In this topic students learn to repeat lines of code a set number of times, using for loops and range.

Learning objectives and goals

Be able to read, comprehend, trace, adapt and create Python code that:

• Iterates a set number of times using for loops
• Uses range with a start, a stop and a step
• Uses the value of the loop counter inside the loop
• Builds a running total inside a loop

Slide deck

This chapter has no slide deck. It was written for Dodona from new material by the course author, after the rest of the course had been converted.

Teacher notes

This chapter is the second half of iteration. The previous chapter covers while loops, where the program repeats until something changes; this one covers for loops, where the number of repetitions is known before the loop starts.

The exam specification calls these count controlled and condition controlled iteration, and both activities and exercises use those terms alongside for and while. It is worth being explicit with students that this is one idea with two shapes, and that the question to ask of any repeated task is “do I know how many times?”.

The pedagogical argument for teaching while first, as this course does, is that a while loop makes every part of the mechanism visible: the counter is created, compared and changed in three separate places, and forgetting the third gives an infinite loop. A for loop does all three at once. Students who have seen the long form first tend to read for counter in range(10): as the short form of something they already understand, rather than as new magic.

Key points

• range(10) counts from 0 and stops before 10, so it produces ten values. The count is the number in the brackets, which is usually what students want.
• range(start, stop) counts from the first number and stops before the second. This is where almost every mistake in this chapter comes from.
• range(start, stop, step) changes the size of the step. Leaving the step out means a step of 1.
• The counter is an ordinary variable and its value can be used inside the loop, which is how one loop prints a whole times table or numbers its own questions.
• A range can be empty. range(0) runs the body zero times, which matters as soon as the user decides how many repetitions there are.

Errors and misconceptions

• Off by one. range(1, 20) repeats 19 times, not 20. Students who count the numbers rather than reading the range get this right; students who read “1 to 20” do not. The Modify task exists mainly to make this mistake visible.
• Including the last number. A program that counts up to a number the user typed needs range(start, end + 1). The Investigate task asks students to explain the + 1 in their own words, which is a better check of understanding than asking them to write it.
• Printing the counter instead of using it. In the repeater exercise the body prints the string the user typed, not the counter. Both are variables in scope and swapping them is a common slip.
• Rebuilding a total inside the loop. In the last exercise total = 0 has to be before the loop. Inside it, the total is reset on every repetition and the program prints only the last number. This is worth tracing on the board.
• Text instead of numbers. Without int(), three answers of 7, 13 and 128 produce Total is 713128 rather than Total is 148. The test suite includes exactly that case, so students see the difference rather than being told.