---
id: program_flow
title: Program Flow
sidebar_label: Program Flow
---

This section covers command that affect the flow of program execution.

## While/Wend loops
A [While]/[Wend] loop continues executing while an expression evaluates to true:
```
While Expression
   Statements
Wend
```

## Repeat/Until loops
A [Repeat]/[Until] loop continues executing until an expression evaluates to true:
```
Repeat
   Statements
Until Expression
```
The statements within a [Repeat]/[Until] loop will always execute at least once, since the
terminating expression is only evaluated at the bottom of the loop.

## Repeat/Forever loops
A [Repeat]/[Forever] loop will simply continue executing forever:
```
Repeat
   Statements
Forever
```

## For/To/Next loops
A [For]/[To]/[Next] loop will continue executing until the value of a numeric index variable goes
beyond an exit value. The index variable is automatically updated every loop iteration by adding a step
value. Note that the exit value is inclusive - the loop will typically execute once with the index variable
equal to the exit value before terminating:
```
For IndexVariable = FirstValue To LastValue Step StepValue
   Statements
Next
```
The [Step] part is optional, in which case the index variable will step in increments of 1. The step
value must also be constant.

The exit value is only evaluated once at the start of the loop. This means it is safe to use complex
or 'slow' code as an exit value.

You may also declare a new local variable to be used as the index variable, by preceding the index variable
with the Local keyword. Such local variables will not be visible outside of the loop when it terminates.


## For/Until/Next loops
A [For]/[Until]/[Next] loop operates in a similar manner to a [For]/[To]/[Next] loop, except that the loop will
terminate when the index variables becomes equal to the last value - in other words, the exit value is exclusive.
This can be useful when you need to count 'up to' a value, without actually including that value:
```
For IndexVariable = FirstValue Until LastValue Step StepValue
   statements
Next
```
The [Step] part is optional, in which case the index variable will step in increments of 1. The step
value must also be constant.

The exit value is only evaluated once at the start of the loop. This means it is safe to use complex or
'slow' code as an exit value.

You may also declare a new local variable to be used as the index variable, by preceding the index variable
with the Local keyword. Such local variables will not be visible outside of the loop when it terminates.

## Exit and Continue
The [Exit] command can be used to exit from a [While], [Repeat] or [For] loop. The loop will be terminated, and program
flow will be transferred to the first command after the loop.

The [Continue] command can be used to force a [While], [Repeat] or [For] loop to resume execution from the 'top'
of the loop, skipping any statements remaining in the loop.

Both [Exit] and [Continue] may be followed by an optional identifier. This identifer must match a previously
declared loop label, which allows you to exit or continue a specific loop, not necessarily the 'nearest' loop.

To declare a loop label, use the syntax **Identifier** immediately preceding a While, Repeat or For loop.
For example, this program will exit both loops when k and j both equal 5:
```blitzmax
Strict
Local k,j
#Label1   'loop label
For k=1 To 4
  #Label2    'another loop label (unused in this example)
  For j=1 To 4
    Print k+","+j
    If k=3 And j=3 Exit Label1
  Next
Next
```

## If/Then blocks
BlitzMax provides both the classic BASIC 'one liner' style of [If]/[Then] statement...
```
If Expression Then Statements Else Statements
...and the more modern block equivalent...
If Expression
   Statements
Else If Expression
   Statements
Else
   Statements
EndIf
```
In both forms, the [Then] keyword is optional.

## Select/Case blocks
A [Select]/[Case] block allows you to simplify complex conditional tests. The select expression is
compared with each of the case expressions and, if found to be equal, the program code within the
appropriate case block is executed. If no matching case is found, the program code within the optional
default block is executed:
```
Select Expression
Case Expressions
   Statements
Default
   Statements
End Select
```
The [Default] section is optional.

The selected value is only evaluated once, before each case is checked.

Multiple expressions for each case may be used by providing a comma separated sequence of expressions.

[While]: ../../api/brl/brl.blitz/#while
[Wend]: ../../api/brl/brl.blitz/#wend
[Repeat]: ../../api/brl/brl.blitz/#repeat
[Until]: ../../api/brl/brl.blitz/#until
[Forever]: ../../api/brl/brl.blitz/#forever
[For]: ../../api/brl/brl.blitz/#for
[To]: ../../api/brl/brl.blitz/#to
[Next]: ../../api/brl/brl.blitz/#next
[Until]: ../../api/brl/brl.blitz/#until
[Step]: ../../api/brl/brl.blitz/#step
[Exit]: ../../api/brl/brl.blitz/#exit
[Continue]: ../../api/brl/brl.blitz/#continue
[If]: ../../api/brl/brl.blitz/#if
[Then]: ../../api/brl/brl.blitz/#then
[Select]: ../../api/brl/brl.blitz/#select
[Case]: ../../api/brl/brl.blitz/#case
[Default]: ../../api/brl/brl.blitz/#default