Pseudocode Tracer: Assignment Is a Copy

Before you start: try it once yourself

Step through the starter, the swap and one exam question yourself once before you start, about ten minutes.

1. Open the lesson. Go to mytekdev.com/try/pseudocode?lesson=assignment-is-a-copy&ref=fam-pseudo-1. The starter program is on the left, with a ? in front of lines 3, 5 and 8.
2. Step and predict. Press Step until it stops at line 3 and asks Line 3 is next. What will score be after it runs? Type 10 and press Check: Not yet. It was 15. Step on to line 5 and answer 10: Right.
3. Finish the run. Keep stepping (line 8 asks about lives: 4). The program displays 15, 10 and 4.
4. Swap. Choose Example, Swapping two values, and step through. The first try displays 8 8, the second 8 3. Open the Trace table tab to see a row for each change.
5. Exam question. Choose Example, Order of operations. Open Teach, then Exam question, and pick a choice. The right one is 18 28 3 2.5.

The tracer stops before a ? line and checks the prediction.

Exam question: each wrong choice is what a real mistake displays.

The ideas, in plain words

Every program on the AP CSP exam is written in the exam reference sheet's notation, in text and in blocks, and Big Idea 3, Algorithms and Programming, is 30 to 35 percent of the multiple-choice section. Topic 3.1 asks learners to determine the value of a variable as a result of an assignment (AAP-1.B). The CED spells the rule out: the arrow "evaluates expression and then assigns a copy of the result to the variable", and the value stored "will be the most recent value assigned" (AAP-1.B.2 and AAP-1.B.3).

The common mistake is to read b ← a as "b is a", a link that keeps them equal. It is an action that happens once: work out the right side, copy the result into the left side, move on. That is why a swap needs a third variable. Without one, the first assignment destroys the value the second one needs.

Topic 3.3 adds expressions (AAP-2.C). Multiplication, division and MOD happen before addition and subtraction. a MOD b is the remainder, defined on the sheet for a 0 or more and b more than 0. Division with / gives the ordinary result, so 17 / 5 is 3.4. Anyone who knows Java or older Python may expect 3. The tracer follows the sheet, and its Outside the sheet page lists every choice it makes where the sheet says nothing.

Do it together

1. Before opening anything, write a ← 1, b ← a, a ← 2, DISPLAY(b) on paper. Ask your teen whether it shows 1 or 2, and why.

   You'll know it worked when: Either answer is fine for now. The rest of the hour settles it.

2. Open mytekdev.com/try/pseudocode?lesson=assignment-is-a-copy&ref=fam-pseudo-1. A short guide runs along the bottom; follow it together. Press Step to run one line at a time and watch the Variables panel on the right.

   If you get stuck: A line with a ? in front of it stops before it runs and asks a question. Type the answer and press Check.

3. At line 3 the tracer asks what score will be. At line 5 it asks what DISPLAY(best) will show. Your teen answers each before checking.

   You'll know it worked when: score becomes 15. DISPLAY(best) shows 10: best got a copy of the old score, so changing score later did not change it. That is also why the paper example shows 1.

4. Choose Example, then Swapping two values. Before stepping, your teen predicts what the first try, lines 1 to 6, displays. Then step through.

   You'll know it worked when: The first try shows 8 and 8, because the 3 was replaced before it was copied. The second, with temp, shows 8 and 3.

5. Open the Trace table tab and copy it onto paper, one row for each change.

   If you get stuck: Each row is one moment. A blank cell means that variable did not change at that moment.

6. Choose Example, then Order of operations. Open Teach, then Exam question, and pick an answer before stepping.

   You'll know it worked when: The right answer is 18 28 3 2.5: multiplication happens before addition, MOD gives the remainder, and / is ordinary division.

7. Finish with one question: after b ← a, why does changing a later leave b alone?

   You'll know it worked when: Your teen explains that the arrow stored a copy of the value at that moment, and the two variables are separate.

Talk about it

Ask these while you work together. A child who has understood the lesson can answer them without help:

• What does b hold right after b ← a? And after a changes?
• Which line of the first swap destroyed a value?
• Why does the second swap need temp?
• What is 23 MOD 4, and why?
• Where would someone who reads left to right go wrong in Order of operations?

Bring it into other subjects

• Math: MOD is the remainder from long division. Find n MOD 2 for n from 1 to 10. What does it tell you about n?
• Computer science: write the starter program in a language you know. Does it display the same three values?
• Science: a lab notebook records the most recent reading of each instrument. Compare a variable to a notebook entry that gets overwritten.
• Careers: software testers trace code by hand to find where a value went wrong. List two questions a tester asks at each line.

Quick check

Name: ______________________________

1. x ← 4, y ← x, x ← x + 3, y ← y * 2, DISPLAY(x), DISPLAY(y). What is displayed? (A) 7 8, (B) 14 14, (C) 7 14.

2. What does DISPLAY(17 MOD 5) display? (A) 3, (B) 3.4, (C) 2.

3. Explain why a swap of two variables needs a third variable.

Answers

Measured in the tracer: the starter displays 15, 10 and 4. Swapping two values displays 8 8 for the first try and 8 3 with temp. Order of operations displays 18 28 3 2.5, and the no-precedence reading gives 28 28 3 2.5. Every choice was worked out by running a program, the question as written and each wrong choice with its mistake built in: question 1's B is the program with y linked to x, and C is y taking x's newest value; question 2's A is MOD read as the quotient, and B is 17 / 5.