Topic 2 : Desk-Checking

Er. Pradip Kharbuja

Er. Pradip Kharbuja

What Makes a Good Algorithm?

• A good algorithm has the following qualities:

1. Complete

2. Efficient

3. Readable

4. Maintainable

5. Documented

6. Robust

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Syntax

• The words and symbols that make up a programming language are

known as its syntax.

• Java, C, Visual Basic and so on all have their own particular

syntax.

• Example of declaring variable as integer

In Java int a;

In Visual Basic Dim a as Integer

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Why Pseudocode?

• No need to worry about programming language

• We can focus on the logic of an algorithm rather than the programming language specific features

• Pseudo code can be useful if you're unfamiliar with the programming language

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Pseudocode Syntax

• There are many different styles of writing pseudocode.

• There is no ‘correct’ form of pseudo code.

• But we will use the following syntax :

1. When we need to hold some value, we use the word data to set up a container for it:

syntax : Data <name> as <type>

e.g. Data myAge as whole number

• Acceptable types for now are ‘whole number’, ‘real number’, ‘string’, ‘character’, ‘boolean'

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Pseudocode Syntax

2. When we need to get information from the user, we use the word input :

syntax : Input <name>e.g. : Input myAge

3. When we need to display anything to the user, we use output. Enclose in quotation marks.

e.g. : Output “Hello there!”• If we need to output the contents of some data, we also use

output but omit the quotation marks:e.g. : Output myAge

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Task

Take following inputs and display them:

1. First Name

2. Last Name

3. Age

4. Salary

Pseudocode

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Pseudocode

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Arithmetic Operators

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% gives remainder

e.g. 10 % 3 gives 1 20 % 4 gives 0 1 % 3 gives 1

Pseudocode Syntax

• We can put information/value in a data container like:

• When we wish to perform arithmetic calculation on numerical data, we need a container for the answer and then we use the arithmetic symbols:

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Pseudocode Example - 1

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Pseudocode Example - 2

• Write a pseudocode to multiply two numbers.

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Desk-Check

Desk checking is a manual (non computerised) technique for checking the logic of an algorithm or pseudo code, ensuring there are no bugs or mistakes present.

This process is done manually without computer assistance because pseudo code is not a real programming language.

Desk checks are useful to check an algorithm (before coding) thereby confirming that the algorithm works as expected.

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What is in a Desk-Check?

Desk-check results are documented on a table for easy reference.

The table typically has columns for

the line number

variables

a condition column, which is either true or false

an input column

an output column, which shows the end results.

and remarks column

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Er. Pradip Kharbuja

Again, there are no specific rules for pseudo code and desk-check

Pseudocode Example - 1(with line number)

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Desk-Checking Pseudocode Example - 1

Line No. myAge myNewAge Input Output Remarks

1 0

2 0 0

3 0 0 "Please enter your age" Output in Screen

4 21 0 21 User enters 21

5 21 22 myNewAge =

myAge + 1

6 21 22 "In next year you will be" Output

7 21 22 22 Output

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Pseudocode Example - 2

• Write a pseudocode to multiply two numbers.

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Desk-Checking Pseudocode Example – 2

Line Num. firstNumber secondNumber answer Input Output Remarks

1 0

2 0 0

3 0 0 0

4 0 0 0 "Please enter a

number"

Output

5 20 0 0 20 Input

6 20 0 0 "Please enter a

second number"

7 20 5 0 5

8 20 5 100 Process

9 20 5 100 "The answer is "

10 20 5 100 100Er. Pradip Kharbuja

Pseudocode Example - 3

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Desk-Checking Pseudocode Example – 3

Line Num. income taxRate myTax myNetPay Input Output Remarks

1 0

2 0 0

3 0 0 0

4 0 0 0 0

5 0 0 0 0 Output

6 20000 0 0 0 20000 User inputs 20000

7 20000 10.00 0 0

8 20000 10.00 200000 0

9 20000 10.00 200000 -180000

10-13 20000 10.00 200000 -180000 -180000 OutputEr. Pradip Kharbuja

Desk-Checking

• Desk-checks help to find the errors in your logic.

• When you have made an error, you can go back to the pseudocode

and correct it.

• By desk-checking complex algorithms, we can be sure the problem

is in our logic.

• In this case, our tax calculation is incorrect. Tax should be 10%.

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Correction

Before After

7 taxRate = 10.00

8 myTax = income * taxRate

9 myNetPay = income - myTax

7 taxRate = 10.00

8 myTax = (income / 100) * taxRate

9 myNetPay = income - myTax

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• Mistakes are going to happen – that’s unavoidable.

• Pseudocode lets us ensure that we fix it at the earliest and

easiest point - before we have written a single line of code.

Again,Desk-Checking Pseudocode Example – 3

Line Num. income taxRate myTax myNetPay Input Output Remarks

1 0

2 0 0

3 0 0 0

4 0 0 0 0

5 0 0 0 0 Output

6 20000 0 0 0 20000 User inputs 20000

7 20000 10.00 0 0

8 20000 10.00 2000 0

9 20000 10.00 2000 18000

10-13 20000 10.00 2000 18000 18000 OutputEr. Pradip Kharbuja

Desk-Checking

With a desk-check we can see where data starts going wrong

Sometimes, errors won’t be uncovered in a single desk-check

When picking user input, be awkward. What happens if you enter a negative number in our previous examples?

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Desk-Checking

• The code (or pseudocode) that we write defines the flow of execution through a program.

This is the order in which code statements are executed when the program is running.

• So far, we have looked only at sequential flow of execution.

Each line of code is executed after the last.

• In later weeks, we will look at ways of representing loops and choices.

These make desk-checking more challenging.

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Commenting

• All programs should contain comments. These are human readable notes in the program that are ignored when it is compiled.

• They make programs much more readable.

• The two main commenting styles in most languages are

1. line-by-line //this is a comment

2. block comments. /* This is block

comment */Er. Pradip Kharbuja

Commenting - 2

• Add comments when:

• You are doing something unusual

• You are doing something that requires some assumptions.

• Comments should detail your intention, not simply describe what the code does.

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Commenting - 3

/*

The following algorithm calculates the

length of the hypotenuse of a right

angled triangle.

*/

/*

The following algorithm takes one value

and squares it and then takes another

value and squares it. It then gives

the square root of the sum of those

squares.

*/

Good Comment Bad Comment

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• A comment should explain to people what you were doing, not

how you were doing it.

Terminologies

• Qualities of Good Algorithm

• Syntax

• Arithmetic Operators

• Desk-check

• Flow of execution

• Commenting

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Pseudocode

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Desk-Check with Null Values

Line num1 num2 sum usertext Output Remarks

1 0

2 0 0

3 0 0 0

4 0 0 0 Null

5 10 0 0 Null

6 10 20 0 Null

7 10 20 30 Null

8 10 20 30 “The answer is”

9 10 20 30 “The answer is” “The answer is” Output

10 10 20 30 “The answer is” 30 Output

Desk Checking

1. Take 3 user inputs and find the sum.

2. Take length and breadth of the rectangle and find the area.

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Any QuestionsEnd of Topic - 02

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