Conditional Statements

Conditional statements allow Python to make decisions based on conditions.They control the flow of execution by running different blocks of code depending on whether a condition is True or False.

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The if Statement

Syntax:

if condition:
    # code block (runs if condition is True)

The if-else Statement

if condition:
    # runs if True
else:
    # runs if False

The if-elif-else Statement (Multiple Conditions)

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

Example:

marks = 85
if marks >= 90:
    grade = "A+"
elif marks >= 80:
    grade = "A"
elif marks >= 70:
    grade = "B"
else:
    grade = "C"
print(grade)  # Output: A

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Nested if Statements

You can place an if inside another if for more complex decision-making.

age = 20
has_id = True
if age >= 18:
    if has_id:
        print("Entry allowed")
    else:
        print("Show your ID")
else:
    print("Not allowed")

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Ternary Operator (One-Line if-else)

Python supports a concise inline conditional expression:

result = "Even" if x % 2 == 0 else "Odd"

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Loops

Loops allow you to repeat a block of code multiple times. They are essential for iterating over data — processing each row in a dataset, each character in a string, or each item in a list.

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The for Loop

Used to iterate over a sequence (list, tuple, string, range, dictionary).

Syntax:

for variable in sequence:
    # code block

Examples:

# Iterate over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# Using range()
for i in range(5):      # 0, 1, 2, 3, 4
    print(i)

for i in range(2, 10, 2):  # 2, 4, 6, 8 (start, stop, step)
    print(i)

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The while Loop

Repeats a block of code as long as the condition remains True.

Syntax:

while condition:
    # code block

Example:

count = 0
while count < 5:
    print(count)
    count += 1  # Don't forget to update! (Otherwise: infinite loop)

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Loop Control Statements

break Example:

for i in range(10):
    if i == 5:
        break       # Stop at 5
    print(i)        # Prints 0, 1, 2, 3, 4

continue Example:

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

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Nested Loops

A loop inside another loop. The inner loop completes all iterations for each iteration of the outer loop.

for i in range(3):
    for j in range(3):
        print(f"({i}, {j})", end=" ")
    print()
# Output:
# (0, 0) (0, 1) (0, 2)
# (1, 0) (1, 1) (1, 2)
# (2, 0) (2, 1) (2, 2)

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List Comprehension (Pythonic Loops)

List comprehension is a concise way to create lists using a single line of code. It is one of Python's most powerful features and is widely used in Data Science.

Syntax: [expression for item in iterable if condition]

Examples:

• Squares: squares = [x**2 for x in range(10)] → [0, 1, 4, 9, 16, ...]
• Even numbers: evens = [x for x in range(20) if x % 2 == 0]
• Upper case: upper = [s.upper() for s in ["hello", "world"]]

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for vs while — When to Use Which?

Summary

• if/elif/else controls decision-making based on conditions.
• for loops iterate over sequences; while loops repeat until a condition is False.
• break exits a loop; continue skips to the next iteration; pass is a placeholder.
• List comprehension provides a Pythonic, concise way to create lists.
• Nested loops are used for multi-dimensional data traversal.