Introduction

Control Structures are blocks of code in which the execution of APL statements follows certain rules and conditions.  Control structures are implemented using a set of control words that all start with the colon symbol (:).  Control Words are case-insensitive.

There are a number of different types of control structures defined by the control words, :If, :While, :Repeat, :For (with the supplementary control words :In and :InEach), :Select, :With, :Trap, :Hold and :Disposable.  Each one of these control words may occur only at the beginning of an APL statement and indicates the start of a particular type of control structure.

Within a control structure, certain other control words are used as qualifiers.  These are :Else, :ElseIf, :AndIf, :OrIf, :Until, :Case and :CaseList.

A third set of control words is used to identify the end of a particular control structure.  These are :EndIf, :EndWhile, :EndRepeat, :EndFor, :EndSelect, :EndWith, :EndTrap, :EndHold and  :EndDisposable.  Although formally distinct, these control words may all be abbreviated to :End.

Finally, the :GoTo, :Return, :Leave and :Continue control words may be used to conditionally alter the flow of execution within a control structure.

Control words, including qualifiers such as :Else and :ElseIf, may occur only at the beginning of a line or expression in a diamond-separated statement. The only exceptions are :In and :InEach which must appear on the same line within a :For expression.

Key to Notation

The following notation is used to describe Control Structures within this section:

aexp an expression returning an array,
bexp an expression returning a single Boolean value (0 or 1),
var loop variable used by :For control structure,
code 0 or more lines of APL code, including other (nested) control structures,
andor either one or more :AndIf statements, or one or more :OrIf statements. For further details, see below.
|
.-----------------------.
|                       |
|<--------------.       |<--------------.
|               |       |               |
code            |       code            |
|               |       |               |
|               |       |               |
:AndIf bexp-----'       :OrIf bexp------'
|                       |
|<----------------------'
|

Notes

Code preceding :OrIf and :AndIf

Code that precedes a :OrIf control statement, e.g. code placed between a :If statement and a subsequent :OrIf, will be executed only if the outer condition is false. If instead the outer condition is true, there is no need to execute the :OrIf statement , so it and any preceding lines of code are skipped.

Code that precedes a :AndIf control statement, e.g. code placed between a :If statement and a subsequent :AndIf, will only be executed if the outer condition is true. If instead the outer condition is false, there is no need to execute the :AndIf statement , so it and any preceding lines of code are skipped.

The above behaviour may be examined using the Tracer.

A potential use for code before a :OrIf or :AndIf is to prepare for the conditional test. This preparatory work will only be done if required. For example:

:If x   ⍝ if x is false, skip everything up to the :EndIf
    y←..⍝ set up stuff for the condition on the next line
    :AndIf y
      do stuff
:EndIf

Warning

With the exception of a diamondised statement, a control statement that should not be followed by an expression will generate an error if an expression is supplied.

A line in a function consisting of a control statement followed by a ⋄ and subsequent expression(s) is not currently disallowed but may exhibit unexpected behaviour. In particular, the line will not honour ⎕STOP and will not be metered by ⎕MONITOR. This syntax is not recommended.