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Structured Programming in Python: Syntax, Logic, and Flow

🎓 SS1 | 📅 Term 1 • Week 8

| ⏱ About 45 minutes

Learning goals: Write valid Python programs applying PEP 8 indentation standards and syntax rules. • Utilize input and output functions (`print()` and `input()`) with proper type casting (`int()`, `float()`). • Implement multi-way conditional branching using `if`, `elif`, and `else`. • Construct repetitive loops using `for` (counted iteration) and `while` (conditional iteration).

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In Weeks 1 through 7, we explored the electronic hardware, number systems, and micro-architectural cycles that power a computer.

Now, we bridge hardware theory with software reality by programming in Python—one of the world's most versatile, expressive, and in-demand programming languages, used globally for artificial intelligence, web development, robotics, and scientific research.

In this lesson, we will master type casting, build dynamic input/output programs, implement multi-way decision branching, and construct counted and conditional loops.

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Learning Outcomes

By the end of this lesson, you will be able to:

1. Write clean Python code adhering to standard 4-space indentation rules.
2. Accept user input and perform Type Casting (int(), float()).
3. Construct multi-way decision trees with if, elif, and else.
4. Implement repetition using for loops and while loops.

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Key Vocabulary

Term	Technical Role
Syntax	The strict grammar and spelling rules of a programming language.
Type Casting	Explicitly converting a value from one data type to another (e.g., converting text "25" to integer 25).
Indentation	The whitespace spaces at the beginning of a code line that define block structure in Python.
for Loop	A counted loop that iterates over a sequence or range of numbers.
while Loop	A conditional loop that repeats as long as a boolean condition remains True.

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Study Notes: Core Python Control Structures

INPUT AND TYPE CASTING:
Keyboard Input: "15" (String) ===( int() )===> 15 (Integer ready for math!)

DECISION STRUCTURE:
if condition_1:
    [ Execute Block 1 ]
elif condition_2:
    [ Execute Block 2 ]
else:
    [ Default Fallback Block ]

COUNTED ITERATION (FOR LOOP):
for i in range(1, 6):  <-- Runs for i = 1, 2, 3, 4, 5
    print(i * 10)

1. The Critical Rule of input() and Type Casting

When you ask a user for their age using input(), Python captures their keystrokes as a String:

# BROKEN CODE:
age = input("Enter your age: ")
next_year = age + 1  # FAILS! TypeError: cannot add string to integer!

To perform arithmetic, you must type cast the string into an integer:

# CORRECT CODE:
age_str = input("Enter your age: ")
age = int(age_str)
next_year = age + 1
print("Next year you will be:", next_year)

Or concisely in a single line:

age = int(input("Enter your age: "))

2. Multi-Way Decisions: if-elif-else

When evaluating multiple mutual conditions, chain them using elif (short for else-if):

score = int(input("Enter exam score (0-100): "))

if score >= 75:
    print("Grade: A - Excellent")
elif score >= 60:
    print("Grade: B - Very Good")
elif score >= 50:
    print("Grade: C - Credit")
elif score >= 40:
    print("Grade: D - Pass")
else:
    print("Grade: F - Fail")

3. Repetition: for and while Loops

The for Loop (Counted Repetition):

Use for when you know in advance how many times an action should repeat:

# Print the 5 times multiplication table up to 5 x 6:
for multiplier in range(1, 7):
    result = 5 * multiplier
    print(f"5 x {multiplier} = {result}")

The while Loop (Conditional Repetition):

Use while when an action should repeat until a condition changes:

# A countdown timer:
count = 5
while count > 0:
    print(count)
    count = count - 1
print("Blastoff! Rocket launches into orbit!")

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Worked Example: A Solar Inverter Energy Budget Calculator

Let us write a complete, practical Python script for a Nigerian home solar installation:

# Solar Energy Budget Calculator
print("=== KINSPIRE SOLAR INVERTER SIZER ===")

fan_watts = int(input("Enter total wattage of fans: "))
light_watts = int(input("Enter total wattage of lights: "))
tv_watts = int(input("Enter total wattage of television: "))

total_load = fan_watts + light_watts + tv_watts
print(f"Total Home Electrical Load: {total_load} Watts")

if total_load > 2000:
    inverter_recommendation = "3.5 kVA Inverter (24V System)"
elif total_load > 1000:
    inverter_recommendation = "2.0 kVA Inverter (24V System)"
else:
    inverter_recommendation = "1.0 kVA Inverter (12V System)"

print(f"Recommended System: {inverter_recommendation}")

Notice: Clear variable names, explicit int() type casting, and structured decision branching!

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Practical Class Activity: The Number Guessing Game

Type and run the following interactive Python script on your computer:

secret_number = 7
guess = 0

print("Welcome to the Number Guessing Game!")
print("I am thinking of a secret number between 1 and 10.")

while guess != secret_number:
    guess = int(input("Take a guess: "))
    if guess < secret_number:
        print("Too low! Try a higher number.")
    elif guess > secret_number:
        print("Too high! Try a lower number.")
    else:
        print("Congratulations! You found the secret number 7!")

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Common Mistakes & Corrections

Common Mistake	Why It Is Incorrect	Proper Correction
"Forgetting to type-cast user input before mathematical comparison."	Comparing strings to numbers ('15' > 10) causes runtime TypeError.	Always wrap numeric input in int() or float().
"Inconsistent indentation (e.g., 2 spaces on line 2 and 5 spaces on line 3)."	Python enforces strict block indentation; mismatched spaces cause IndentationError.	Always use 4 consistent spaces for indented code blocks.

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Notebook Summary

Copy Into Your Computing Notebook

• Use input() to accept user input; always type cast numbers using int() or float().
• Decision logic: if, elif, and else define mutually exclusive execution branches.
• Indentation (4 spaces) defines code block membership in Python.
• Use for loops with range() for counted iteration; use while loops for conditional iteration.

Check your understanding

What is the return data type of Python's built-in 'input()' function when a user types a response on the keyboard?

• String (str) — text characters
• Integer (int)
• Float (float)
• Boolean (bool)

What function is used in Python to convert the string '45' into a mathematical integer number?

• int('45')
• str('45')
• float('45')
• convert('45')

How does Python define code blocks belonging inside an 'if' statement or a 'for' loop?

• Using consistent indentation (typically 4 spaces)
• Using curly braces { }
• Using square brackets [ ]
• Using capital letters

What will be the output of the following Python loop? for i in range(1, 4): print(i, end=' ')

• 1 2 3
• 1 2 3 4
• 0 1 2 3
• 1 4

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