Strand notation, or stranding, is the convention that multiple (more than one) arrays written next to each other are automatically combined into a vector. Stranding can make code easier to read by eliminating the need for punctuation when writing small arrays in APL. It can also cause frustration when programming if unrelated arrays are stranded together. This issue occurs when operators are allowed to take array operands, and can be resolved by inserting extra parentheses or identity functions into the expression.
Several variations on stranding exist:
- In Dyalog APL, NARS2000 and others, arrays are stranded before operator or function evaluation.
- In APL2, APLX and others, arrays are stranded after operator evaluation and bracket indexing, but before function evaluation.
- In APL\360, SHARP APL and J stranding is called vector notation and is a part of token formation rather than execution. Only plain numbers are stranded.
An example in which stranding interferes with the most obvious way of writing a program is shown below. Consider applying the function
0.8 three times using the Power operator:
In a language which strands before function application, this expression is equivalent to the derived function
f⍣(3,0.8). Not what was intended! The two numbers must be separated somehow, for instance with parentheses or a tack function.
For operators that take array operands, such as the Rank operator, stranding before operator application can be beneficial. Without it, a function with two ranks such as
⊥⍤0 1 would have to be written with parentheses
|APL syntax |
|Overview||Comparison with traditional mathematics ∙ Precedence|
|Array||Numeric literal ∙ String ∙ Strand notation ∙ Object literal ∙ Array notation|
|Function||Argument ∙ Function valence ∙ Derived function ∙ Function train ∙ Monadic function ∙ Dyadic function ∙ Ambivalent function|
|Operator||Operand ∙ Operator valence ∙ Derived function ∙ Derived operator|
|Assignment||Multiple ∙ Indexed ∙ Selective ∙ Modified|
|User definition||Tradfn ∙ Dfn ∙ Tacit|
|Other||Function axis ∙ Branching ∙ System command ∙ User command ∙ Keyword ∙ Dot notation|