Arrays are extremely useful in Visual Basic, and are present in many other programming languages. Arrays are used to group similar data together, to make it easier to search and sort through this data. The best way to look at an array is to show you one. Say you want to make a phone book for your computer. Rather than make a new variable with a new name whenever you want to add a name, you can make an array. Arrays consist of the variable name and a subscript, which is put in parentheses for computer use. So, if you want to make a phone book of say, 100 people, rather than make a bunch of variables named 'person1, person2, person3....person100', you may use an array. All you have to do is dimension the array (see above). So now those 100 variables turn into person(1 to 100) in a dim statement. Now you may be asking, what do I do now? Each object stored in an array is called an element. What you have done is created an array which can hold 100 elements. If you want to print the 32nd person in your phone book, you simply would type:

 Debug.Print Person(32)

Arrays have far more uses other than just making phone books. But before we get into specific uses, let's look at some of the basic techniques used when working with arrays. The first thing you may want to do is learn how to easily fill an array. Filling an array with random numbers is a popular technique for starting off programs, and testing other techniques such as sorting. To fill an array, a simple For Loop may be used.

OptionExplicit
DimlngIndexasLong
DimstrArray(0to9)asString' Create an array with 10 values
DimintCounterasInteger
intCounter=1
ForlngIndex=LBound(strArray)toUBound(strArray)
intCounter=intCounter+1
strArray(lngIndex)=intCounter
NextlngIndex

The above shows an example of read-write iteration over an array. For Each loop can be used for read-only iteration over an array:

DimMyArray(9)
Fori=0To9
MyArray(i)=i
Next
ForEachItemInMyArray
Debug.PrintItem
Next

Per default, array indices start at 0, unless "Option Base 1" declaration is used. Without the declaration used, an array declared as "Dim MyArray(5)" has 6 elements: 0, 1, 2, 3, 4, 5.

The index range of an array has to be a continuous sequence of integers, including negative numbers. Thus, the following is possible:

DimMyArray1(-5to5)
DimMyArray2(-10to-5)
Fori=LBound(MyArray1)toUBound(MyArray1)
MyArray1(i)=i+5
Debug.Printi,MyArray1(i)
Next
Fori=LBound(MyArray2)toUBound(MyArray2)
MyArray2(i)=i+5
Debug.Printi,MyArray2(i)
Next

The size of an array can be obtained using LBound and UBound as follows:

DimMyArray1(-5to5)
DimMyArray2(10-1)
Size1=UBound(MyArray1)-LBound(MyArray1)+1
Size2=UBound(MyArray2)+1'For a same array with indexing starting at zero
Debug.PrintSize1,Size2

Keywords: length, item count, element count.

An array with the number of elements specified upon its declaration, as in Dim Names(0 to 9), is a static one: the number of its elements cannot be changed in runtime. By contrast, an array declared without the number of elements, as in Dim Names(), is a dynamic array, and its number of elements can be changed using ReDim. To preserve the element content of the array when using ReDim, Preserve keyword has to be used after ReDim.

Say you have a phone book program running and it has an array of your friends' names, like this:

OptionExplicit
DimStrNames(0to2)AsString
StrNames(0)="Alec"
StrNames(1)="Jack"
StrNames(2)="Pie"

But say you want to add more friends to your phone book, when a button is clicked, for example. What do you do? You can define a dynamic array instead:

DimNames()AsString

This array initially has no elements. You can add more elements, by using the ReDim keyword:

ReDimNames(0to1)AsString

This would create two new elements. You can assign names to these elements in the same way as before:

Names(0)="Alec"
Names(1)="Jack"

If you want to add more entries then use, for example:

ReDimNames(0to2)AsString

But you'll end up losing all the old records! If you want to keep them, you must also use the Preserve keyword:

ReDimPreserveNames(0To3)AsString
Names(1)="Eugene H / Orage"'Whoa this person's name is too long to remember

To iterate AKA loop over the elements of a dynamic array, you have to use LBound and UBound functions or For Each loop, as described at #Use of Arrays.

To add an element to an initialized dynamic array, you can proceed as follows, using UBound function:

ReDimPreserveNames(0ToUBound(Names)+1)AsString
Names(UBound(Names))="my new friend"

However, UBound does not work with an un-initialized dynamic array, so the following would cause a run-time error:

DimMyArray()AsString
Debug.PrintUBound(MyArray)

One way to avoid this run-time error is to keep a flag:

DimArrayIsEmptyAsBoolean
DimMyArray()AsString

ArrayIsEmpty=True

IfNotArrayIsEmptyThen
ReDimPreserveMyArray(0ToUBound(MyArray)+1)AsString
Else
ReDimPreserveNames(0To0)AsString
ArrayIsEmpty=False
EndIf
Names(UBound(Names))="my new friend"

Another way to avoid the run-time error is to catch the error created by the use of "UBound" on an uninitialized array:

DimMyArray()AsString

NumberOfElements=0
OnErrorResumeNext
NumberOfElements=UBound(MyArray)+1
OnErrorGoTo0
ReDimPreserveMyArray(0ToNumberOfElements)AsString
MyArray(UBound(MyArray))="my new friend"

Finally, you can use the following hack to check if a dynamic array has been initialized:

DimMyArray()AsString

If(NotMyArray)=-1Then
NumberOfElements=0'The array is uninitialised
Else
NumberOfElements=UBound(MyArray)+1
EndIf
ReDimPreserveMyArray(0ToNumberOfElements)AsString
MyArray(UBound(MyArray))="my new friend"

Variant arrays are dynamic arrays declared using the Variant type, and initialized using "= Array()". Their advantage is that, after they are initialized using "= Array()", LBound and UBound functions work with them even when they have no elements, as follows:

DimVariantArrayAsVariant

VariantArray=Array()

Debug.PrintLBound(VariantArray)'Prints 0
Debug.PrintUBound(VariantArray)'Prints -1

As a consequence, adding an element is straightforward, with no need to check for UBound failure:

ReDimPreserveVariantArray(0ToUBound(VariantArray)+1)AsVariant
VariantArray(UBound(VariantArray))="Jennifer"

Variant arrays can be initialized to a set of values:

DimVariantArrayAsVariant
VariantArray=Array(1,2.3,"Hey")

For very large arrays, the overhead of using variant arrays, which have the element type of Variant rather than String or other narrower type, may well be restrictive, in which case conventional dynamic arrays should be used. What seems also much faster than variant arrays for adding items one at a time are Collections.

Arrays can be defined to have any number of dimensions (or indices), by listing the sizes of each dimension:

DimFunkyArray(2,3,1,4)AsString

An element can be referenced like this:

FunkyArray(1,2,0,3)=24

Dynamic arrays can also be re-dimensioned to have any number of dimensions:

DimVeryFunkyArray()asString

ReDimVeryFunkyArray(2,2,2)AsString

The LBound and UBound functions can be used to find the bounds of a particular dimension:

Debug.PrintUBound(FunkyArray,4)

Would give 4.

Using UBound without the second parameter gives the first dimension

Debug.PrintUBound(FunkyArray)
' Displays 2
Debug.PrintUBound(VeryFunkyArray)
' Displays 2

Any type of array can be re-set to empty by using:

EraseSomeArray

The real power of arrays comes when defining arrays of arrays. What does this mean? Declare an array:

DimVariantArray()AsVariant

VariantArray=Array()

ReDimVariantArray(1)AsVariant

VariantArray(0)=Array(1,2,3)
VariantArray(1)=Array(4,5,6)

What do we have here? Essentially two arrays inside another. They can be referenced like this:

Debug.PrintVariantArray(0)(2)

Would show 3.

You can nest arrays like this to any depth and in any order and they can be of any size. A note of caution must be taken when using the ReDim statement, however. You cannot specifically re-dimension a particular dimension of an array; instead you need to temporarily copy to a variant.

DimvtempAsVariant

vtemp=VariantArray(0)

ReDimvtemp(1+UBound(vtemp))AsVariant
vtemp(UBound(vtemp))=7

VariantArray(0)=vtemp

See also the section #Multi-Dimensional Arrays.

Matrices are not as commonly used as arrays, but are an important element of programming. Rather than just one dimension, a matrix may have 2 or more. So, to make a matrix, the Dim statement would be:

DimMatrix(0To9,0To9)asInteger

This will create a matrix that is 10 by 10 and comprised of integers. In reality, a matrix is much like an array of arrays. The first thing you'll want to do is know how to create a loop for reading and filling a matrix. But before that, even, you should know how matrices are structured. An example matrix would be:

 1 2 3 4 5 6 7 8 9 
 
 1 1 1 1 1 1 1 1 1 1 
 2 1 1 1 1 1 1 1 1 1 
 3 1 1 1 1 1 1 1 1 1 
 4 1 1 1 1 1 1 1 1 1

This is a 4 by 9 matrix. When talking about matrices, rows are always stated before columns. You will also notice column numbers travel from left to right, while row numbers travel from top to bottom. This is very important. And of course, matrices need not consist of merely 1s. So, now you want to fill a matrix? Its very similar to an array.

ForRow=0To3
ForColumn=0To8
Matrix(Row,Column)=1
Next
Next

This will of course fill the matrix with nothing but ones, but random numbers may be used (see references). As you may notice, this involves nested loops. The loop fills left to right, top to bottom (the same directions we use when reading).

For sorting variant arrays, see http://stackoverflow.com/questions/152319/vba-array-sort-function.