Logarithm

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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

See also

External links

Documentation

APL built-ins [edit]
Primitives (Timeline) Functions
Scalar
Monadic ConjugateNegateSignumReciprocalMagnitudeExponentialNatural LogarithmFloorCeilingFactorialNotPi TimesRollTypeImaginarySquare Root
Dyadic AddSubtractTimesDivideResiduePowerLogarithmMinimumMaximumBinomialComparison functionsBoolean functions (And, Or, Nand, Nor) ∙ GCDLCMCircularComplexRoot
Non-Scalar
Structural ShapeReshapeTallyDepthRavelEnlistTableCatenateReverseRotateTransposeRazeMixSplitEncloseNestCut (K)PairLinkPartitioned EnclosePartition
Selection FirstPickTakeDropUniqueIdentityStopSelectReplicateExpandSet functions (IntersectionUnionWithout) ∙ Bracket indexingIndexCartesian ProductSort
Selector Index generatorGradeIndex OfInterval IndexIndicesDealPrefix and suffix vectors
Computational MatchNot MatchMembershipFindNub SieveEncodeDecodeMatrix InverseMatrix DivideFormatExecuteMaterialiseRange
Operators Monadic EachCommuteConstantReplicateExpandReduceWindowed ReduceScanOuter ProductKeyI-BeamSpawnFunction axis
Dyadic BindCompositions (Compose, Reverse Compose, Beside, Withe, Atop, Over) ∙ Inner ProductDeterminantPowerAtUnderRankDepthVariantStencilCutDirect definition (operator)
Quad names Index originComparison toleranceMigration levelAtomic vector