Converting algorithms into programs

What you'll learn

• How to translate flowcharts and pseudocode into Python 3 programs.
• How to map algorithm ideas like sequence, selection and iteration into code.
• How to choose sensible variables, data types and indentation.
• How to test that your program still matches the original algorithm.

From algorithm to program

An algorithm is a step-by-step method for solving a problem. It can be written in different forms, such as a flowchart or pseudocode.

A program is an algorithm written in a programming language so that a computer can run it. In Paper 2, Edexcel assesses programming using Python 3.

The key skill in this topic is not inventing a new solution. It is keeping the same logic while changing the representation from flowchart or pseudocode into working code.

The basic building blocks

Most GCSE algorithms are built from three control structures.

Sequence

Sequence means instructions happen one after another, in order.

For example:

name = input("Enter your name: ")
print("Hello", name)

The input happens first. The output happens second.

Selection

Selection means the program chooses between different paths depending on a condition. In Python, this usually uses if, elif and else.

A condition is an expression that is either True or False, such as score >= 50.

if score >= 50:
    print("Pass")
else:
    print("Resit")

Iteration

Iteration means repetition. A section of code runs more than once.

A count-controlled loop repeats a known number of times, often using for.

for count in range(5):
    print("Hello")

A condition-controlled loop repeats while a condition is true, often using while.

while password != "letmein":
    password = input("Try again: ")

Mapping flowcharts and pseudocode to Python

Flowcharts use shapes and arrows. Pseudocode uses informal English-like instructions. Python uses exact syntax.

Here are some common conversions.

Converting a decision

Suppose the algorithm says:

• Input a score.
• If the score is at least 50, output Pass.
• Otherwise, output Resit.

This becomes a Python selection.

The completed Python program is:

score = int(input("Enter score: "))

if score >= 50:
    print("Pass")
else:
    print("Resit")

Notice the indentation. In Python, indentation is part of the syntax. It shows which statements belong inside the if or else block.

Converting assignment and calculations

An assignment stores a value in a variable. A variable is a named memory location that holds data while a program runs.

In pseudocode you might see:

SET total TO price * quantity

In Python this becomes:

total = price * quantity

The equals sign in Python means “store the value on the right in the variable on the left”. It does not mean exactly the same thing as equals in maths.

Useful Python arithmetic operators include:

Python version:

price = float(input("Enter price: "))
quantity = int(input("Enter quantity: "))

total = price * quantity

print(total)

Converting count-controlled loops

A count-controlled loop repeats a fixed number of times.

Pseudocode often uses wording like:

SET total TO 0
FOR count FROM 1 TO 5
    INPUT mark
    SET total TO total + mark
ENDFOR
OUTPUT total

This algorithm inputs 5 marks and adds them together.

Python version:

total = 0

for count in range(1, 6):
    mark = int(input("Enter mark: "))
    total = total + mark

print(total)

Converting condition-controlled loops

A condition-controlled loop repeats until something changes. In Python, this is usually a while loop.

Example algorithm:

INPUT password
WHILE password != "secret"
    OUTPUT "Incorrect"
    INPUT password
ENDWHILE
OUTPUT "Access granted"

Python version:

password = input("Enter password: ")

while password != "secret":
    print("Incorrect")
    password = input("Enter password: ")

print("Access granted")

The input appears before the loop and again inside the loop. This is because the condition needs a first value to test, and then the user needs another chance after an incorrect password.

A careful conversion method

When you are given a flowchart or pseudocode algorithm, work through it in this order.

1. Identify inputs and outputs

Look for values that enter or leave the algorithm.

• Inputs usually become input(...).
• Outputs usually become print(...).
• Numeric inputs usually need int(...) or float(...).

2. Identify variables

Choose meaningful variable names, such as total, score, largest or number_of_items.

Avoid names like x unless the algorithm already uses them or the value is genuinely temporary.

3. Identify control structures

Ask yourself:

• Are the steps simply in order? Use sequence.
• Is there a decision? Use if, elif or else.
• Is there repetition? Use for or while.

4. Translate one block at a time

Do not try to write the whole program at once. Convert each section, then check that the order still matches the original algorithm.

5. Test using sample data

A test value is an input chosen to check whether a program works correctly.

Use values that test different paths, such as:

• a normal value
• a boundary value, such as exactly 50 in a pass mark algorithm
• a value that should take the else path