Flowchart, Pseudocode & Decomposition

Ways to represent an algorithm

We can write an algorithm in plain steps (as in the previous lesson), but two standard forms are clearer: a flowchart (a picture) and pseudocode (structured English).

Flowchart symbols

Flowchart: sequence

Steps run one after another, top to bottom. Example: find the simple interest SI = P × R × T ÷ 100.

Flowchart: selection (if-else)

A diamond chooses one of two paths. Example: print "Pass" if marks ≥ 33, otherwise "Fail".

Flowchart: repetition (loop)

An arrow goes back to an earlier step so the steps repeat while a condition is true. Example: sum of numbers from 1 to N.

n = int(input('Enter N: '))
total = 0
i = 1
while i <= n:
    total = total + i
    i = i + 1
print('Sum =', total)

Pseudocode

Pseudocode ("fake code") describes an algorithm using simple keywords in capital letters — INPUT, COMPUTE, PRINT, IF … THEN … ELSE, WHILE, FOR — with indentation to show blocks.

INPUT marks
IF marks >= 90 THEN
    PRINT "Grade A"
ELSE IF marks >= 60 THEN
    PRINT "Grade B"
ELSE
    PRINT "Grade C"
END IF

INPUT N
SET fact = 1
FOR i = 1 TO N
    fact = fact * i
END FOR
PRINT fact

Flow of control

The order in which steps run is the flow of control. Every algorithm is made from just three building blocks:

Decomposition

Decomposition means breaking a big problem into smaller parts. Each part is solved separately (often by a different team member) and the parts are then combined. A school result system can be decomposed like this:

📁 result-system/
├── read_marks          # input marks of each student
├── total_and_percent   # calculations
├── grade               # A1, A2, B1 ...
├── rank                # sort students
└── print_report_card   # output

In Python, each sub-problem becomes a function:

# Decomposition: one big task split into small functions
def read_marks():
    return [78, 85, 92, 66, 81]          # sample marks

def total_of(marks):
    return sum(marks)

def grade_of(percent):
    if percent >= 90: return 'A1'
    if percent >= 80: return 'A2'
    if percent >= 70: return 'B1'
    return 'B2'

marks = read_marks()
t = total_of(marks)
p = t / len(marks)
print('Total:', t, '| Percent:', p, '| Grade:', grade_of(p))

Summary & practice

• Flowchart symbols: oval (start/stop), parallelogram (I/O), rectangle (process), diamond (decision), arrows (flow).
• Pseudocode uses keywords like INPUT, IF, WHILE, PRINT with indentation.
• Flow of control: sequence, selection, repetition.
• Decomposition splits a big problem into smaller ones.