Segment II prints nothing. In Segment I, x > 5 is true, so x becomes 4; the second if is a separate statement checked after that, and 4 <= 5 is true, so it prints small. In Segment II, the else body runs only when x > 5 is false. It is true, so only the first body runs and the else is skipped.

A complete answer gives both outputs and says that separate if statements are each checked, but an if-else runs exactly one body.