Answers

F2.5 Count-controlled loops: for

Decisions and loops · GCSE · OCR J277 2.2.1, AQA 8525 3.2.2, Edexcel 1CP2 6.2.2 · about 15 min

What this lesson is about

range, the loop variable, and repeating a known number of times.

• for and range
• Counting from zero
• The loop variable is useful
• Loops and decisions together

Questions 9 marks in all

1. [1 mark]What does this program print?

   for i in range(4):
       print(i)

   Answer:

   0
   1
   2
   3

   range(4) is 0, 1, 2, 3: four numbers, starting at 0 and never reaching 4.

2. [1 mark]What does this program print?

   for n in range(1, 4):
       print(n, n * n)

   Answer:

   1 1
   2 4
   3 9

   range(1, 4) starts at 1 and stops before 4.

3. [1 mark]What does this print? total = 0 then for i in range(5): total = total + i, then print(total)

   Answer: 10. 0 + 1 + 2 + 3 + 4 is 10.

4. [1 mark]A square has four sides. Which loop drives it?

   1. Afor i in range(4): with forward and turn_right(30, angle=90) inside
   2. Bfor i in range(3): with forward and turn_right(30, angle=90) inside
   3. Cfor i in range(90): with forward inside
   4. Dfor i in 4: with forward and turn_right(30, angle=90) inside
   Answer: A. range(4) runs the block four times. for i in 4 is an error: a number is not something to loop over.

5. [1 mark]How many times does the block under for i in range(10): run?

   Answer: 10. range(10) is 0 to 9: ten numbers.

6. [1 mark]Put the lines in order to print corner 1, corner 2, corner 3.

   Number the lines 1 to 3 to put them in the right order.

   1. for i in range(3):
   2. print("corner", corner)
   3. corner = i + 1
   Answer:

   for i in range(3):
       corner = i + 1
       print("corner", corner)

   The for line comes first, and the lines of the block are indented under it.

7. [1 mark]What does this program print?

   for n in range(0, 20, 5):
       print(n)

   Answer:

   0
   5
   10
   15

   The third number is the step: 0, 5, 10, 15, stopping before 20.

8. [1 mark]In OCR's Reference Language, for i = 1 to 5 ... next i. How many times does the loop run?

   Answer: 5. The Reference Language includes the end number: 1, 2, 3, 4, 5. Python's range(1, 5) would stop at 4.

9. [1 mark]What does AQA call a count-controlled loop?

   1. ADefinite iteration
   2. BIndefinite iteration
   3. CSelection
   4. DRecursion
   Answer: A. Definite: the number of times is known before it starts.

The task: count the corners

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

for i in range(4):
    forward(60, distance=30)

from bugbot import *
connect()
for i in range(4):
    forward(60, distance=30)
    turn_right(30, angle=90)
    print('corner', i + 1)