Loops and Iteration

A for loop asks an iterable for successive values. range(start, stop, step) excludes stop, which makes it fit zero-based indexing but creates boundary mistakes when guessed. enumerate supplies both positions and values without maintaining a counter.

The central idea in this lesson is controlled repetition. Read the rule, predict a concrete outcome, and then run the smallest example that can confirm or reject the prediction. This separates knowledge from familiarity with syntax.

A while loop is appropriate for retry, sentinel, or state-driven processes. Ensure something in the body can make its condition false. Break exits the nearest loop; continue begins the next iteration, but too many control jumps can obscure the main rule.

temperatures = [18.5, 21.0, 19.5, 24.0]
total = 0.0

for position, value in enumerate(temperatures, start=1):
  print(f"Reading {position}: {value:.1f}°C")
  total += value

print(f"Average: {total / len(temperatures):.1f}°C")

Reading 1: 18.5°C
Reading 2: 21.0°C
Reading 3: 19.5°C
Reading 4: 24.0°C
Average: 20.8°C

Trace the example before running it. Identify each input, transformation, returned value, and side effect. Then change one boundary value and explain why the new behavior follows from the rule rather than memorizing the output.

Do not remove items from a list while iterating over that same list. Build a filtered result or iterate over a copy. Test zero iterations, one iteration, the final boundary, and early termination.

Turn the concept into a repeatable workflow: write down the expected behavior, implement one coherent change, run a representative example, and retain a regression check. If the result surprises you, capture the exact input and error before changing anything.

Review the program for names, boundaries, and hidden side effects. A solution is complete when another developer can understand its contract, reproduce its setup, and verify both the successful path and one meaningful failure path.