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Week 3: For Loops, Lists, and Tuples

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Week 3: For Loops, Lists, and Tuples

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Course: Jan 2026 - Python Difficulty: Intermediate Setup Focus: Iteration over collections, mutability vs immutability.


Structure Visualizer
Demystifying Python Collections
[0"Python"
142
2True
]
Dynamic Injection
Code Generation
my_list = ["Python", 42, True]
# Mutability allowed

Operator Precedence (PEMDAS vs BODMAS)

In Python, mathematical expressions follow a strict hierarchy. While you might know it as BODMAS or PEMDAS, the logic is identical:

Parentheses (): Highest priority.
Exponents **: Evaluated next.
Unaries +x, -x: Positive/Negative signs.
Multiplication *, Division /, Floor Division //, Modulo %: Evaluated from left to right.
Addition +, Subtraction -: Lowest priority, evaluated left to right.

Why it matters: 10 + 5 / 5 results in 11.0, not 3.0, because division happens before addition!

1. The `for` Loop

While a while loop runs based on a condition, a for loop iterates over a sequence (like a string, list, or range). It executes the block of code once for each item in the sequence.

The

f in print(f"...") The f stands for Formatted String Literal (or f-string). It was introduced in Python 3.6 to make string interpolation faster and more readable.

How it works: Python looks inside { } curly braces and evaluates the expression within them at runtime.
Efficiency: F-strings are faster than % formatting or .format() because they are evaluated as part of the constant folding or more direct bytecode instructions.

1.1 The `range()` Function

range() is commonly used with for loops to generate a sequence of numbers.

range(stop): 0 up to (but not including) stop.
range(start, stop): From start to stop - 1.
range(start, stop, step): Increment by step.


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

# Prints even numbers: 2, 4, 6, 8
for i in range(2, 10, 2):
    print(i)

1.2 Loop Control: `break` and `continue`

break: Immediately exits the entire loop.
continue: Skips the rest of the current iteration and jumps to the next loop cycle.

2. Lists

A List is an ordered, mutable (changeable) collection of items. Lists can contain mixed data types and are defined using square brackets [].

2.1 Creating and Accessing


python
fruits = ["apple", "banana", "cherry"]
print(fruits[1])  # 'banana'

2.2 List Mutability (Changing Elements)

Unlike strings, you can modify elements of a list in-place.


python
fruits[1] = "blueberry"
print(fruits) # Output: ['apple', 'blueberry', 'cherry']

Memory Efficiency of Lists

Lists are over-allocated. When you append, Python allocates more space than needed so subsequent appends are

O(1)

amortized.

2.3 Common List Methods

append(item): Adds an item to the end.
insert(index, item): Inserts item at a specific position.
remove(item): Removes the first occurrence of the item.
pop(index): Removes and returns the item at the index.
- Scenario with no index: pop() defaults to the last item. This is used when implementing a Stack (Last-In, First-Out).
len(list): Returns the number of items.

3. Tuples

A Tuple is similar to a list: it is an ordered collection of items. However, tuples are immutable (they cannot be changed after creation). They use parentheses ().

Why use Tuples as Dictionary Keys?

To be a key in a Python dictionary, an object must be hashable. Hashability requires that the object's hash value never changes during its lifetime.

Lists: Can change ( $\to$ hash changes $\to$ unstable key).
Tuples: Cannot change (if they contain only immutable items) $\to$ stable hash $\to$ valid key.
Example: Coordinates (x, y) are often used as keys to map a 2D position to a value.

4. Nested Loops

You can place a loop inside another loop. The inner loop finishes all its iterations for every single iteration of the outer loop.


python
for i in range(3):       # Outer loop (runs 3 times)
    for j in range(2):   # Inner loop (runs 2 times per outer loop)
        print(f"i={i}, j={j}")

What do

i and j mean? These are just variable names. By convention:

i is for Index (often used for the outermost loop).
j is the next letter in the alphabet, used for the second level.
k is used for the third level. You can name them row and col for better clarity in 2D grids! (Useful for handling matrices, 2D grids, or generating combinations).

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