Programming – count-controlled repetition

Your turn: read a range

Three values this time. Say which is the start, which is the stop and which is the step before you pick.

range(2, 11, 2): which pair gives its first and last values?

Work backwards

This time you know the numbers you want and have to choose the range.

You're printing a times table up to 12 x 5, so the loop variable has to run from 1 to 12. Which call gives exactly that?

Computer Science · Programming

A loop inside a loop

Step through and watch what the inner loop does every time the outer loop moves on.

Before you step: the outer loop repeats 3 times and the inner loop repeats 2 times. How many times will the print line run?

 1  for outer in range(3):
 2      for inner in range(2):
 3          print(outer, inner)

What you need to know

• Iteration means repeating a sequence of instructions inside a program loop.
• Count-controlled iteration uses a count value to decide how many times the instructions repeat. In Python, that's a for loop.
• A variable like count takes each value in the range in turn. The code inside the loop runs once for each value.
• When the sequence of values runs out, the loop ends by itself.
• Have a goYour mate Sam says: 'A for loop that goes through 4 values prints four times, then sits there waiting for someone to stop it.' What do you tell Sam?

  Sam's wrong: the loop ends by itself once the sequence of values runs out.

  After the last value has been used, the sequence has run out, so the loop finishes without anyone stopping it.

• With one value, range() uses it as the stop and starts at 0: range(5) gives 0, 1, 2, 3, 4.
• Have a goBe the computer: range(6) has just one value. What numbers does it give?

  0, 1, 2, 3, 4, 5

  One value is the stop, so counting starts at 0. Like range(5) giving 0 to 4, the stop itself isn't in the list.

• Two values are start and stop. Three are start, stop and step, so range(0, 10, 2) steps up by 2.
• The stop value is never included. range(0, 14, 2) gives 0, 2, 4, 6, 8, 10 and 12, so 12 is the last number printed.
• A for loop removes repeated code, and the number of repeats can come from a variable, such as num_users.
• Have a goIf the number of users changes every time the program runs, why does one loop beat 30 copied input lines?

  The repeat count can come from a variable like num_users, so the same loop fits any number of users.

  Copied lines fix the number of repeats in advance. A loop can read the count from a variable.

• Nested loops put one loop inside another. The outer loop controls how many times the inner loop is repeated.
• The inner loop runs completely, from its beginning, each time the outer loop runs once. Outer 3, inner 2: 3 x 2 = 6 runs.

The big picture

A for loop repeats code a set number of times, and range() decides which values the loop variable takes. The stop value is never reached, and that's the bit that catches people out. You'll also see how a loop replaces copied code and what a loop inside a loop really does.

⚠ Watch out

Believing the stop value gets used, and that the inner loop runs just once for each outer repetition. The sequence never includes the stop value, and the inner loop runs completely every time the outer loop runs once.

Memory hook

The stop value is the number the loop never reaches.

Check yourself

Think it through: for i in range(2, 20, 5), what is the last value i takes, and does the loop ever reach 20?