Answers

F3.4 Iterating over a list

Strings, lists and records · GCSE · OCR J277 2.2.3, AQA 8525 3.2.6, Edexcel 1CP2 6.2.2 · about 15 min

What this lesson is about

Loops over items and indexes, totals, counts, min and max, and linear search.

• for over the items
• for over the indexes
• The patterns, over a list
• Readings from the robot
• Searching a list

Questions 6 marks in all

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

   total = 0
   for r in [4, 7, 9]:
       total = total + r
   print(total / 3)

   Answer:

   6.666666666666667

   The running total is 20, and the average is 20 divided by 3.

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

   colours = ["red", "green", "blue"]
   for i in range(len(colours)):
       print(i, colours[i])

   Answer:

   0 red
   1 green
   2 blue

   range(len(colours)) gives every valid index, 0 to 2.

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

   readings = [41, 35, 30, 25]
   count = 0
   for r in readings:
       if r < 32:
           count = count + 1
   print(count)

   Answer:

   2

   Only 30 and 25 are under 32.

4. [1 mark]A linear search finds its target at index 2. Why does it use break?

   1. AThere is no need to look at the rest of the list
   2. BTo restart the search
   3. Cbreak is needed in every loop
   4. DTo print the index
   Answer: A. Once the item is found, looking further only wastes time.

5. [1 mark]How does a linear search work?

   1. AIt checks each item in turn, from the start, until it finds the target or runs out
   2. BIt jumps to the middle and halves the list
   3. CIt sorts the list first
   4. DIt only checks the first and last items
   Answer: A. Linear search looks at the items one by one. It works on any list, sorted or not.

6. [1 mark]What does this program print?

   readings = [40, 36, 25]
   for i in range(1, len(readings)):
       print(readings[i - 1] - readings[i])

   Answer:

   4
   11

   Starting at index 1 means each item can be compared with the one before it.

The task: sensor log

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

readings = []

from bugbot import *
connect()
readings = []
for i in range(5):
    readings.append(distance())
    forward(50, distance=8)
print("readings:", readings)
print("smallest:", min(readings))
print("largest:", max(readings))
print("average:", sum(readings) / len(readings))