Understanding Basic Program Output
This guide explains how a simple QBASIC program works step by step. It is designed for beginners who are just starting to learn programming.
Program Code
Below is the QBASIC program we are analyzing:
CLS
FOR a = 1 TO 5
FOR b = 1 TO a
PRINT b;
NEXT b
PRINT
NEXT a
ENDWhat Does This Program Do?
This program prints numbers in a specific pattern. It uses loops to repeat actions multiple times. Let’s break it down step by step:
-
CLS: This command clears the screen. It removes anything that was displayed before, so you start fresh. -
FOR a = 1 TO 5: This is the outer loop. It controls how many rows of numbers will be printed. The variableastarts at 1 and increases by 1 each time until it reaches 5. -
FOR b = 1 TO a: This is the inner loop. It controls how many numbers will be printed in each row. The variablebstarts at 1 and increases by 1 until it reaches the value ofa. -
PRINT b;: This command prints the value ofb. The semicolon (;) keeps the cursor on the same line, so the numbers are printed next to each other. -
NEXT b: This tells the program to go back to the start of the inner loop and increase the value ofb. -
PRINT: After the inner loop finishes, this command moves the cursor to the next line. This is why each row of numbers appears on a new line. -
NEXT a: This tells the program to go back to the start of the outer loop and increase the value ofa. -
END: This command stops the program.
How Does the Program Work?
The program uses two loops:
- The outer loop controls the rows.
- The inner loop controls the numbers printed in each row.
For example:
- When
a = 1, the inner loop runs once and prints1. - When
a = 2, the inner loop runs twice and prints1 2. - When
a = 3, the inner loop runs three times and prints1 2 3. - This continues until
a = 5.
Dry Run Table
A dry run is a step-by-step simulation of how the program works. It helps us understand what happens at each step.
Outer Loop (a) | Inner Loop (b) | Action | Screen Output |
|---|---|---|---|
| 1 | |||
| 1 | PRINT 1; | 1 | |
PRINT (newline) | 1 | ||
| 2 | 1 | ||
| 1 | PRINT 1; | 1 1 | |
| 2 | PRINT 2; | 1 1 2 | |
PRINT (newline) | 1 1 2 | ||
| 3 | 1 1 2 | ||
| 1 | PRINT 1; | 1 1 2 1 | |
| 2 | PRINT 2; | 1 1 2 1 2 | |
| 3 | PRINT 3; | 1 1 2 1 2 3 | |
PRINT (newline) | 1 1 2 1 2 3 | ||
| 4 | 1 1 2 1 2 3 | ||
| 1 | PRINT 1; | 1 1 2 1 2 3 1 | |
| 2 | PRINT 2; | 1 1 2 1 2 3 1 2 | |
| 3 | PRINT 3; | 1 1 2 1 2 3 1 2 3 | |
| 4 | PRINT 4; | 1 1 2 1 2 3 1 2 3 4 | |
PRINT (newline) | 1 1 2 1 2 3 1 2 3 4 | ||
| 5 | 1 1 2 1 2 3 1 2 3 4 | ||
| 1 | PRINT 1; | 1 1 2 1 2 3 1 2 3 4 1 | |
| 2 | PRINT 2; | 1 1 2 1 2 3 1 2 3 4 1 2 | |
| 3 | PRINT 3; | 1 1 2 1 2 3 1 2 3 4 1 2 3 | |
| 4 | PRINT 4; | 1 1 2 1 2 3 1 2 3 4 1 2 3 4 | |
| 5 | PRINT 5; | 1 1 2 1 2 3 1 2 3 4 1 2 3 4 5 | |
PRINT (newline) | 1 1 2 1 2 3 1 2 3 4 1 2 3 4 5 | ||
END | Program stops. |
Final Output
The final output displayed on the screen will be:
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5Key Takeaways
- Loops are used to repeat actions multiple times.
- The outer loop controls the rows, while the inner loop controls the numbers in each row.
- A dry run helps you understand how a program works step by step.
This program is a great example of how loops can be used to create patterns in programming!