FreeBasic Do Until Keyword

last modified June 16, 2025

The FreeBasic Do Until keyword creates a loop that continues executing until a specified condition becomes true. It's useful when you need to repeat code while waiting for a particular state.

Basic Definition

In FreeBasic, Do Until is a loop control structure that checks the condition after each iteration. The loop runs at least once before evaluating the exit condition.

The syntax consists of Do Until condition to start the loop and Loop to mark its end. Code between these statements executes repeatedly until the condition evaluates to true.

Simple Do Until Loop

This basic example demonstrates the fundamental structure of a Do Until loop.

do_until_simple.bas

Dim counter As Integer = 1

Do Until counter > 5
    Print "Iteration: "; counter
    counter += 1
Loop

Print "Loop finished"

The loop continues until counter exceeds 5. Each iteration prints the current counter value and increments it. Notice the condition is checked after each complete iteration of the loop body.

User Input Validation

Do Until is excellent for validating user input until acceptable data is entered.

do_until_input.bas

Dim age As Integer

Do Until age >= 18 And age <= 120
    Input "Enter your age (18-120): ", age
    If age < 18 Then
        Print "You're too young!"
    ElseIf age > 120 Then
        Print "Please enter a realistic age!"
    End If
Loop

Print "Valid age entered: "; age

This loop repeatedly prompts for age until a value between 18 and 120 is provided. It demonstrates how Do Until can combine with conditional logic for robust input validation.

File Reading

Do Until loops work well for reading files until the end is reached.

do_until_file.bas

Dim fileNum As Integer = FreeFile()
Open "data.txt" For Input As #fileNum

Dim line As String
Dim lineCount As Integer = 0

Do Until EOF(fileNum)
    Line Input #fileNum, line
    lineCount += 1
    Print "Line "; lineCount; ": "; line
Loop

Close #fileNum
Print "Total lines read: "; lineCount

This example reads a file line by line until EOF (End Of File) is true. The Do Until structure ensures we process all file content while handling the termination condition automatically.

Random Number Generation

Do Until can generate values until a specific random number appears.

do_until_random.bas

Randomize Timer
Dim target As Integer = 7
Dim attempts As Integer = 0
Dim roll As Integer

Print "Rolling dice until we get "; target
Do Until roll = target
    roll = Int(Rnd() * 6) + 1
    attempts += 1
    Print "Attempt "; attempts; ": "; roll
Loop

Print "Got "; target; " after "; attempts; " attempts"

The loop simulates dice rolls until the target number appears. This shows how Do Until handles unpredictable termination conditions in simulations or games.

Password Verification

Do Until works well for implementing limited password attempts.

do_until_password.bas

Dim correctPassword As String = "secret123"
Dim userPassword As String
Dim attempts As Integer = 0
Dim maxAttempts As Integer = 3
Dim authenticated As Boolean = False

Do Until authenticated Or attempts >= maxAttempts
    Input "Enter password: ", userPassword
    attempts += 1
    
    If userPassword = correctPassword Then
        authenticated = True
        Print "Access granted!"
    ElseIf attempts < maxAttempts Then
        Print "Incorrect. Attempts left: "; maxAttempts - attempts
    End If
Loop

If Not authenticated Then
    Print "Maximum attempts reached. Access denied."
End If

This security example combines Do Until with multiple conditions. The loop continues until either authentication succeeds or attempts are exhausted. Complex conditions are common in real-world Do Until scenarios.

Game Loop

Do Until serves as an excellent game loop controller.

do_until_game.bas

Dim playerHealth As Integer = 100
Dim enemyHealth As Integer = 75
Dim gameOver As Boolean = False

Print "Combat begins!"
Do Until gameOver
    ' Player attack
    Dim playerDamage As Integer = Int(Rnd() * 20) + 5
    enemyHealth -= playerDamage
    Print "You hit for "; playerDamage; " damage"
    
    ' Enemy attack if still alive
    If enemyHealth > 0 Then
        Dim enemyDamage As Integer = Int(Rnd() * 15) + 3
        playerHealth -= enemyDamage
        Print "Enemy hits for "; enemyDamage; " damage"
    End If
    
    ' Check win/lose conditions
    If playerHealth <= 0 Then
        gameOver = True
        Print "You have been defeated!"
    ElseIf enemyHealth <= 0 Then
        gameOver = True
        Print "Enemy defeated! You win!"
    End If
    
    Print "Player: "; playerHealth; " HP | Enemy: "; _
        IIf(enemyHealth > 0, enemyHealth, 0); " HP"
    Print "--------------------------------"
    Sleep 1500
Loop

This combat simulation uses Do Until to control the game flow. The loop continues until either character's health drops to zero. The example shows how Do Until can manage complex game state transitions.

Nested Do Until Loops

Do Until loops can be nested to handle multi-dimensional problems.

do_until_nested.bas

Dim rows As Integer = 3
Dim cols As Integer = 4
Dim row As Integer = 1
Dim col As Integer

Do Until row > rows
    col = 1
    Print "Row "; row; ": ";
    
    Do Until col > cols
        Print row * col; " ";
        col += 1
    Loop
    
    Print
    row += 1
Loop

This nested loop example creates a multiplication table. The outer loop controls rows while the inner loop handles columns. Nested Do Until loops are powerful for grid-based operations and multi-pass algorithms.

Best Practices

Clear Conditions: Make exit conditions obvious and well-documented.
Avoid Infinite Loops: Ensure the condition can become true.
Initialization: Initialize all variables used in the condition.
Modification: Update condition variables within the loop body.
Readability: Use meaningful variable names in conditions.
Complex Conditions: Break complex conditions into variables.
Error Handling: Include safeguards against unexpected states.

This tutorial covered the FreeBasic Do Until keyword with practical examples showing its usage in different programming scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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