Power (operator)

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Power (⍣) is a primitive dyadic operator that performs bounded looping, unbounded looping, and function inverses in NARS2000, Dyalog APL, and related implementations like ngn/apl, dzaima/APL, and Extended Dyalog APL.

Description

A call to Power is of the form <syntaxhighlight lang=apl inline>X(f⍣g)Y</source>, where

  • <syntaxhighlight lang=apl inline>X</source> is an optional argument.
  • <syntaxhighlight lang=apl inline>f</source> is a function. If <syntaxhighlight lang=apl inline>X</source> is given, then it is bound to <syntaxhighlight lang=apl inline>f</source> so <syntaxhighlight lang=apl inline>X f⍣g Y</source> is equivalent to <syntaxhighlight lang=apl inline>X∘f⍣g Y</source>.
  • <syntaxhighlight lang=apl inline>g</source> can be an array or a function.

Power repeatedly applies <syntaxhighlight lang=apl inline>f</source> to <syntaxhighlight lang=apl inline>Y</source> based on the type of operand <syntaxhighlight lang=apl inline>g</source>:

  • Function: Must be dyadic and must return a boolean singleton. The previous iteration value is provided as the right argument to <syntaxhighlight lang=apl inline>f</source>, and the current iteration value is given as the left argument. <syntaxhighlight lang=apl inline>f</source> is repeatedly applied until this function returns 1.
  • Integer: Applies <syntaxhighlight lang=apl inline>f</source> <syntaxhighlight lang=apl inline>g</source> times to <syntaxhighlight lang=apl inline>Y</source>. If <syntaxhighlight lang=apl inline>g</source> is negative, then the inverse of <syntaxhighlight lang=apl inline>f</source> (if available) is applied.
  • Integer Array: In Extended Dyalog APL, <syntaxhighlight lang=apl inline>g</source> can be an integer array. Each integer <syntaxhighlight lang=apl inline>i</source> in <syntaxhighlight lang=apl inline>g</source> will be replaced by <syntaxhighlight lang=apl inline>⊂(f⍣i)Y</source>.

Examples

Some basic examples: <syntaxhighlight lang=apl> 1 (+⍣3) 5 ⍝ Fixed number of iterations 8 

     (2∘×⍣3) 5 ⍝ No X given

40 

     1 +∘÷⍣= 1 ⍝ iterate till fixed point

1.618033989</source> A well-known use for Power is iterating until a fixed point is reached. <syntaxhighlight lang=apl> 

     (∨.∧⍨∨⊢)⍣≡3 3⍴0 0 1 1 0 1 1 0 1 ⍝ Transitive closure of an adjacency matrix

1 0 1 1 0 1 1 0 1</source> Power is also used to access function inverses. <syntaxhighlight lang=apl> 2(⊥⍣¯1)5 1 0 1</source>

External Links

Lessons

Documentation

APL built-ins [edit]
Primitives (Timeline) Functions
Scalar
Monadic Conjugate ∙ Negate ∙ Signum ∙ Reciprocal ∙ Magnitude ∙ Exponential ∙ Natural Logarithm ∙ Floor ∙ Ceiling ∙ Factorial ∙ Not ∙ Pi Times ∙ Roll ∙ Type ∙ Imaginary ∙ Square Root ∙ Round
Dyadic Add ∙ Subtract ∙ Times ∙ Divide ∙ Residue ∙ Power ∙ Logarithm ∙ Minimum ∙ Maximum ∙ Binomial ∙ Comparison functions ∙ Boolean functions (And, Or, Nand, Nor) ∙ GCD ∙ LCM ∙ Circular ∙ Complex ∙ Root
Non-Scalar
Structural Shape ∙ Reshape ∙ Tally ∙ Depth ∙ Ravel ∙ Enlist ∙ Table ∙ Catenate ∙ Reverse ∙ Rotate ∙ Transpose ∙ Raze ∙ Mix ∙ Split ∙ Enclose ∙ Nest ∙ Cut (K) ∙ Pair ∙ Link ∙ Partitioned Enclose ∙ Partition
Selection First ∙ Pick ∙ Take ∙ Drop ∙ Unique ∙ Identity ∙ Stop ∙ Select ∙ Replicate ∙ Expand ∙ Set functions (Intersection ∙ Union ∙ Without) ∙ Bracket indexing ∙ Index ∙ Cartesian Product ∙ Sort
Selector Index generator ∙ Grade ∙ Index Of ∙ Interval Index ∙ Indices ∙ Deal ∙ Prefix and suffix vectors
Computational Match ∙ Not Match ∙ Membership ∙ Find ∙ Nub Sieve ∙ Encode ∙ Decode ∙ Matrix Inverse ∙ Matrix Divide ∙ Format ∙ Execute ∙ Materialise ∙ Range
Operators Monadic Each ∙ Commute ∙ Constant ∙ Replicate ∙ Expand ∙ Reduce ∙ Windowed Reduce ∙ Scan ∙ Outer Product ∙ Key ∙ I-Beam ∙ Spawn ∙ Function axis ∙ Identity (Null, Ident)
Dyadic Bind ∙ Compositions (Compose, Reverse Compose, Beside, Withe, Atop, Over) ∙ Inner Product ∙ Determinant ∙ Power ∙ At ∙ Under ∙ Rank ∙ Depth ∙ Variant ∙ Stencil ∙ Cut ∙ Direct definition (operator) ∙ Identity (Lev, Dex)
Quad names Index origin ∙ Comparison tolerance ∙ Migration level ∙ Atomic vector
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