Algorithm Detective

Algorithm Detective 🕵️

Give a computer a problem and it needs a plan — an **algorithm**. Your job in this case: read the plan, work out what it does, and prove it. By the end you'll trace an algorithm line by line and name its purpose in one sentence — exactly what the exam rewards.

Three big ideas

• **Algorithm** — a precise sequence of steps that solves a problem. • **Decomposition** — breaking a big problem into smaller, easier sub-problems. • **Abstraction** — removing detail that doesn't matter, to focus on what does (like a map showing roads, not every tree).

Spot the abstraction

Which of these is **abstraction**?

• Ignoring detail that doesn't matter, to focus on the key idea
• Breaking a problem into smaller sub-problems
• A precise sequence of steps to solve a problem

Inputs → process → outputs

Almost every algorithm has the same shape: • **Inputs** — the data it's given. • **Process** — what it does with that data. • **Outputs** — the result it produces.

Label the parts

To find the average of some marks: the marks are the _____, adding them up and dividing is the _____, and the average is the _____.

How we write algorithms

Two common ways: **pseudo-code** (plain, code-like steps) and **flowcharts** (boxes and arrows). You need to read *and* complete both. Here's pseudo-code that reports the larger of two numbers: `INPUT a` · `INPUT b` · `IF a > b THEN OUTPUT a ELSE OUTPUT b`

Read it

Using that algorithm, what is output when `a = 4` and `b = 7`?

• 4
• 7
• 11

Dry-run with a trace table

To work out what an unfamiliar algorithm does, **trace** it: go line by line and write down every change, updating each column **in order** (left to right). Watch a running total build as a loop adds `1`, then `2`, then `3`.

Trace it yourself

An interactive activity.

Now — what does it DO?

Grade-9 detectives don't describe every line. After tracing, state the **overall purpose** in one precise sentence. That algorithm didn't just "add `i` to `total`" — step back: it worked out a **running total**.

Name its purpose

In one sentence, what is that algorithm's purpose?

• It adds up the whole numbers from 1 to 3
• It counts up to 3
• It outputs 6 three times

Good tracing habits

Pick the TWO things a good trace does.

• Update each column in order, left to right
• Write down every change to a variable
• Describe what each line does, separately
• Guess the final output

In the exam 🎯

Case closed. Grade-9 habits for algorithms: • Trace **accurately under time** — one column at a time, in order (the classic slip is updating out of order). • State the **overall purpose** in one sentence, not a line-by-line retelling. • Know your trio: **algorithm**, **decomposition**, **abstraction**.