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• Every array has a data type, which differs from the data type of its elements. There is no single data type for all arrays. Instead, the data type of an array is determined by the number of dimensions, or rank, of the array, and the data type of the elements in the array.Two array variables are of the same data type only when they have the same rank and their elements have the.
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An array is a set of values, which are termed elements, that are logically related to each other. For example, an array may consist of the number of students in each grade in a grammar school; each element of the array is the number of students in a single grade. Similarly, an array may consist of a student's grades for a class; each element of the array is a single grade.

It is possible individual variables to store each of our data items. For example, if our application analyzes student grades, we can use a separate variable for each student's grade, such as englishGrade1, englishGrade2, etc. This approach has three major limitations:

• We have to know at design time exactly how many grades we have to handle.
• Handling large numbers of grades quickly becomes unwieldy. This in turn makes an application much more likely to have serious bugs.
• It is difficult to maintain. Each new grade that we add requires that the application be modified, recompiled, and redeployed.

By using an array, you can refer to these related values by the same name, and use a number that’s called an index or subscript to identify an individual element based on its position in the array. The indexes of an array range from 0 to one less than the total number of elements in the array. When you use Visual Basic syntax to define the size of an array, you specify its highest index, not the total number of elements in the array. You can work with the array as a unit, and the ability to iterate its elements frees you from needing to know exactly how many elements it contains at design time.

Some quick examples before explanation:

Array elements in a simple array

Let's create an array named students to store the number of students in each grade in a grammar school. The indexes of the elements range from 0 through 6. Using this array is simpler than declaring seven variables.

The following illustration shows the students array. For each element of the array:

• The index of the element represents the grade (index 0 represents kindergarten).

• The value that’s contained in the element represents the number of students in that grade.

The following example contains the Visual Basic code that creates and uses the array:

The example does three things:

• It declares a students array with seven elements. The number 6 in the array declaration indicates the last index in the array; it is one less than the number of elements in the array.
• It assigns values to each element in the array. Array elements are accessed by using the array name and including the index of the individual element in parentheses.
• It lists each value of the array. The example uses a For statement to access each element of the array by its index number.

The students array in the preceding example is a one-dimensional array because it uses one index. An array that uses more than one index or subscript is called multidimensional. For more information, see the rest of this article and Array Dimensions in Visual Basic.

Creating an array

You can define the size of an array in several ways:

• You can specify the size when the array is declared:

• You can use a New clause to supply the size of an array when it’s created:

If you have an existing array, you can redefine its size by using the ReDim statement. You can specify that the ReDim statement keep the values that are in the array, or you can specify that it create an empty array. The following example shows different uses of the ReDim statement to modify the size of an existing array.

For more information, see the ReDim Statement.

Storing values in an array

You can access each location in an array by using an index of type Integer. You can store and retrieve values in an array by referencing each array location by using its index enclosed in parentheses. Indexes for multidimensional arrays are separated by commas (,). You need one index for each array dimension.

The following example shows some statements that store and retrieve values in arrays.

Populating an array with array literals

By using an array literal, you can populate an array with an initial set of values at the same time that you create it. An array literal consists of a list of comma-separated values that are enclosed in braces ({}).

When you create an array by using an array literal, you can either supply the array type or use type inference to determine the array type. The following example shows both options.

When you use type inference, the type of the array is determined by the dominant type in the list of literal values. The dominant type is the type to which all other types in the array can widen. If this unique type can’t be determined, the dominant type is the unique type to which all other types in the array can narrow. If neither of these unique types can be determined, the dominant type is Object. For example, if the list of values that’s supplied to the array literal contains values of type Integer, Long, and Double, the resulting array is of type Double. Because Integer and Long widen only to Double, Double is the dominant type. For more information, see Widening and Narrowing Conversions.

Note

You can use type inference only for arrays that are defined as local variables in a type member. If an explicit type definition is absent, arrays defined with array literals at the class level are of type Object[]. For more information, see Local type inference.

Note that the previous example defines values as an array of type Double even though all the array literals are of type Integer. You can create this array because the values in the array literal can widen to Double values.

You can also create and populate a multidimensional array by using nested array literals. Nested array literals must have a number of dimensions that’s consistent with the resulting array. The following example creates a two-dimensional array of integers by using nested array literals.

When using nested array literals to create and populate an array, an error occurs if the number of elements in the nested array literals don't match. An error also occurs if you explicitly declare the array variable to have a different number of dimensions than the array literals.

Just as you can for one-dimensional arrays, you can rely on type inference when creating a multidimensional array with nested array literals. The inferred type is the dominant type for all the values in all the array literals for all nesting level. The following example creates a two-dimensional array of type Double[,] from values that are of type Integer and Double.

For additional examples, see How to: Initialize an Array Variable in Visual Basic.

Iterating through an array

When you iterate through an array, you access each element in the array from the lowest index to the highest or from the highest to the lowest. Typically, use either the For...Next Statement or the For Each...Next Statement to iterate through the elements of an array. When you don't know the upper bounds of the array, you can call the Array.GetUpperBound method to get the highest value of the index. Although lowest index value is almost always 0, you can call the Array.GetLowerBound method to get the lowest value of the index.

The following example iterates through a one-dimensional array by using the For...Next statement.

The following example iterates through a multidimensional array by using a For...Next statement. The GetUpperBound method has a parameter that specifies the dimension. GetUpperBound(0) returns the highest index of the first dimension, and GetUpperBound(1) returns the highest index of the second dimension.

The following example uses a For Each...Next Statementto iterate through a one-dimensional array and a two-dimensional array.

Array size

The size of an array is the product of the lengths of all its dimensions. It represents the total number of elements currently contained in the array. For example, the following example declares a 2-dimensional array with four elements in each dimension. As the output from the example shows, the array's size is 16 (or (3 + 1) * (3 + 1).

Note

This discussion of array size does not apply to jagged arrays. For information on jagged arrays and determining the size of a jagged array, see the Jagged arrays section.

You can find the size of an array by using the Array.Length property. You can find the length of each dimension of a multidimensional array by using the Array.GetLength method.

You can resize an array variable by assigning a new array object to it or by using the ReDim Statement statement. The following example uses the ReDim statement to change a 100-element array to a 51-element array.

There are several things to keep in mind when dealing with the size of an array.

The array type

Every array has a data type, which differs from the data type of its elements. There is no single data type for all arrays. Instead, the data type of an array is determined by the number of dimensions, or rank, of the array, and the data type of the elements in the array. Two array variables are of the same data type only when they have the same rank and their elements have the same data type. The lengths of the dimensions of an array do not influence the array data type.

Every array inherits from the System.Array class, and you can declare a variable to be of type Array, but you cannot create an array of type Array. For example, although the following code declares the arr variable to be of type Array and calls the Array.CreateInstance method to instantiate the array, the array's type proves to be Object[].

Also, the ReDim Statement cannot operate on a variable declared as type Array. For these reasons, and for type safety, it is advisable to declare every array as a specific type.

You can find out the data type of either an array or its elements in several ways.

• You can call the GetType method on the variable to get a Type object that represents the run-time type of the variable. The Type object holds extensive information in its properties and methods.
• You can pass the variable to the TypeName function to get a String with the name of run-time type.

The following example calls the both the GetType method and the TypeName function to determine the type of an array. The array type is Byte(,). Note that the Type.BaseType property also indicates that the base type of the byte array is the Array class.

Arrays as return values and parameters

To return an array from a Function procedure, specify the array data type and the number of dimensions as the return type of the Function Statement. Within the function, declare a local array variable with same data type and number of dimensions. In the Return Statement, include the local array variable without parentheses.

To specify an array as a parameter to a Sub or Function procedure, define the parameter as an array with a specified data type and number of dimensions. In the call to the procedure, pass an array variable with the same data type and number of dimensions.

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In the following example, the GetNumbers function returns an Integer(), a one-dimensional array of type Integer. The ShowNumbers procedure accepts an Integer() argument.

In the following example, the GetNumbersMultiDim function returns an Integer(,), a two-dimensional array of type Integer. The ShowNumbersMultiDim procedure accepts an Integer(,) argument.

Jagged arrays

Sometimes the data structure in your application is two-dimensional but not rectangular. For example, you might use an array to store data about the high temperature of each day of the month. The first dimension of the array represents the month, but the second dimension represents the number of days, and the number of days in a month is not uniform. A jagged array, which is also called an array of arrays, is designed for such scenarios. A jagged array is an array whose elements are also arrays. A jagged array and each element in a jagged array can have one or more dimensions.

The following example uses an array of months, each element of which is an array of days. The example uses a jagged array because different months have different numbers of days. The example shows how to create a jagged array, assign values to it, and retrieve and display its values.

The previous example assigns values to the jagged array on an element-by-element basis by using a For...Next loop. You can also assign values to the elements of a jagged array by using nested array literals. However, the attempt to use nested array literals (for example, Dim valuesjagged = {{1, 2}, {2, 3, 4}}) generates compiler error BC30568. To correct the error, enclose the inner array literals in parentheses. The parentheses force the array literal expression to be evaluated, and the resulting values are used with the outer array literal, as the following example shows.

A jagged array is a one-dimensional array whose elements contain arrays. Therefore, the Array.Length property and the Array.GetLength(0) method return the number of elements in the one-dimensional array, and Array.GetLength(1) throws an IndexOutOfRangeException because a jagged array is not multidimensional. You determine the number of elements in each subarray by retrieving the value of each subarray's Array.Length property. The following example illustrates how to determine the number of elements in a jagged array.

Zero-length arrays

Visual Basic differentiates between a uninitialized array (an array whose value is Nothing) and a zero-length array or empty array (an array that has no elements.) An uninitialized array is one that has not been dimensioned or had any values assigned to it. For example:

A zero-length array is declared with a dimension of -1. For example:

You might need to create a zero-length array under the following circumstances:

• Without risking a NullReferenceException exception, your code must access members of the Array class, such as Length or Rank, or call a Visual Basic function such as UBound.

• You want to keep your code simple by not having to check for Nothing as a special case.

• Your code interacts with an application programming interface (API) that either requires you to pass a zero-length array to one or more procedures or returns a zero-length array from one or more procedures.

Splitting an array

In some cases, you may need to split a single array into multiple arrays. This involves identifying the point or points at which the array is to be split, and then spitting the array into two or more separate arrays.

Note

This section does not discuss splitting a single string into a string array based on some delimiter. For information on splitting a string, see the String.Split method.

The most common criteria for splitting an array are:

• The number of elements in the array. For example, you might want to split an array of more than a specified number of elements into a number of approximately equal parts. For this purpose, you can use the value returned by either the Array.Length or Array.GetLength method.

• The value of an element, which serves as a delimiter that indicates where the array should be split. You can search for a specific value by calling the Array.FindIndex and Array.FindLastIndex methods.

Once you've determined the index or indexes at which the array should be split, you can then create the individual arrays by calling the Array.Copy method.

The following example splits an array into two arrays of approximately equal size. (If the total number of array elements is odd, the first array has one more element than the second.)

The following example splits a string array into two arrays based on the presence of an element whose value is 'zzz', which serves as the array delimiter. The new arrays do not include the element that contains the delimiter.

Joining arrays

You can also combine a number of arrays into a single larger array. To do this, you also use the Array.Copy method.

Note

This section does not discuss joining a string array into a single string. For information on joining a string array, see the String.Join method.

Before copying the elements of each array into the new array, you must first ensure that you have initialized the array so that it is large enough to accommodate the new array. You can do this in one of two ways:

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• Use the ReDim Preserve statement to dynamically expand the array before adding new elements to it. This is the easiest technique, but it can result in performance degradation and excessive memory consumption when you are copying large arrays.
• Calculate the total number of elements needed for the new large array, then add the elements of each source array to it.

The following example uses the second approach to add four arrays with ten elements each to a single array.

Since in this case the source arrays are all small, we can also dynamically expand the array as we add the elements of each new array to it. The following example does that.

Collections as an alternative to arrays

Arrays are most useful for creating and working with a fixed number of strongly typed objects. Collections provide a more flexible way to work with groups of objects. Unlike arrays, which require that you explicitly change the size of an array with the ReDim Statement, collections grow and shrink dynamically as the needs of an application change.

When you use ReDim to redimension an array, Visual Basic creates a new array and releases the previous one. This takes execution time. Therefore, if the number of items you are working with changes frequently, or you cannot predict the maximum number of items you need, you'll usually obtain better performance by using a collection.

For some collections, you can assign a key to any object that you put into the collection so that you can quickly retrieve the object by using the key.

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If your collection contains elements of only one data type, you can use one of the classes in the System.Collections.Generic namespace. A generic collection enforces type safety so that no other data type can be added to it.

For more information about collections, see Collections.

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