Statements

Conditional Execution

If Statement

The if statement executes a block of code if a specified condition evaluates to true. The various syntax permutations for the if statement are as follows.

if <Condition>
<CodeBlock>
end

if <Condition1>
<CodeBlock1>
else
<CodeBlock2>
end

if <Condition1>
   <CodeBlock1>
elseif <Condition2>
   <CodeBlock2>
else
   <CodeBlock3>
end

If the condition does not evaluate to true, then code execution branches to the next elseif or else conditional. If none of the conditionals evaluate to true, then the code branches to the code after the end of the if statement.

if ($myVar > 20)
   $myOtherVar = 1
elseif ($myVar < -20)
   $myOtherVar = 0
else
   $myOtherVar = -1
end

Switch Statement

The switch statement allows control to be transferred to one of various sections of code depending on the value of a given expression. The basic syntax of the switch statement is as follows.

switch <Expression>
   case <Constant1>
      <CodeBlock1>

   case <Constant2>
      <CodeBlock2>
    case <Constant3>
      <CodeBlock3>

   ...

   default
      <CodeBlockDefault>
end

If the value of the <Expression> matches the value of one of the case <Constant> values, then the statements under the matching case label will be executed. If the optional default label is specified, and the <Expression> does not match any of the specified case <Constant> values, then the statements under the default label will be executed.

The <Expression> must resolve to one of the following primitive data types:

integer
real
axis
enum

Each of the <Constant> values must be a constant value. A constant value is any value that can be determined at the time that the program is compiled. Constants include literals such as 3 or 2.5, as well as simple mathematical expressions containing only literals, such as (5 - 2), (0x1 << 3), or (2.5 + 3.0).

The data type of each <Constant> value must be the same data type as the <Expression>, or a data type that can be converted to the data type of the <Expression>. In addition, each of the <Constant> values must be unique. A constant value may appear in only one case within a switch statement.

An optional default label can be used to specify statements that should execute when the <Expression> does not equal any of the case <Constant> values.

An example of a switch statement is as follows.

switch ($myVar)
   case 0
      $rglobal[0] = 0

   case 1
      $rglobal[0] = $myVar

   default
      $rglobal[0] = -$myVar
end

Each case or default label must have code specified to execute when the switch expression evaluates to the value of that label. For example, the following switch statement would cause a compilation error because the default label does not have any code specified.

switch ($myVar)
   case 0
      $rglobal[0] = $myVar

   default
      // There is no code specified here
      // This is an error
end

Unlike switch statements found in other programming languages, the switch statement in this language does not have the concept of fallthrough execution because the end of a code section specified for a case or default label is the next case or default label, or the end specified for the switch. Some other programming languages use the break keyword to denote the end of a block of code within a switch statement, but the break keyword does not have this effect in AeroScript.

However, multiple case labels may be specified to execute the same code within a switch statement. In the following example, $iglobal[0] will be set to 0 if the value of $myVar is either 0 or 1.

switch ($myVar)
   case 0
   case 1
     $iglobal[0] = 0

   case 2
      $iglobal[0] = 1

   default
      $iglobal[0] = 2
end

Iterative Execution

While Loop

The while loop executes a block of code while a specified condition is true. The syntax of a while loop is as follows.

while
<Condition>
end

The <Condition> is considered to be true if it evaluates as true (e.g., relational operators such as > and <= evaluate as true/false in a conditional) or if it has a nonzero value (e.g., 2.0).

If the specified condition evaluates as false when the while statement is first executed, the block of code in the body of the while statement is never executed.

while ($iterations < 100)
$myVar[$iterations] = $rglobal[0]
$iterations++
end

For Loop

The for loop executes a block of code in an iterative fashion. The syntax of a for loop is as follows.

for <CounterVariable> = <StartValue> to <EndValue>
<CodeBlock>
end
for <CounterVariable> = <StartValue> to <EndValue> step <StepValue>
<CodeBlock>
end

The <CounterVariable> is set to the specified <StartValue>, and the loop will continue to iterate until the counter variable reaches the <EndValue>. The <CounterVariable> must be declared before it can be used in a for loop.

for $myVar = 0 to 9
$rglobal[$myVar] = $myVar
end

The <StepValue> is an optional argument that can be used to determine how much the <CounterVariable> will increment/decrement with each loop iteration. If the <StepValue> is not specified, the <CounterVariable> will increment by 1 with each iteration of the loop.

// Set only the global real values whose index is even (0, 2, 4, ...)
for $myVar = 0 to 9 step 2
$rglobal[$myVar] = $myVar
end

If the <StartValue> is greater than the <EndValue> (or less than the <EndValue> if the <StepValue> is negative), then the loop will not be entered and code execution will immediately transfer to after the end keyword.

for $myVar = 0 to 9 step -1
// This code is never executed
$rglobal[$myVar] = $myVar
end

ForEach Loop

The foreach loop executes a block of code for each element in a given array variable, in order from the lowest index to the highest index. The syntax of a foreach loop is as follows.

foreach var <IterationVariable>in<ArrayVariable>
<CodeBlock>
end

The <ArrayVariable> is the variable that is iterated over within the loop. It must be a 1-dimensional, 2-dimensional, or 3-dimensional array variable. The <IterationVariable> is created to have the same data type as the <ArrayVariable> when the <ArrayVariable> is accessed with the array access operator ([]). For example, if the <ArrayVariable> is a 2-dimensional array of real values, then the <IterationVariable> will be declared as a 1-dimensional array of real values. The <IterationVariable> exists only within the foreach loop for which it was specified. The following is an example of using a foreach loop to iterate over each element of a 1-dimensional array variable.

var $axes[] as axis = [X, Y, Z]

// Perform the same actions for each axis in the array of axes
foreach var $axis in $axes
   Enable($axis)
   Home($axis)
   AnalogOutputSet($axis, 0, 2.5)
end

The following example shows how a foreach loop can be used to iterate over a 2-dimensional array, which can be useful when performing motion over a predetermined set of positions. In the example, the $point iterator variable is unlike any variable that can be declared normally because it is able to access different arrays within the foreach loop.

var $axes[] as axis = [X, Y]
var $positions[4][2] = [
   [10.0, 20.0],
   [20.0, 40.0],
   [30.0, 60.0],
   [40.0, 80.0]]

// Enable the axes
Enable($axes)

// Iterate over each sub-array in the 2-dimensional $positions[][] variable
foreach var $point in $positions
   MoveLinear($axes, $point)
end

A foreach loop can be used to iterate over an array of any data type, including a user-defined struct.

struct
   $position as real
   $offset as real
end

var $axisData[4] as AxisData

foreach var $data in $axisData
   $data.position = 0.0
   $data.offset = 0.0
end

Repeat Loop

The repeat loop is used to execute a block of code a specified number of times. The syntax of a repeat loop is as follows.

repeat <Count>
<CodeBlock>
end

The <Count> expression specifies the number of times that the repeat loop will execute. This expression must be an integer expression or a real expression that evaluates to an integer value (for example, 3.0).

repeat 10
$rglobal[0] = $rglobal[0] * 2
end

The <Count> expression will only be evaluated the first time that the repeat loop is executed, and not each time that a loop iteration completes. This means that event if you modify the variable that was specified as the <Count> of a repeat loop within the <CodeBlock>, the number of iterations of the loop will not change.

var $myVar as integer = 10

repeat $myVar // This loop repeats 10 times
$myVar = $myVar + 1
end

If the <Count> expression is less than or equal to zero, code execution will never enter the loop and will immediately branch to the end specified for the repeat loop.

Conditional and Iterative Statement Nesting

Many control flow statements can be nested within each other to give more flexibility. For example, an if statement can be placed inside of a repeat statement, and vice versa.

repeat 10
   if $myVar == 0
      Enable(X)
   else
      Disable(X)
   end
end

Labels

A label can be the target of a Goto Statement. Labels have the following syntax.

<LabelName>:

The <LabelName> must be a valid identifier (see Valid Identifier Format), and cannot be the same as any of the Reserved Keywords or an axis name.

A label can be specified on a line by itself.

programStart:
RunProgram()

goto programStart

A label may also directly precede a statement.

setIO: AnalogOutputSet(X, 0, 2.5)

In addition, multiple labels can be specified on the same line, but must precede any statements on that line.

label1: label2: Enable(X)

Labels are case-sensitive. For example, myLabel and MYLABEL refer to different labels.

It is generally discouraged to use the goto statement, and therefore labels. Refer to the Goto Statement for more information.

Unconditional Branching

Goto Statement

A goto statement is used to unconditionally transfer program execution to a line of code with a given Labels. The following is the syntax for a goto statement.

goto <LabelName>

A goto statement must be specified in the same functional region as the destination <Label> of the statement. A functional region includes the following:

A program block
A function body
The program global scope (if no program block is specified)

For more information about the program block, refer to the User-Defined Functions section.

A goto statement may be defined either before or after the <Label> to which it is referring. The following example shows a case where a label is defined before the goto statement that references it.

myLabel:
   G1X10
   Dwell(0.5)
   goto myLabel

The example that follows shows a case where a label is defined after the goto statement that references it.

   if ($iglobal[0] == 0)
      goto skipDwell
   end

   Dwell(0.5)

skipDwell:
   G1X10

One instance where the goto statement can be useful is breaking out of deeply-nested loops.

var $i, $j, $k as integer
var $num as integer = 0

for $i = 0 to 9
   for $j = 0 to 9
      for $k = 0 to 9
         if ($rglobal[($i * 100) + ($j * 10) + $k] < 0.0)
            goto found
         else
            $num++
         end
      end
   end
end

found:

The goto statement cannot be used to transfer control into constructs that require initialization:

The body of a repeat loop
The body of a for loop
The body of a foreach loop
The body of a function

The reason that goto statements cannot be used to transfer control into the listed loops is because the initialization of the loops would not be performed, causing the program to behave incorrectly. For example, using a goto statement to transfer control to the body of a for loop would cause the for loop counter variable to be uninitialized.

As an example, the following usage of a goto statement will cause an error during compilation because the code execution would branch over the initialization of the repeat loop.

goto myLabel

repeat 10
myLabel:
Dwell(0.01)
end

While the goto statement cannot be used to enter certain regions of code, it may still be used to exit those regions. For example, a goto statement may be used to exit a repeat loop.

In addition, the goto statement cannot be used to transfer control past the initialization of a variable because the value of a variable cannot be determined. As an example, the following usage of a goto statement will cause an error during compilation.

goto myLabel

var $myVar as integer = 3

myLabel:
$myVar++

The use of the goto statement is generally discouraged as its use can make programs difficult to read or debug. In many cases, the use of a goto statement can be replaced with a proper while, for, or foreach loop.

Break Statement

Break statements allow for a control flow loop (while, for, foreach, or repeat) to be immediately terminated. The syntax for the break statement is as follows.

break

The break statement is used to break out of the nearest containing loop. The break statement cannot be used outside of a while, for, foreach, or repeat loop.

repeat 5
   while ($cnt > 10)
      $cnt--
      if ($iVal == 0)
         // Break out of the while loop
       break
      end
   end
end

Continue Statement

The continue statement allows for control flow to jump to the beginning of a control flow loop (while, for, foreach, or repeat) to start executing its next iteration, skipping any code remaining in the loop for the current iteration. The syntax for the continue statement is as follows.

continue

The continue statement is used to branch to the beginning of the nearest containing loop. The continue statement cannot be used outside of a while, for, foreach, or repeat loop.

$skipIndex = 10
for $index = 0 to 31
   if ($index == $skipIndex)
      // Skip setting the 10th index
      continue
   end
    $rglobal[$index] = $index
end

Return Statement

The return statement allows for control flow to exit the current function, returning a value (if specified) to its caller. If the control of execution is within a program block, or is at the program global scope, then a return statement will cause the program to end.

The following are the various syntax permutations for the return statement.

return
return <Value>

The return syntax without a <Value> argument should be used to return early from a function that does not return a value.

function SetValue($value as real)
   if ($value < 0.0)
      // Return from the function early to prevent the value from being
      // Set if it is negative
      return
   end

   $rglobal[0] = $value
end

A return statement without a return value can also be specified in a program block, or at the program global scope (if no program block was defined) in order to stop the execution of a program before execution reaches the end. A value cannot be specified to a return statement in these cases.

The return <Value> syntax should be used to return a value from a function that returns a value. The value returned from a function must match (or be convertible to) the data type of the function return value.

function MyFunc () as integer
// This is valid because the function returns an integer
return 123
end

function MyFunc()
// This is not valid because the function does not return a value
return 123
end

function MyFunc() as integer
   // This is not valid because the function has a return with the integer
   // data type, but a string is being returned
   return "Hello"
end

Wait Statement

The wait statement is used to wait for a specified condition to evaluate as true before continuing program execution. The syntax for the wait statement is as follows.

wait(<Condition>)
<return> = wait(<Condition>, <Timeout>)

The program will wait on the wait statement program line forever until the specified <Condition> evaluates as true.

// Wait forever until $iglobal[0] has a value of 1
wait($iglobal[0] == 1)

An optional <Timeout> value, in milliseconds, may be specified. If the <Condition> continues to evaluate as false for longer than the <Timeout> duration, then the wait statement will return a value of 1. If the <Condition> evaluates as true before the <Timeout> duration, then the wait statement will return a value of 0. The return value may be ignored.

// $timedOut will have a value of 1 if the wait statement executes for
// more than 2 seconds (2000 milliseconds)
$timedOut = wait($signalVar == 1, 2000)

// The return value of a wait statement may be ignored if the timeout
// status is not useful
wait($signalVar == 0, 150)

// Specifying a value of "false" for the conditional allows a wait
// statement to be used to delay program execution for the specified
// timeout
wait(false, 1000)

The wait statement is syntactic sugar, or syntax that makes it easier to express an idea using the language. The following two examples have the same behavior, but the wait statement allows for the code to be expressed more concisely.

var $timeout as integer
var $timedOut as integer = false

CriticalSectionStart()

$timeout = 100
while (StatusGetAxisItem(X, AxisStatusItem.VelocityCommand) != 0.0)
   $timeout--
   if ($timeout == 0)
      $timedOut = true
      break
   end
   Dwell(0.001)
end

CriticalSectionEnd()

if ($timedOut == 1)
   AppMessageDisplay("A timeout occurred.")
end

var $timedOut as integer

$timedOut = wait(StatusGetAxisItem(X, AxisStatusItem.VelocityCommand) == 0, 100)
if ($timedOut == 1)
AppMessageDisplay("A timeout occurred.")
end

OnError Statement

The onerror statement causes the specified function to be executed when a task error occurs during program execution. The syntax of an onerror statement that sets an error handling function is as follows.

onerror(<FunctionName>())

An onerror statement can be specified without a function to clear any active onerror handler. The syntax for clearing an onerror handler is as follows.

onerror()

If an onerror handler is active and a task error occurs while a program is active (in either the running, paused, or feedheld states), program execution will immediately branch to the function specified by the <FunctionName> when the onerror statement was specified. The function specified by the <FunctionName> must not accept any arguments, nor may it return a value.

program
   // Configure the onerror handler to call the ErrorHandler() function if
   // an error occurs
   onerror(ErrorHandler())

   // Any errors that occur here will cause the ErrorHandler() function to
   // be executed automatically
   Enable([X, Y, Z])
   Home([X, Y, Z])
   MoveLinear([X, Y], [10, 20])

   // Disable the active onerror handler
   onerror()
end

function ErrorHandler()
   // Clear errors and start the program again
   AcknowledgeAll()
   ProgramRestart()
end

Errors can be handled within the called function and then the programmer can decide whether to restart program execution or end the program. If the error condition is not cleared within the function, or a new error condition occurs within the function, then the program will terminate.

Next Topic: Functions