1.2: Designing Algorithms

Exam Board:
OCR

Specification:
2020

What is an algorithm?

An algorithm is a set of instructions, presented in a logical sequence.

In an exam you may be asked to read and understand an algorithm that has been written. To prove your understanding you may be asked to respond by actions such as listing the outputs of the algorithm, correcting errors or identifying an error within it.

Programmers create algorithm designs as a method of planning a program before writing any code. This helps them to consider the potential problems of the program and makes it easier to start creating source code.

 

There are two main methods of defining algorithms

Defining Algorithms - Pseudocode & Flowcharts

Pseudocode

Pseudocode is not a specific programming language but a more general method of describing instructions. It should be unambiguous, and it should not resemble any particular kind of programming language (e.g. Python or Java), so it can theoretically be turned into working code in any language.

Generally, pseudocode can be written in any way that is readable and clearly shows its purpose. However, the Eduqas exam board advises that pseudocode for the programming exam should follow the conventions below:

Annotation

{Write your comment in curly brackets}

Define data type

price is integer

firstname is string

Declare a variable's value

set price = 100

set firstname = "Marcella"

Input / output

output "Please enter your first name"

input firstname

Selection (must have indentation)

if firstname = "Steven" then​

       output "Hello" + firstname

elif firstname = "Steve" then

      output "Please use full name"

else output "Who are you?"

end if

Iteration (while loop)

while firstname != "Steven"

      output "Guess my name."

      input firstname

repeat

Iteration (for loop)

for i in range 10

      input item

next i

Define a subroutine

Declare Sub1

     [Subroutine content indented]

End Sub1

Call a subroutine

call Sub1

Flowcharts

A flowchart can be used to visually represent an algorithm. The flowchart symbols are:

alg.png
alg2.png

Algorithm Examples

Below are two different methods for representing the same algorithm - a program to encourage people to buy items cheaply at a supermarket.

 

The program allows the price of items in a supermarket to be entered until the total reaches 100. The total price and the number of items entered are tracked as the program loops. Once the total reaches 100 or more, an if statement checks how many items have been entered and a different message is printed if there are 20 or more items, 30 or more items or less than 20 items. 

Pseudocode

{This is a program to see how many items you can buy in a supermarket
before you spend over £100}

total is integer, itemsentered is integer, itemprice is integer

set total = 0

set itemsentered = 0

while total < 100

      output "enter the price of the next item"

      input itemprice

      total = total + itemprice

      itemsentered = itemsentered + 1

repeat

 

if itemsentered >= 20 then

     output "You are on your way to saving money."

elif itemsentered => 30 then

     output "You're a real money saver."

else output "Look for better deals next time."

end if      

Flowchart

Reading Algorithms

In an exam you may be asked to read an algorithm and prove your understanding, most commonly by listing the outputs.

Start from the first line and follow the program line by line, recording the value of variables as you go.

When you encounter a for loop, repeat the indented code as many times as stated in the range.

Example Algorithm:

Start NewProgram

 

number is integer

maxvalue is integer

input maxvalue

for i = 1 to maxvalue

     output (i * i)

???????

output 'program finished'

End NewProgram

Example Questions:

1. List the outputs produced by the algorithm if the 'maxvalue' input is 5.

2. State the code that has been replaced by '???????' and what the code's purpose is.

Example Answers:

1.

Outputs:

1

4

9

16

25

program finished

2.

Missing Code: next i

Purpose: Moves the loop to the next iteration.

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Truth Tables

Create and use trace tables to follow an algorithm

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1.2 - Designing Algorithms:

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