Chapter 7: Statements in QBasic

💡 1. Fundamentals of QBasic

1. QBasic stands for Quick Beginners All-purpose Symbolic Instruction Code.
2. It is a high-level programming language developed by Microsoft for MS-DOS in 1991.
3. QBasic is an interpreter-based language that executes programs line by line.
4. Each QBasic program is made of statements telling the computer what to do.

📘 2. Some Basic Statements in QBasic

Statement	Purpose	Syntax	Example
LET	Assigns a value to a variable	LET variable = expression	LET A = 10
PRINT	Displays output	PRINT expression	PRINT "HELLO"
INPUT	Takes input from user	INPUT "Prompt", variable	INPUT "Enter name"; N$
END	Stops execution	END	END
REM	Adds comments	REM comment	REM This is a comment

⚙️ 3. Control Statements in QBasic

➥ Control statements change the sequence of execution in a program. They are of two types:

1. Conditional Control Statements
2. Looping (Repetition) Statements

🧩 A. Conditional Control Statements

➯Used to test a condition.
➯ If the condition is true, a statement is executed; otherwise, it is skipped.

(i) IF...THEN Statement

IF condition THEN statement

Example: IF A > B THEN PRINT "A is greater"

(ii) IF...THEN...ELSE Statement

➯ Used when there are two possible actions (True or False).

IF condition THEN
   statement1
ELSE
   statement2
END IF

➭ Example:-

IF A > B THEN PRINT "A is greater" ELSE PRINT "B is greater" END IF

(iii) IF...THEN...ELSEIF...ELSE Statement

➯Used when there are multiple conditions to be tested one after another.

IF condition1 THEN
   statement1
ELSEIF condition2 THEN
   statement2
ELSE
   statement3
END IF

➭ Example:-

IF marks >= 90 THEN PRINT "Grade A" ELSEIF marks >= 75 THEN PRINT "Grade B" ELSEIF marks >= 50 THEN PRINT "Grade C" ELSE PRINT "Fail" END IF

(iv) SELECT CASE Statement

➯ Used when we want to test one variable for many possible values.

SELECT CASE variable
   CASE value1
      statement1
   CASE value2
      statement2
   CASE ELSE
      statement3
END SELECT

➭ Example:-

SELECT CASE day$ CASE "Mon" PRINT "Monday" CASE "Tue" PRINT "Tuesday" CASE ELSE PRINT "Invalid day" END SELECT

🔁 B. Looping Statements in QBasic

➥ Looping means repeating a set of instructions until a condition is met.

(i) FOR...NEXT Loop

➯ Executes a block of statements a fixed number of times.

FOR counter = start TO end
   statement(s)
NEXT counter

➭ Example:-

FOR I = 1 TO 5 PRINT I NEXT I

(ii) FOR...NEXT with STEP

➯ Used to increase or decrease the counter by a specific value.

FOR counter = start TO end STEP value
   statement(s)
NEXT counter

➭ Example:-

FOR I = 2 TO 10 STEP 2 PRINT I NEXT I (Prints even numbers 2, 4, 6, 8, 10)

(iii) Nested FOR...NEXT

➯ When one loop is inside another loop.

FOR i = 1 TO n
   FOR j = 1 TO m
      statement(s)
   NEXT j
NEXT i

➭ Example:-

FOR i = 1 TO 3 FOR j = 1 TO 3 PRINT i; j NEXT j NEXT i

(iv) WHILE...WEND Loop

➯ Executes a block of statements as long as a condition is true.

WHILE condition
   statement(s)
WEND

➭ Example:-

N = 1 WHILE N <= 5 PRINT N N = N + 1 WEND

(v) DO...LOOP

➯ Repeats statements while or until a condition is true/false.

DO
   statement(s)
LOOP WHILE condition

➭ Example:-

DO WHILE condition statement(s) LOOP

🧠 Summary Table

Type	Statement	Purpose
Conditional	IF...THEN	Executes statement if true
Conditional	IF...THEN...ELSE	Chooses one of two actions
Conditional	SELECT CASE	Tests variable for many values
Looping	FOR...NEXT	Fixed repetitions
Looping	WHILE...WEND	Repeats while true
Looping	DO...LOOP	Repeats until/while true

😊

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