for Loops

A Sequence of Small Steps

A for loop is useful when the same block of code must be repeated for each item in a sequence. Instead of treating the repetition as one large action, the loop visits one item, assigns that item to a loop variable, runs the loop body, and then moves to the next item. This continues until the sequence has no items left.

The Loop Variable Moves

The loop variable is the name that receives the current item. On the first iteration, it receives the first item in the sequence. After the loop body finishes, it receives the next item, and the body runs again. The loop variable therefore represents the item currently being processed, not the entire sequence at once.

for color in ["red", "blue", "green"]:
    print(color)

red
blue
green

Reading the for...in Pattern

for item in sequence: process(item)

Generating Number Sequences

The range() function generates a sequence of numbers. With range(start, stop), the sequence begins at start and continues up to, but does not include, stop. If no step is supplied, the sequence increases by 1. A third parameter controls the step size, so a range can count by larger increments or count backwards.

for number in range(2, 8, 2):
    print(number)

2
4
6

Tracing Each Iteration

for number in range(1, 4): print(number)

Mistakes Beginners Make

• Assuming that the stop value is included.

  The range sequence goes up to but does not include its stop value.

  Fix: Trace the generated values as 1, 2, and 3.

• Treating the loop variable as the whole sequence.

  The loop variable receives one current item at a time.

  Fix: Track its value separately for each iteration.

• Forgetting that the loop body repeats.

  The loop body runs for each item in the sequence.

  Fix: Count the sequence items and trace one body execution for each.

• Using the wrong step direction.

  The step controls how the generated sequence changes, including whether it counts by different increments or backwards.

  Fix: Choose a step that matches the intended direction and increment.

Choosing the Structure

Choose a for loop when the task requires the same processing for each item in a known sequence. The sequence may provide its items directly, or range() may generate a sequence of numbers with the required start, stop, and step. Before writing the loop, identify the sequence, the loop variable, and the action that should happen once per item.

1. Identify the sequence that should be visited.
2. Choose a loop variable name for the current item.
3. Write the loop body as the action that should occur once per item.
4. Trace the first item, the next item, and the final item to check the expected result.

Key Takeaways

1. A for loop repeats its body once for each item in a sequence.
2. The loop variable receives one current item at a time.
3. range(start, stop) begins at start, excludes stop, and uses a step of 1 by default.
4. The third range() parameter controls the step, allowing different increments or backwards counting.
5. Tracing the sequence of loop-variable values makes the loop's output predictable.