Strand notation

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 APL2 arrays are stranded before operator or function evaluation.
• In APL*PLUS arrays are stranded after operator evaluation but before function evaluation.
• In 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 f to 0.8 three times using the Power operator:

f⍣3 0.8

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.

(f⍣3)0.8
f⍣3⊢0.8

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 ⊥⍤(0 1).

External links

Documentation

• Dyalog
• APLX