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myInt=myArray(5)

This will set the value of myInt to the current value of column 5 in myArray.

Two-Dimensional Arrays

A two-dimensional array is an array that has more than one row, along with the

defined columns. A two-dimensional array is like a table, with a defined number of rows,

where each row has a defined number of columns. The following code snippet defined an

array using the default method.

DimmyArray(2,10)asInteger

The first dimension defines the number of rows in the array, while the second

dimension defines the number of columns in each row. In this example, the array has 3

rows, numbered 0 to 2, and each row has 11 columns, numbered 0 to 10, if Option Base

has not been defined. If Option Base 1 had been used, then the row indexes would range

from 1 to 2, and the column indexes would range from 1 to 10.

You can also define the lower and upper bounds of the array.

DimmyArray(1to2,1to10)asInteger

This definition would set the number of rows to 2, numbered 1 to 2 and the

number of columns to 10, numbered 1 to 10.

Two-Dimensional Array Indexes

To access the array elements you would use two indexes. The first index selects

the row, and the second index selects a column within that row.

myArray(1,5)=7

This code would set column 5 in row 1 to 7.

myInt=myArray(1,5)

This code would set myInt to the current value contained within column 5 of row 1

of myArray.

Multi-Dimensional Arrays

For arrays of three or more dimensions, you would use the same format as listed

above, taking into account the progression of the array dimensions. For a three-

dimensional array, the first dimension would be the row, the second the column, the third

would be the z-order, or depth, of each column. For example, to define a cube in space,

you would use the y,x,z format, where y defines the vertical axis, x defines the horizontal

axis and z defines the depth axis. To create an array in this format you could define the

array as DimmyCube(y,x,z)asInteger. MyCube(10,10,10) would create a cube

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with 11 vertical units, 0 to 10, 11 horizontal units, 0 to 10 and 10 depth units, 0 to 10. To

access the center of the cube, you would use iCenter=myCube(5,5,5).

You will probably never need to use arrays of more than three dimensions, unless

you are doing some advanced mathematical calculations. However, if you need to use

higher-dimensional arrays, the same principles apply.

The following program illustrates creating and accessing a two dimensional array.

OptionExplicit

'Createatwodimensionalarray

DimAsIntegermyArray(1To2,1To10),i,j

'Loadsomedataintothearray

Fori=1To2

Forj=1To9

myArray(i,j)=Rnd*10

Next

'Printdatainarray

Fori=1To2

Forj=1To9

Print"row:";i;"col:";j;"value:";myArray(i,j)

Next

Sleep

End

Listing 12.1: arrayindex.bas

Analysis:

Line 4 creates a two-dimensional array with two rows, with each row

having ten columns. The working variables i and j are also declared on the same line.

Lines 7 through 11 load some random data into the array. A nested For-Next loop is the

common way to access multi-dimensional arrays. The variable i will select the row

indexes while the j variable will access the column indexes.

Lines 14 through 18 will print the values stored in the array. The format is identical

to the load code. The program is closed in the usual way.

When you run the program you should see the following output.

row:1col:1value:0

row:1col:2value:6

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row:1col:3value:2

row:1col:4value:8

row:1col:5value:6

row:1col:6value:5

row:1col:7value:4

row:1col:8value:9

row:1col:9value:8

row:2col:1value:7

row:2col:2value:2

row:2col:3value:9

row:2col:4value:7

row:2col:5value:5

row:2col:6value:3

row:2col:7value:0

row:2col:8value:1

row:2col:9value:4

Output 12.1: Output of arrayindex.bas

Dynamic Arrays

The arrays described above are static arrays; the array size cannot change during

program execution. You can also create dynamic arrays that can change size during

execution. Dynamic arrays are useful for creating data structures such as stacks or

queues.

In order to use dynamic arrays in your program you need to include the Option

Dynamic directive in your program. Static arrays, the arrays described above, are kept on

the heap, but dynamic arrays are allocated from the computer's pool of memory so

Option Dynamic is needed to tell the compiler to dynamically allocate the array. You can

dimension a dynamic array in two ways. The first is to declare the array size in the Dim

statement and then resize it using Redim or RedimPreserve. The second is to not specify

the array size, use empty parenthesis, and then use Redim or Redim Preserve to size the

array. Redim will size the array and clear the array contents. Redim Preserve will size the

array and keep any existing data in th array.

There are a couple of exceptions to this. When declaring an array using variables for the

index values, the array is implicitly dynamic. You can also declare an array without

specifying index values, that is using an empty parentheses in the array declaration,

and then use Redim to resize the array without specifying Option Dynamic.

The following program creates a simple integer stack and then manipulates

the stack.

OptionExplicit

OptionDynamic

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'Createanintegerstack.The0indexwillbeouremptymarker.

DimAsIntegerstack(0),top=0,i,ivalue

'Thiswillpushavalueonthestack,updatethetopofstack

SubPushValue(astack()AsInteger,stacktopAsInteger,valueAsInteger)

'Incrementthetopofthestack

stacktop+=1

'Resizethestack

RedimPreserveastack(stacktop)

astack(stacktop)=value

EndSub

'Thiswillpopavalueoffthestack,updatetopofstack

FunctionPopValue(astack()AsInteger,stacktopAsInteger)AsInteger

DimAsIntegerret



Ifstacktop=0Then

ret=0

Else

ret=astack(stacktop)

stacktop=1

RedimPreserveastack(stacktop)

EndIf

Returnret

EndFunction

'Pushfivevaluesontostack

Print"Pushingvaluesonstack..."

Fori=1To5

ivalue=i*2

PushValuestack(),top,ivalue

Print"Stacktop:";Ubound(stack),"Value:";ivalue

'Popthevaluesoffthestack

Print"Poppingvaluesfromstack..."

ivalue=PopValue(stack(),top)

DoWhileivalue>0

Print"StackValue:";ivalue

ivalue=PopValue(stack(),top)

Loop

'Checkstacksize

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Print"Stacksizeafterpops:";Ubound(stack)

Sleep

End

Listing 12.2: stack.bas

Analysis:

In line the Option Dynamic directive is used to create the stack array

using dynamic memory. Line 5 dimensions the working variables. The 0 index in the stack

array will be used to indicate that the stack is empty. Even though this wastes one

integer, it makes the code easier to implement. Line 8 through 14 defines the code to

push a value on to the the stack. The parameters of the subroutine are the stack array,

the top-of-stack variable, and the value to push on to the stack. You could determine the

top of the stack using the Ubound function, however in a real stack implementation, you

will probably need to pass the top-of-stack to other functions, and keeping the value in a

variable will reduce the number of calculations the program must do.

Line 10 increments the stack top variable, and line 12 uses Redim Preserve to

resize the array, while keeping the existing data in the array. Line 13 set the new array

location to the value passed to the subroutine.

Line 16 through 27 pops the top value of the stack, if the stack index is grater than

zero, updates the top-of-stack variable and then returns the popped value. Line 19 and

20 checks to make sure that the stack has some data. If the index is 0, the stack is

empty. Lines 22 through 24 get the current value of the top stack item, updates the stack

pointer variable and then resizes the stack using Redim Preserve. Lines 31 through 35

use a For-Next loop to push 5 values on to the stack. Lines 40 through 44 pop the values

off the stack. A Do-Loop is used to pop the values since when the stack is empty, the

return value will be 0. Line 48 then cheks to see if all the values have been popped from

the stack.

The program is closed in the usual way.

When you run the program you will see the following output.

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Pushingvaluesonstack...

Stacktop:1Value:2

Stacktop:2Value:4

Stacktop:3Value:6

Stacktop:4Value:8

Stacktop:5Value:10

Poppingvaluesfromstack...

StackValue:10

StackValue:8

StackValue:6

StackValue:4

StackValue:2

Stacksizeafterpops:0

Output 12.2: Output of stack.bas

Stacks have a wide range of uses, however most implementations use pointers

since it is much faster to manipulate a pointer than it is an array. The advantage of an

array is the simplicity of use, and the readability of the code, but if you need speed, you

should use a pointer memory array for stacks.

You can only Redim or Redim Preserve the first index, that is the row, in a

multidimensional array.

Array Functions

There are a number of functions that you can use to manage arrays. The following

table lists the array functions available in FreeBasic.

Function

Syntax

Comment

Clear

Clear array, value,

num_bytes

Sets num_bytes in array to

byte value.

Erase

Erase array

Erases dynamic arrays from

memory, or clears static

arrays.

Lbound

B = Lbound(array)

B = Lbound(array,

dimension)

Returns the lowest index of

a single or multidimensional

array. The dimension

parameter is the dimension

to check in a

multidimensional array

where the first dimension is

156

Function

Syntax

Comment

1, the second dimension is

2, and so on.

Option Dynamic

Allocates arrays into

dynamic memory by

default.

Option Static

Clears a previous Option

Dynamic. All subsequent

arrays will be allocated on

the heap.

Preserve

Redim Preserve

Preserves data in an array

when resized using the

Redim statement.

Redim

Redim array(dimensions) as

DataType

Resizes an array to size

dimensions.

Ubound

B = Lbound(array)

B = Lbound(array,

dimension)

Returns the highest index of

a single or multidimensional

array. The dimension

parameter is the dimension

to check in a

multidimensional array

where the first dimension is

1, the second dimension is

2, and so on.

Table 12.1: Array Functions

Out of all the functions, you will probably use the Lbound and Ubound functions

the most, especially when working with dynamic arrays. The following program uses both

functions to get the size of a two dimensional array.

OptionExplicit

OptionDynamic

'Createadynamicarray

DimAsIntegermyArray(),i,nb

'Resizearraywithtwodimensions

RedimmyArray(1To1,1To5)

'Printupperandlowerboundsforeachrowandcolumn

Print"FirstRedim"

Print""

Print"MinRowIndex:";Lbound(myArray,1);

Print"MaxRowIndex:";Ubound(myArray,1)

Print"MinColumnindex:";Lbound(myArray,2);

Print"MaxColumnindex:";Ubound(myArray,2)

157

Print"AdditionalRedims"

Print""

'Redimarrayfivetimes

Fori=1To5

nb=Ubound(myArray,1)+1

RedimmyArray(1Tonb,1To5)

'Printnewarraysize

Print"MinRowIndex:";Lbound(myArray,1);

Print"MaxRowIndex:";Ubound(myArray,1)

Print"MinColumnindex:";Lbound(myArray,2);

Print"MaxColumnindex:";Ubound(myArray,2)

Print

Sleep

End

Listing 12.3: ulbound.bas

Analysis: Line 2 set the compiler directive Option Dynamic so that the array is

allocated in dynamic memory. Line 5 creates a dynamic array with no dimensions

specified. Line 7 uses Redim to create a two-dimensional array. Lines 10 through 15 print

the upper and lower bounds using Lbound and Ubound respectively. Notice that the

dimension parameter is used to specify which dimension to get the lower and upper

bounds. 1 is used to get the bounds for the first dimension, and 2 is used to get the

second dimension.

Lines 20 through 29 resize the first dimension of the array and print the new lower

and upper indexes. Line 21 gets the current upper bound of the first dimension, 1 is

added to that value and then Redim is used in line 22 to resize the first dimension of the

array. Lines 24 through 27 get the new bounds and print the values to the screen.

The program is closed in the usual way.

When you run the program you should see the following output.

FirstRedim

MinRowIndex:1MaxRowIndex:1