Lecture 5 - Pseudo Code & Desk Checking

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Objectives

• To introduce Desk Checking as a way to check the correctness of algorithms
• To discuss a number of different problems and possible solutions

Contents

• Desk Checking
• Example Problems
  • Problem 1: Hello user-name
  • Problem 2: Square and cube of x
  • Problem 3: Supermarket Item Value
• What's Next
• Key Points
• Further Reading

This lecture focuses on step 5 of the Steps in Program Development - Desk Checking

1. Desk Checking

A desk check is a manual (non computerised) technique for checking the logic of an algorithm. The person performing the desk check effectively acts as the computer, using pen and paper to record results. The desk checker carefully follows the algorithm being careful to rigidly adhere to the logic specified. The desk check can expose problems with the algorithm.

Desk checks are useful to check an algorithm (before coding) thereby confirming that the algorithm works as expected. This can save time as the programmer doesn't waste time writing a program that doesn't do what is intended.

A desk check is done as a table, with columns in the order shown below:

1. A pseudo code line number column. Pseudo code doesn't normally have lines numbers, but these are necessary in a desk check to specify the line being executed.
2. One column per variable used. The variables should be in alphabetic order. State the variable name at the top of the column. As the algorithm is executed, the new values of the variables are put in the appropriate column. Show the working for all calculations.
3. A condition column. Show the working for the evaluation of any condition in this column. The result of the condition will be True (T) or False (F).
4. An Input/Output column is used to show what is input by the user and displayed by the program. Show inputs with: the variable name, followed by a "?" and the value input e.g. number1 ? 10. Show outputs with the variable name, an =, and the value displayed e.g. total = 13

Pseudo Code (with line numbers shown)

1 addTwoNumbers()
2     Input number1, number2
3     total = number1 + number2
4     Display total
5 STOP

Test data: number1 = 10, number2 = 3, total = 13

A more compact version of the above is normally used and is shown below. As line 1 doesn't perform any action (it would be a blank row in the desk check), it is combined with next line of code to be executed and shown in a single row in the desk check.

Example Problems

It is important to have a go at problems. Attempting and solving problems is essential in learning to program. With time you will begin to develop a knowledge base of problem types to draw on. These can be used to help solve problems.

Don't expect to get it right the first time. Look at your solution - does it do what is required? A Desk Check is a useful tool for checking that the algorithm performs as required. Modify your solution as necessary. Learn from your successes and your mistakes.

Problem 1: Hello user-name

Write a program that allows a user to input their name, then displays Hello student-name.

Example Runs

Run
Name ? Jane
Hello Jane

Run
Name ? Peter Smith
Hello Peter Smith

1. Understand the Problem

1. Read the Problem
2. Check understanding

name = "Jane"
Display "Hello Jane"

2. Specify a High Level Solution (Defining Diagram)

1. Determine the output(s) (result) required:
2. What processing is required to get to obtain the result:
3. What inputs are needed:
4. Develop the Defining Diagram

3. Develop the Outline

This is a simple problem and requires no more detail.

4. Develop the Algorithm (Pseudo Code and Data Dictionary)

hello()
    Input name
    Display "Hello ", name
STOP

Data Dictionary

5. Test the Algorithm (Desk Checking)

1 hello()
2     Input name
3     Display "Hello ", name
4 STOP

Inputs: name = Jane;
Correct results: Hello Jane

Inputs: name = Peter Smith;
Correct results: Hello Peter Smith

Problem 2: Square and Cube of x

A program is required to display the square and cube of a number input by a user..

Example Runs

Run
x ? 2
x squared = 4
x cubed = 8

Run
x ? 5
x squared = 25
x cubed = 125

1. Check understanding

2. Specify a High Level Solution (Defining Diagram)

3. Develop the Outline

4. Algorithm (Pseudo Code & Data Dictionary)

Data Dictionary

5. Test the Algorithm (Desk Checking)

Inputs: x = 2;
Correct results: xSquared = 4, xCubed = 8

Inputs: x = 5;
Correct results: xSquared = 25, xCubed = 125

Problem 3: Supermarket Item Value

It can be difficult to compare the price of items in a supermarket when the items are different sizes. In some countries, supermarkets display a price per 100 grams for items to make comparisons easier. For example, which is the better value, 250g of honey at a cost of $5.00, or 500g for $8.00, or 375g for $5.95? Write a program to allow the user to input the weight in grams and the cost of an item, and display the cost per 100g. Hint: the weight needs to be converted to lots of 100g (e.g. divide the weight in grams by 100).

Example Runs

Run
Weight (g) ? 250
Cost ($) ? 5.00
Cost per 100g = $2.00

Run
Weight (g) ? 500 
Cost ($) ? 8.00
Cost per 100g = $1.60

1. Check understanding

weight = 250, cost = $5.00.
weight in lots of 100 grams = 250 / 100 = 2.5.
cost per 100g  = cost / weight in lots of 100 grams = 5.0 / 2.5 = $2.00

weight = 500, cost = $8.00:
weight in lots of 100 grams = 500 / 100 = 5.0.
cost per 100g  = cost / weight in lots of 100 grams = 8.00 / 5.0 = $1.60

2. Specify a High Level Solution (Defining Diagram)

3. Develop the Outline

This is a simple problem and requires no more detail.

4. Algorithm (Pseudo Code & Data Dictionary)

Data Dictionary

5. Test the Algorithm (Desk Checking)

Inputs: weight = 250, cost = 5
Correct results: costPer100Grams = 2.0

Inputs: weight = 500, cost = 8
Correct results: costPer100Grams = 1.6

What's Next

In the next lecture we will look at converting an algorithm into a program and introduce programming in Visual Basic.

Key Points

• An algorithm is a series of steps to accomplish a task
• Pseudo Code is a commonly used method to express an algorithm
• A Data Dictionary is used to clarify the use of data in an algorithm
• A Desk Check is a manual check of an algorithm
• A Desk Check has a column for the line number, a column for each variable, a column for conditions and a column for input/output
• It is important to have a go at problems. Don't expect to get it right the first time. With practise you will improve
• Learn from your successes and your mistakes.

Further Reading

• Defining Diagram Guide
• Pseudo Code Guide
• Data Dictionary Guide
• Desk Check Guide
• Robertson, L. A. (2003) Simple Program Design: A Step-by-Step Approach. 4th ed. Melbourne: Thomson. chp 1-3

Written by Tim Whitfort.