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:''This page describes the dyadic arithmetic function. For the monadic natural logarithm function, see [[Natural Logarithm]].''
:''This page describes the dyadic arithmetic function. For the monadic natural logarithm function, see [[Natural Logarithm]].''


{{Built-in|Logarithm|⍟}}, or '''Log''', is a [[dyadic]] [[scalar function]] which computes the [[wikipedia:logarithm|logarithm]] of the two [[argument|arguments]]. More precisely, <source lang=apl inline>X⍟Y</source> computes how much [[power]] of X equals Y, i.e. the value of R that satisfies <source lang=apl inline>Y=X*R</source>. Logarithm shares the [[glyph]] <source lang=apl inline>⍟</source> with the monadic arithmetic function [[Natural Logarithm]]. The [[glyph]], a composition of the glyphs for [[Circular]] (<source lang=apl inline>○</source>) and [[Power]] (<source lang=apl inline>*</source>), indicating its close mathematical ties with these two functions, while also being a stylised tree log.
{{Built-in|Logarithm|⍟}}, or '''Log''', is a [[dyadic]] [[scalar function]] which computes the [[wikipedia:logarithm|logarithm]] of the two [[argument|arguments]]. More precisely, <source lang=apl inline>X⍟Y</source> computes how much [[power]] of X equals Y, i.e. the value of R that satisfies <source lang=apl inline>Y=X*R</source>. Logarithm shares the [[glyph]] <source lang=apl inline>⍟</source> with the monadic arithmetic function [[Natural Logarithm]]. The [[glyph]], a composition of the glyphs for [[Circular]] (<source lang=apl inline>○</source>) and [[Power]] (<source lang=apl inline>*</source>) to indicate its close mathematical ties with these two functions, is a stylised tree log.


== Examples ==
== Examples ==

Revision as of 09:01, 2 June 2020

This page describes the dyadic arithmetic function. For the monadic natural logarithm function, see Natural Logarithm.
⍟

Logarithm (⍟), or Log, is a dyadic scalar function which computes the logarithm of the two arguments. More precisely, X⍟Y computes how much power of X equals Y, i.e. the value of R that satisfies Y=X*R. Logarithm shares the glyph ⍟ with the monadic arithmetic function Natural Logarithm. The glyph, a composition of the glyphs for Circular (○) and Power (*) to indicate its close mathematical ties with these two functions, is a stylised tree log.

Examples

      2⍟0.5 1 2 32 1024
¯1 0 1 5 10

Logarithm can be used to determine how many digits are needed to write a positive integer Y in base X: 

      Digits←{1+⌊⍺⍟⍵}
      ToBase←⊥⍣¯1
      (2 Digits 100) (2 ToBase 100)
┌─┬─────────────┐
│7│1 1 0 0 1 0 0│
└─┴─────────────┘
      (10 Digits 100) (10 ToBase 100)
┌─┬─────┐
│3│1 0 0│
└─┴─────┘
Works in: Dyalog APL

Properties

By definition, logarithm is the inverse of the power with the same base (left argument). 

      2*1 2 3 4 5
2 4 8 16 32
      2⍟2 4 8 16 32
1 2 3 4 5
      2 (*⍣¯1 ≡ ⍟) ⍳10
1
Works in: Dyalog APL

Reciprocal on the left or right argument gives the negated result. 

      2⍟÷2 4 8 16 32
¯1 ¯2 ¯3 ¯4 ¯5
      (÷2)⍟2 4 8 16 32
¯1 ¯2 ¯3 ¯4 ¯5

External links

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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