For...Next Statement (Visual Basic)

Artykuł
11/16/2012

Repeats a group of statements a specified number of times.

For counter [ As datatype ] = start To end [ Step step ]
    [ statements ]
    [ Exit For ]
    [ statements ]
Next [ counter ]

Parts

counter
Required in the For statement. Numeric variable. The control variable for the loop.
datatype
Required if counter is not already declared. Data type of counter.
start
Required. Numeric expression. The initial value of counter.
end
Required. Numeric expression. The final value of counter.
step
Optional. Numeric expression. The amount by which counter is incremented each time through the loop.
statements
Optional. One or more statements between For and Next that run the specified number of times.
Exit For
Optional. Transfers control out of the For loop.
Next
Required. Terminates the definition of the For loop.

Remarks

Use a For...Next structure when you want to repeat a set of statements a set number of times.

A While...End While Statement (Visual Basic) or Do...Loop Statement (Visual Basic) works well when you do not know in advance how many times you need to run the statements in the loop. However, when you expect to run the loop a specific number of times, a For...Next loop is a better choice. You determine the number of iterations when you first enter the loop.

The value of step can be either positive or negative. It determines loop processing as follows:

Step value	Loop executes if
Positive or zero	counter <= end
Negative	counter >= end

If not specified, step defaults to 1.

Rules

Data Types. The data type of counter is usually Integer but can be any type that supports the greater than or equal to (>=), less than or equal to (<=), addition (), and subtraction (-) operators. It can even be a user-defined type provided it supports all these operators.

The start, end, and step expressions usually evaluate to type Integer but can evaluate to any data type that widens to the type of counter. If you use a user-defined type for counter, this means you might have to define the CType conversion operator to convert the types of start, end, or step to the type of counter.
Declaration. If counter has not been declared outside this loop, you must declare it within the For statement. In this case, the scope of counter is the body of the loop. However, you cannot declare counter both outside and inside the loop.
Number of Iterations. Visual Basic evaluates the iteration values start, end, and step only once, before the loop begins. If your statement block changes end or step, these changes do not affect the iteration of the loop.
Nesting Loops. You can nest For loops by placing one loop within another. However, each loop must have a unique counter variable. The following construction is valid.
```
For i As Integer = 1 To 10
    For j As Integer = 1 To 10
        For k As Integer = 1 To 10
            ' Insert statements to operate with current values of i, j, and k.
        Next k
    Next j
Next i
```
You can also nest different kinds control structures within one another. For more information, see Nested Control Structures.

Note

If a Next statement of an outer nesting level is encountered before the Next of an inner level, the compiler signals an error. However, the compiler can detect this overlapping error only if you specify counter in every Next statement.
Identifying the Control Variable. You can optionally specify counter in the Next statement. This improves the readability of your program, especially if you have nested For loops. You must specify the same variable as the one that appears in the corresponding For statement.
Transferring Out of the Loop. The Exit Statement (Visual Basic) transfers control immediately to the statement following the Next statement. You might want to exit a loop if you detect a condition that makes it unnecessary or impossible to continue iterating, such as an erroneous value or a termination request. Also, if you catch an exception in a Try...Catch...Finally, you can use Exit For at the end of the Finally block.

You can place any number of Exit For statements anywhere in the For loop. Exit For is often used after evaluating some condition, for example in an If...Then...Else structure.
Endless Loops. One use of Exit For is to test for a condition that could cause an endless loop, which is a loop that could run an extremely large or even infinite number of times. If you detect such a condition, you can use Exit For to escape the loop. For more information, see Do...Loop Statement (Visual Basic).

Behavior

Entry into the Loop. When execution of the For...Next loop begins, Visual Basic evaluates start, end, and step for the only time. It then assigns start to counter. Before it runs the statement block, it compares counter to end. If counter is already past the end value, the For loop terminates and control passes to the statement following the Next statement. Otherwise the statement block runs.
Iterations of the Loop. Each time Visual Basic encounters the Next statement, it increments counter by step and returns to the For statement. Again it compares counter to end, and again it either runs the block or terminates the loop depending on the result. This process continues until counter passes end or an Exit For statement is encountered.
Termination of the Loop. The loop does not terminate until counter has passed end. If counter is equal to end, the loop continues. The comparison that determines whether to run the block is counter <= end if step is positive and counter >= end if step is negative.
Changing Iteration Values. Changing the value of counter while inside a loop can make it more difficult to read and debug your code. Changing the value of start, end, or step does not affect the iteration values determined when the loop was first entered.

Example

The following example demonstrates nested For...Next structures with different step values.

Dim words, digit As Integer 
Dim thisString As String = "" 
For words = 10 To 1 Step -1
    For digit = 0 To 9
        thisString &= CStr(digit)
    Next digit
    thisString &= " " 
Next words

The preceding example creates a string that contains 10 instances of the numbers 0 through 9, each string separated from the other by a single space. The outer loop decrements a loop counter variable each time through the loop.

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For...Next Statement (Visual Basic)

Parts

Remarks

Rules

Behavior

Example

See Also

Tasks

Concepts

Reference

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