Chapter 3: Control Flow

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1. Conditional Statements

Conditional statements execute different code blocks based on conditions.

1.1 if Statement

The simplest form of control flow. If a condition is True, the code inside the if block is executed.

Syntax:

if condition:
    # Code to execute if the condition is True

Example:

age = 18
if age >= 18:
    print("You are an adult.")

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1.2 if-else Statement

Provides an alternative code block if the condition is False.

Syntax:

if condition:
    # Code to execute if the condition is True
else:
    # Code to execute if the condition is False

Example:

num = 10
if num % 2 == 0:
    print("Even number.")
else:
    print("Odd number.")

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1.3 if-elif-else Statement

Tests multiple conditions in sequence.

Syntax:

if condition1:
    # Code if condition1 is True
elif condition2:
    # Code if condition2 is True
else:
    # Code if none of the above conditions are True

Example:

score = 85
if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
else:
    print("Grade: C")

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1.4 Nested if Statements

if statements can be nested for more complex conditions.

Example:

age = 20
has_id = True

if age >= 18:
    if has_id:
        print("Access granted.")
    else:
        print("ID required.")
else:
    print("Access denied.")

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1.5 Ternary Conditional Expression

A one-liner for if-else statements.

Syntax:

value = true_value if condition else false_value

Example:

age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)

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1.6 Precautions with Conditionals

1. Proper Indentation: Python relies on indentation to define blocks. A mismatch will cause errors.

2. Avoid Overcomplicating: Use elif instead of nesting multiple if statements.

3. Boolean Values: Avoid redundancy. For example, instead of:

   if x == True:

   Simply use:

   if x:

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2. Loops

Loops allow you to execute code repeatedly.

2.1 while Loop

Repeats as long as a condition is True.

Syntax:

while condition:
    # Code to execute repeatedly

Example:

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

Precautions:

• Infinite Loops: Ensure the condition eventually becomes False.

• Use a break statement to exit loops if necessary.

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2.2 for Loop

Iterates over a sequence (e.g., list, string, range).

Syntax:

for variable in sequence:
    # Code to execute for each item in the sequence

Example:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

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2.3 range() in for Loops

The range() function generates a sequence of numbers, commonly used in for loops.

Examples:

for i in range(5):  # 0 to 4
    print(i)

for i in range(2, 10, 2):  # Start at 2, end at 9, step by 2
    print(i)

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2.4 break and continue

• break: Exit the loop entirely.

• continue: Skip to the next iteration.

Examples:

for i in range(5):
    if i == 3:
        break
    print(i)  # Stops when i == 3

for i in range(5):
    if i == 2:
        continue
    print(i)  # Skips i == 2

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2.5 Nested Loops

Loops inside other loops.

Example:

for i in range(3):
    for j in range(2):
        print(f"i={i}, j={j}")

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3. Control Flow Tools

3.1 The pass Statement

Does nothing; acts as a placeholder.

Example:

if True:
    pass  # Placeholder for future code

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3.2 The else Clause in Loops

A for or while loop can have an else clause. The else block runs if the loop completes normally (i.e., no break).

Example:

for i in range(5):
    if i == 3:
        break
else:
    print("Loop completed.")

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4. Comprehensions

4.1 List Comprehensions

A concise way to create lists.

Example:

squares = [x**2 for x in range(5)]
print(squares)  # [0, 1, 4, 9, 16]

4.2 Dictionary Comprehensions

Create dictionaries in one line.

Example:

squared_dict = {x: x**2 for x in range(5)}
print(squared_dict)  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

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5. Precautions and Tricks

1. Avoid Infinite Loops:

   • Always ensure the loop condition will eventually be False.

2. Keep Logic Simple:

   • Use comprehensions where possible for clarity and efficiency.

3. Don’t Modify a Collection While Iterating:

   • If you need to modify a list while looping, iterate over a copy:

     for item in list(items):
         items.remove(item)

4. else in Loops:

   • Remember that the else block executes only if the loop wasn't terminated by break.

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Summary Table: Control Flow Tools

Control Flow Tool	Description	Example
if	Executes block if condition is True.	if x > 5: print(x)
if-else	Adds an alternative block.	if x > 5: ... else: ...
if-elif-else	Tests multiple conditions.	if x > 5: ... elif x == 5: ...
while	Repeats as long as condition is True.	while x < 5: ...
for	Iterates over a sequence.	for item in list: ...
break	Exits the loop entirely.	if x == 3: break
continue	Skips to the next iteration.	if x == 3: continue
pass	Does nothing (placeholder).	if x > 5: pass
else in loops	Executes if loop completes normally.	for x in range(5): ... else: ...
Comprehensions	One-line list or dict creation.	[x**2 for x in range(5)]

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