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F2.7 Loop patterns

Decisions and loops · GCSE · OCR J277 2.2.1, AQA 8525 3.2.2, Edexcel 1CP2 1.2.4 · about 20 min

What this lesson is about

Running totals, counting, finding the largest, nested loops and trace tables.

• Running total
• Counting
• Finding the largest
• Loops inside loops
• Tracing a loop

Questions 7 marks in all

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

   total = 0
   for d in [40, 35, 30]:
       total = total + d
   print(total, total / 3)

   Answer:

   105 35.0

   A running total adds each value: 105. Dividing by how many gives the average, 35.0.

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

   count = 0
   for d in [50, 20, 35, 10]:
       if d < 30:
           count = count + 1
   print(count)

   Answer:

   2

   Only 20 and 10 are under 30, so the count is 2.

3. [1 mark]What does this program print?

   most = 0
   for d in [12, 51, 30]:
       if d > most:
           most = d
   print(most)

   Answer:

   51

   most is replaced whenever a bigger value turns up, so it ends as the largest, 51.

4. [1 mark]An outer loop runs 3 times and the loop inside it runs 4 times each time. How many times does the inner block run?

   Answer: 12. The inner loop runs all the way through for every time round the outer loop: 3 times 4.

5. [1 mark]A program puts total = 0 inside the loop instead of before it. What goes wrong?

   1. AThe total is reset every time round, so it ends as just the last value
   2. BNothing
   3. CA syntax error
   4. DThe loop never ends
   Answer: A. The starting value belongs before the loop, so it is set once.

6. [1 mark]What is a trace table for?

   1. AFollowing the values of variables step by step as an algorithm runs
   2. BDrawing the robot's path
   3. CListing a program's errors
   4. DStoring data in a program
   Answer: A. One column per variable, one row each time something changes: it shows exactly what an algorithm does.

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

   for row in range(1, 3):
       for col in range(1, 4):
           print(row, col)

   Answer:

   1 1
   1 2
   1 3
   2 1
   2 2
   2 3

   For each row, the inner loop goes through all three columns before the row changes.

The task: times grid

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

for row in range(1, 6):
    print(row)

from bugbot import *
connect()
for row in range(1, 6):
    for col in range(1, 6):
        print(row * col, end=' ')
    print()