Loops: Let the Program Handle the Repetition

What You Might Be Wondering

"Can't I just copy-paste a few lines to get the same result?"

Sure, for tiny scripts. But in real projects, copy-paste spirals out of control fast. A loop applies the same rule to a batch of data — that's its entire purpose.

One-Line Definition

Loops repeat code execution based on a condition or a sequence.

Real-Life Analogy

An assembly line: as long as there are items to process, the station keeps doing the same operation.

Minimal Working Example

for i in range(1, 6):
    print(i)

for vs while

# for: when you know what you're iterating over
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# while: condition-driven
count = 3
while count > 0:
    print(count)
    count -= 1

break and continue

for n in range(1, 8):
    if n == 3:
        continue
    if n == 6:
        break
    print(n)

Quick Quiz (5 min)

1. Print all multiples of 3 from 1 to 100.
2. Use while to calculate the sum from 1 to n.
3. Allow max 3 password attempts, exit early on success.

Quiz Rubric & Grading Criteria

• Direction: write runnable code that covers the core requirements and edge cases from the prompt.
• Criterion 1 (Correctness): main flow produces correct results, key branches execute.
• Criterion 2 (Readability): clear variable names, no excessive nesting.
• Criterion 3 (Robustness): basic protection against empty values, type errors, or unexpected input.

Take-Home Task

Build an "order counter":

• Input: a list of order amounts
• Loop to calculate the total
• Count how many orders are over 100

Acceptance Criteria

You can independently:

• Choose between for and while based on the situation
• Use break/continue correctly
• Avoid infinite loops

Common Errors & Debugging Steps (Beginner Edition)

• Error message looks like gibberish: read the last line for the error type (TypeError, NameError, etc.), then trace back to the offending line.
• Not sure what a variable holds: drop a temporary print(variable, type(variable)) to check.
• Changed code but nothing happened: make sure you saved the file, you're running the right file, and your terminal environment (venv) is correct.

Common Misconceptions

• Misconception: forgetting to update the state variable inside while.

• Reality: every iteration must advance the condition. Otherwise you've got an infinite loop.

• Misconception: stuffing heavy logic inside the loop body.

• Reality: keep loops for orchestration. Extract complex processing into functions.