Predict the Output

A short program goes on the board. Before anyone touches a keyboard, you commit to a prediction — the exact output, in ink. Then we run it. Being wrong costs nothing; refusing to commit costs everything — the gap between prediction and reality is where the learning lives.

How to run it

1. Read the program twice, silently — no talking yet.
2. Write the exact output you expect, character for character.
3. Compare with a neighbour; the code, not volume, settles disputes.
4. Run it. Surprised? The line that fooled you is today’s lesson.

One program, three defensible readings (click to expand)

x = "5"
y = x + x
print(y)

10 — five plus five. 55 — the quotation marks make x text, and + glues text together. A crash — you cannot add words. Only one reading is what Python does; the quotation marks decide it.

Expressions and operator precedence (click to expand)

a = 10 + 4 * 2 ** 3
b = (10 + 4) * 2 ** 3
print(a, b)

42 112 — in a, exponentiation ($2^3=8$) and multiplication ($4\times 8=32$) happen before addition ($10+32=42$). In b, parentheses force addition first ($14\times 8=112$). Order of operations decides every compound expression.

One variation

Reverse it: show only the output; pairs write a program that would produce it. There is never just one answer — which is the point.

Commit before you run

A guess never written down cannot surprise you, and code that never surprises you never teaches you — Debugging Is the Job says why.

Curriculum connection

C1.3

identify various types of data and explain how they are used within programs

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C1.4

determine the appropriate expressions and instructions to use in a programming statement, taking into account the order of operations

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C2.6

interpret program errors and implement strategies to resolve them

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C3.1

analyze existing code to understand the components and outcomes of the code

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