FreeBasic LBound and UBound Keywords

last modified June 16, 2025

The FreeBasic LBound and UBound keywords are used to determine the lower and upper bounds of an array. These functions are essential for working with arrays of unknown or variable sizes.

Basic Definition

LBound returns the smallest valid index (lower bound) of an array dimension. UBound returns the largest valid index (upper bound) of an array dimension.

Both functions take the array as their first parameter and an optional dimension number as the second parameter. For single-dimensional arrays, the dimension parameter can be omitted.

Basic Array Bounds

This example shows the simplest usage of LBound and UBound with a 1D array.

basic_bounds.bas

Dim numbers(1 To 5) As Integer

Print "Lower bound: "; LBound(numbers)
Print "Upper bound: "; UBound(numbers)

We declare an array with explicit bounds from 1 to 5. LBound returns 1 and UBound returns 5. These functions help make code more flexible when array bounds might change.

Default Array Bounds

FreeBasic arrays have default bounds when not explicitly specified.

default_bounds.bas

Dim values(5) As Double

Print "Default lower bound: "; LBound(values)
Print "Default upper bound: "; UBound(values)

When bounds aren't specified, FreeBasic uses 0 as the default lower bound. This array has indices from 0 to 5. Always check bounds to avoid off-by-one errors in your code.

Multi-dimensional Arrays

LBound and UBound work with multi-dimensional arrays by specifying the dimension.

multi_dimension.bas

Dim matrix(1 To 3, 1 To 4) As Integer

Print "First dimension bounds:"
Print "LBound: "; LBound(matrix, 1)
Print "UBound: "; UBound(matrix, 1)

Print "Second dimension bounds:"
Print "LBound: "; LBound(matrix, 2)
Print "UBound: "; UBound(matrix, 2)

This 2D array has different bounds for each dimension. The second parameter specifies which dimension to check. Dimension numbering starts at 1 for the first dimension.

Dynamic Arrays

LBound and UBound are particularly useful with dynamic arrays whose size changes.

dynamic_array.bas

Dim dynamicArray() As String
ReDim dynamicArray(5 To 10)

Print "Initial bounds:"
Print LBound(dynamicArray), UBound(dynamicArray)

ReDim Preserve dynamicArray(5 To 15)
Print "After resize:"
Print LBound(dynamicArray), UBound(dynamicArray)

The array is initially dimensioned from 5 to 10, then resized to 5 to 15. LBound and UBound automatically reflect these changes. This prevents hardcoding array sizes in your code.

Iterating Through Arrays

Using LBound and UBound makes array iteration more robust and maintainable.

array_iteration.bas

Dim fruits(3 To 7) As String
fruits(3) = "Apple"
fruits(4) = "Banana"
fruits(5) = "Cherry"
fruits(6) = "Date"
fruits(7) = "Elderberry"

For i As Integer = LBound(fruits) To UBound(fruits)
    Print fruits(i)
Next

This loop works correctly regardless of the array's lower and upper bounds. If the array bounds change later, the loop will still work without modification. This is more reliable than hardcoded loop limits.

Function Parameters

LBound and UBound work with arrays passed to functions and procedures.

function_parameters.bas

Sub PrintArrayBounds(arr() As Integer)
    Print "Array bounds in function:"
    Print LBound(arr), UBound(arr)
End Sub

Dim testArray(10 To 20) As Integer
PrintArrayBounds(testArray)

The function correctly reports the bounds of the passed array. This allows functions to work with arrays of any bounds. The array bounds information is preserved when passed to functions.

Checking Empty Arrays

LBound and UBound can help detect uninitialized or empty arrays.

empty_array.bas

Dim emptyArray() As Integer

On Error Goto ErrorHandler
Print "Lower bound: "; LBound(emptyArray)
Print "Upper bound: "; UBound(emptyArray)
Exit Sub

ErrorHandler:
Print "Array is not dimensioned"

Attempting to get bounds of an undimensioned array causes an error. This can be used to check if an array has been initialized. Always dimension arrays before using LBound/UBound on them.

Best Practices

Always use: Prefer LBound/UBound over hardcoded array bounds.
Dimension checking: Verify arrays are dimensioned before use.
Loop variables: Use the same type as array indices for loop counters.
Function parameters: Document expected array bounds for functions.
Error handling: Include checks for array bounds in critical code.

This tutorial covered the FreeBasic LBound and UBound keywords with practical examples showing their usage in different 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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