Control structure

Some modern versions of APL include a set of keywords for controlling flow of execution. Such keywords allow programmers to create control structures which are commonly used in procedural programming languages.

Example

Dyalog APL has a rich set of flow control keywords, including :If, :While, :Repeat, :For (with the supplementary control words :In and :InEach), :Select, :With, :Trap, :Hold and :Disposable. The use of control structures defined by these keywords is only allowed in defined functions.

For example, a function implementing the Trabb Pardo–Knuth algorithm using control structures:

∇ {res}←trabb;f;S;i;a;y
  f←{(0.5*⍨|⍵)+5×⍵*3}
  S←,⍎{⍞←⍵ ⋄ (≢⍵)↓⍞}'Please, enter 11 numbers: '
  :For i a :InEach (⌽⍳≢S)(⌽S)
      :If 400<y←f(a)
          ⎕←'Too large: ',⍕i
      :Else
          ⎕←i,y
      :EndIf
  :EndFor
∇

External links

Tutorials

• APL Cultivation: Control structures
• Bernard Legrand. Mastering Dyalog APL (page 189). Dyalog Ltd. November 2009.

Documentation

• Dyalog
• APLX