Workshop 3: Control Flow and Loops in Python

Prerequisites:

Completion of Workshop 1 and Workshop 2
Basic understanding of Python syntax, variables, and data types
Python installed on your computer
IDE or text editor

What will you learn in this workshop?

Control Flow
Loops
Practice Problems

Control Flow

Conditional Statements

Conditional statements allow your program to make decisions based on conditions.

Basic if-elif-else

age = 18

if age < 18:
    print("You are a minor.")
elif age == 18:
    print("You just became an adult!")
else:
    print("You are an adult.")

Multiple conditions example

grade = 85

if grade >= 90:
    print("A - Excellent!")
elif grade >= 80:
    print("B - Good job!")
elif grade >= 70:
    print("C - Not bad.")
elif grade >= 60:
    print("D - You need to study more.")
else:
    print("F - You failed.")

Ternary operator (one-line if statement)

temperature = 35
status = "Hot" if temperature > 30 else "Not hot"
print(status)  # Prints: Hot

Comparison and Logical Operators

Comparison operators are used to compare values:

== - Equal to
!= - Not equal to
< - Less than
> - Greater than
<= - Less than or equal to
>= - Greater than or equal to

Logical operators combine conditions:

and - Both conditions must be true
or - At least one condition must be true
not - Inverts the condition

Comparison operators example

x = 10
y = 20

# Using comparison operators
print(x == y)  # False
print(x != y)  # True
print(x < y)   # True

Logical operators example

x = 10
y = 20

# Using logical operators
if x > 5 and y < 30:
    print("Both conditions are true")

if x > 15 or y > 15:
    print("At least one condition is true")

if not x > y:
    print("x is not greater than y")

Nested Conditions

You can nest conditional statements inside each other:

Nested if-else example

age = 25
has_license = True

if age >= 18:
    if has_license:
        print("You can drive a car.")
    else:
        print("You are old enough, but you need a license.")
else:
    print("You are too young to drive.")

Complex nested condition example

score = 85
attendance = 95
bonus_points = 5

if score >= 70:
    if attendance >= 80:
        final_grade = score + bonus_points
        if final_grade >= 90:
            print("Your grade is A")
        else:
            print("Your grade is B")
    else:
        print("Good score, but attendance too low")
else:
    print("You need to improve your score")

Loops

For Loops and range()

For loops iterate over a sequence (like a range, string, or other iterable). The range() function is particularly useful with for loops to generate sequences of numbers.

Basic range() function

# range(stop) - Generates numbers from 0 up to (but not including) stop
for i in range(5):
    print(i)  # Prints 0, 1, 2, 3, 4

Range with start and stop values

# range(start, stop) - Generates numbers from start up to (but not including) stop
for i in range(2, 8):
    print(i)  # Prints 2, 3, 4, 5, 6, 7

Range with start, stop, and step values

# range(start, stop, step) - Generates numbers with given step
for i in range(1, 10, 2):
    print(i)  # Prints 1, 3, 5, 7, 9

Range with negative step (counting backwards)

for i in range(10, 0, -1):
    print(i)  # Prints 10, 9, 8, 7, 6, 5, 4, 3, 2, 1

Iterating through a string

for char in "Python":
    print(char)  # Prints P, y, t, h, o, n on separate lines

Using enumerate for index and value

for index, char in enumerate("Hello"):
    print(f"Character at position {index}: {char}")

For loop with else clause

# The else clause executes after the loop completes normally (not via break)
for i in range(3):
    print(i)
else:
    print("Loop finished successfully")

Using range with conditional statements

# Finding even numbers
for i in range(1, 11):
    if i % 2 == 0:
        print(f"{i} is even")  # Prints 2, 4, 6, 8, 10 with labels

Other useful range() operations

# Sum of numbers using range
total = sum(range(1, 101))
print(f"Sum of numbers 1-100: {total}")  # Prints: 5050x

# Using range for specific patterns
print("Multiples of 3 from 0 to 30:")
for i in range(0, 31, 3):
    print(i, end=" ")  # Prints: 0 3 6 9 12 15 18 21 24 27 30

While Loops

While loops continue as long as a condition is true:

Basic while loop

count = 0
while count < 5:
    print(count)
    count += 1  # Increment count by 1

Using while with a sentinel value

# Note: this would normally require user input
answer = ""
while answer != "quit":
    print("Current answer:", answer)
    answer = "quit"  # Normally this would be input("Enter 'quit' to exit: ")

Infinite loop with break

# This pattern is common when requiring continuous user interaction
while True:
    # user_input = input("Type 'exit' to quit: ")
    user_input = "exit"  # Simulating input for documentation
    print("You entered:", user_input)
    if user_input.lower() == 'exit':
        break

While loop with else clause

counter = 0
while counter < 3:
    print(counter)
    counter += 1
else:
    print("While loop completed!")

Loop Control

Control statements for loops:

break - Exits the loop completely
continue - Skips the current iteration and continues with the next

Using break

# Using break to exit a loop early
print("Break example:")
for i in range(10):
    if i == 5:
        print("Found 5! Breaking the loop.")
        break
    print(i)  # Prints 0, 1, 2, 3, 4

Using continue

print("Continue example:")
for i in range(10):
    if i % 2 == 0:
        continue  # Skip even numbers
    print(i)  # Prints 1, 3, 5, 7, 9

Nested loops with break

print("Nested loops with break:")
for i in range(3):
    print(f"Outer loop: {i}")
    for j in range(3):
        if j == 1:
            print(f"  Found j=1, breaking inner loop")
            break
        print(f"  Inner loop: {j}")

While loop with break

print("While loop with break:")
counter = 0
while True:  # Infinite loop
    counter += 1
    if counter > 5:
        print("Counter exceeded 5, breaking out")
        break
    print(f"Counter: {counter}")

Practice Problems

Temperature Converter
- Write a program that converts temperature from Celsius to Fahrenheit
- Formula: F = C * 9/5 + 32
- Ask the user to enter a temperature in Celsius
- Use an if statement to tell the user if it's hot (>30°C), moderate (15-30°C), or cold (<15°C)
- Example output:
```
Enter temperature in Celsius: 35
35°C is 95.0°F
It's hot!
```
Number Guessing Game
- Create a simple game where the computer generates a random number (between 1-100)
- The user tries to guess it
- Use loops to allow multiple guesses
- Give hints whether the guess is too high or too low
- Example output:
```
I'm thinking of a number between 1 and 100.
Your guess: 50
Too high!
Your guess: 25
Too low!
Your guess: 42
Correct! It took you 3 guesses.
```
FizzBuzz
- Write a program that prints numbers from 1 to 100
- For multiples of 3, print "Fizz" instead of the number
- For multiples of 5, print "Buzz" instead of the number
- For multiples of both 3 and 5, print "FizzBuzz"
- Example output:
```
1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
...
```
Pattern Generator
- Use nested loops to generate patterns
- Create at least two different patterns using for loops and range()
- Example pattern:
```
*
**
***
****
*****
```

Simple Calculator

Create a calculator that performs basic operations (+, -, *, /) on two numbers
Use if/elif/else to determine which operation to perform
Use a while loop to allow multiple calculations

Example output:

Welcome to Simple Calculator

Enter first number: 10
Enter operation (+, -, *, /): +
Enter second number: 5
10 + 5 = 15

Calculate again? (yes/no): yes

Enter first number: 20
Enter operation (+, -, *, /): *
Enter second number: 3
20 * 3 = 60

Calculate again? (yes/no): no
Thank you for using Simple Calculator!

Nested Loops

You can create nested loops (loops inside loops):

Simple multiplication table

print("Multiplication table from 1 to 5:")
for i in range(1, 6):
    for j in range(1, 6):
        print(f"{i} x {j} = {i*j}")
    print("---")  # Separator between rows

Creating a pattern with nested loops

print("Number pattern:")
rows = 5
for i in range(1, rows + 1):
    for j in range(1, i + 1):
        print(j, end=" ")
    print()  # New line after each row

Output:

Coordinate grid with nested loops

print("Coordinate grid (x,y):")
for x in range(1, 4):
    for y in range(1, 4):
        print(f"({x},{y})", end=" ")
    print()  # New line after each row