Programming structures or Logic in QBASIC

Following are the three types of flowchart structures.

a)      Sequence structure

b)      Selection/conditional structure

c)      Looping /Iteration structure

 a) Sequence Structure:

A sequential structure is a logic that executes statements one after another sequentially from top to bottom.

Syntax:

 

 Example: Display the average of given two numbers

 

b) Selection/ Conditional structure:

A selection structure is a logic that depends upon a condition and makes a choice between two alternate paths.

Syntax:

 

                                                          

Example: To check whether result is pass or fail.  (Assume pass percentage is 40)

 

c) Looping / Iteration Structure:

The looping structure is used when one or more instructions may be executed several times depending on some conditions.

We can pass loop structure in two ways.

Syntax:

Some examples:

1. To input any three subjects marks and display the average marks

Algorithm:

STEP – 1 Start

 

STEP – 2 Input first subject marks

 

STEP – 3 Input second subject marks

 

STEP – 4 Input third subject marks.

 

STEP – 5 Calculate the total of all 3 subjects

 

STEP – 6 Calculate average by dividing  total by 3

 

STEP – 7 End

Flowchart:

2. To display the greatest number among the input three numbers.

Algorithm:                                                                              

STEP 1: Start

STEP 2:  Input any three numbers

STEP 3: If a>b then

     If a>c then

    Print “a” is greatest

     Else

    Print “c” is greatest

     Else

    If b>c then

    Print “B” is greatest

     Else

Print “c” is greatest

STEP 4: End

Flowchart:

3.  To display the series of number 9, 7, 5……..1

Algorithm                                                                              

STEP-1:           Start

STEP-2: Assign value of x is 9

STEP-3:           Display x

STEP-4:           Decrease the value of x by 2

STEP-5            Is the value of x is greater than equal to 1

            If “yes” then goto step 3 and repeat it.

            If “no” then GOTO step 6

STEP-6            End

Flowchart