Planning & pseudocode

Plan before you code

• An algorithm is a clear list of steps that solves a problem.
• Good programmers plan the steps before they type code.
• A plan helps you spot mistakes early and explain your idea.

Decomposition

• Decomposition means breaking a big problem into smaller parts.
• Solve each small part on its own.
• Small parts are easier to write, test, and fix.

Abstraction

• Abstraction means keeping only the details that matter.
• You hide the parts you do not need right now.
• A map is an abstraction: it shows roads, not every tree.

Structured English

• Structured English writes the steps as plain numbered sentences.
• It is not real code, so anyone can read it.

1. Read the number.
2. If it can be divided by 2 with no remainder, it is even.
3. Otherwise it is odd.
4. Output the answer.

Flowcharts

• A flowchart draws the steps as boxes joined by arrows.
• A rectangle is a step; a diamond is a decision (yes / no).

   ( start )
       |
   [ read n ]
       |
  < n > 0 ? > --no--> [ output "not positive" ]
       | yes
   [ output "positive" ]
       |
    ( end )

Three building blocks

• Sequence: steps run one after another.
• Selection: if / elif / else chooses a path.
• Iteration: for / while repeats steps.

In Cambridge pseudocode

• The same plan in pseudocode, then in Python below.

total ← 0
FOR i ← 0 TO LENGTH(numbers) - 1
    total ← total + numbers[i]
NEXT i
average ← total / LENGTH(numbers)
OUTPUT average

numbers = [4, 8, 6]
total = 0
for n in numbers:
    total = total + n
average = total / len(numbers)
print(average)

Now you try

• Each task gives you a plan in words. Turn it into Python.
• Press Check answer to test your code.