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F5.1 What an algorithm is

Algorithms · GCSE · OCR J277 2.1.2, AQA 8525 3.1.1, Edexcel 1CP2 1.2.1 · about 12 min

What this lesson is about

Inputs, processes and outputs; what makes an algorithm precise; programs implement algorithms.

• An algorithm is not a program
• Inputs, processes and outputs
• What makes a good algorithm
• Many algorithms, one problem

Questions 5 marks in all

1. [1 mark]What is an algorithm?

   1. AA set of step-by-step instructions to solve a problem
   2. BA program written in Python
   3. CA type of computer
   4. DA list of variables
   Answer: A. An algorithm is the method. A program is that method written in a language a computer can run.

2. [1 mark]"Drive towards the wall and stop when close." What is wrong with this as an algorithm?

   1. AIt is ambiguous: towards and close are not precise
   2. BIt is too short
   3. CIt has no loop
   4. DNothing is wrong
   Answer: A. Every step must mean exactly one thing: how fast, and how close is close?

3. [1 mark]A program asks for a number of sides, works out the angle and drives the shape. Which are inputs?

   Tick every answer that is true.

   1. AThe number of sides
   2. BThe angle
   3. CThe shape driven
   4. DThe message printed
   Answer: A. Only the number of sides comes in. The angle is processing; the shape and the message are outputs.

4. [1 mark]What does it mean for an algorithm to be finite?

   1. AIt always finishes
   2. BIt uses few variables
   3. CIt only works on small numbers
   4. DIt has exactly one loop
   Answer: A. An algorithm must end, rather than run forever.

5. [1 mark]In which forms can an algorithm be written?

   Tick every answer that is true.

   1. AA flowchart
   2. BPseudocode
   3. CProgram code
   4. DA trace table
   Answer: A, B, C. A trace table follows an algorithm to check it; it does not describe the algorithm.

The task: any shape from an answer

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

# 1. Ask how many sides.
# 2. Work out the angle.
# 3. Repeat for each side: drive 20 cm, turn.
# 4. Say how many sides were drawn.

from bugbot import *
connect()
sides = int(input("How many sides? "))
angle = 360 / sides
for i in range(sides):
    forward(60, distance=20)
    turn_right(30, angle=angle)
print("drew a shape with", sides, "sides")