Control and Loop Structure in QBasic with Programs : QBasic Tutorial

Control and Loop Structure in QBasic with Programs : QBasic Tutorial, Control and Loop Structure in QBasic

Control Structure

The process of departure of the control of the program Conditionally or Unconditionally from its sequential flow depending upon the result of the condition is known as Branching or Jumping. 

Control Structure are the statement the structure that are used to handle different condition and iteration.

It is used to control the flow of program. They are Sequential Structure , Selection Structure and Loop Structure.

Control and Loop Structure

Sequential Structure:

A structure in which the statements are executed sequentially one after another without changing the flow of program.

Example,

CLS
INPUT A
INPUT B
LET S = A + B
PRINT " SUM OF NUMBER"; S
END

Selection Structure:

It is also known as branching structure that allows you to transfer the program control from one part to another on the basis  of specified condition or with out condition.

Types of Selection Structure:

i)  If Statement 
ii) Select Case Statement

If Statement

This is a decision making statement that decides which  statement has to be executed on the basis of specified condition.

CLS
INPUT A
INPUT B
LET S = A + B
PRINT " SUM OF NUMBER"; S
END

1. If.....Then Statement

It is one way decision making statement that evaluates a condition and executes the statement if the result of the condition id true.

Syntax,

If < Condition > Then
Statement 1
End If

2. If.....Then....Else Statement

It is the two way decision making statement that can decide which part of computer it has executed when the condition is true or false. It executes one part of a program if the condition is true and another part if the condition is not true or false.

Syntax:

If <  Condition > Then
Statement 1
Else 
Statement 2
End if

Write a program to check whether the number is divisble by 4 and 5

CLS
INPUT" ENTER THE NUMBER" ; N
IF N MOD 4 = 0 AND N MOD 6 = 0 THEN
PRINT" NUMBER IS DIVISIBLE IS DIVISIBLE BY 4 AND 6"
ELSE
PRINT" NUMBER IS NOT DIVISIBLE BY 4 AND 6"
END IF
END 
Mod=(divisible sign)

3.  If.....Then....Else If......Else

It is multi-way decision making statement that is used when there is two or more conditions to be evaluated.

Syntax
If < Condition 1 > Then
Statement 1
Else If < Condition 2 > Then
Statement 2
-----------------------
--------------------
Else Statement- N
End If

( N= Any Number)

Loop Structure


a. For........Next Statement

It repeats a block of statement for specified number of times.
1,2,3,4.....10
CLS 
FOR K = 1 TO 10
PRINT K ;
NEXT K
END

b. While.....Wend Statement

It executes a  block of statements repeatedly till the specific condition is true.

1,4,7,10.......25

CLS 
A = 1
WHILE A < = 25 
PRINT A
A = A +3
WEND
END

c. Do....Loop Statement

It repeats a part of program while a condition is true or until the condition becomes true.
100,90,80......up to 10

CLS 
S = 100
DO
PRINT A;
A  = A -10
LOOP WHILE A > = 10
END

Nested Loop

The Loop inside the loop structure is called Nested loop. It contains an outer loop and inner loop.
7  7  7  7  7  7  7  7
6  6  6  6  6  6  6  
5  5  5  5  5  5  
4  4  4  4  4  
3  3  3
2  2
1
CLS 
FOR A = 7 TO 1 STEP -1
FOR B = 1 TO A
PRINT A;
NEXT B
PRINT
NEXT A
END

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