# Inner Product: Difference between revisions

 .

Inner Product (.) is a dyadic operator that produces a dyadic function when applied with two dyadic functions. It's a generalisation of the matrix product, allowing not just addition-multiplication, but any dyadic functions given as operands.

## Examples

<syntaxhighlight lang=apl>

     x ← 1 2 3
y ← 4 5 6
x ,.(⊂,) y ⍝ visualizing of pairing


┌─────────────┐ │┌───┬───┬───┐│ ││1 4│2 5│3 6││ │└───┴───┴───┘│ └─────────────┘

     x {⊂⍺,'+',⍵}.{⊂⍺,'×',⍵} y ⍝ visualizing function application in matrix multiplication


┌───────────────────────────┐ │┌─────────────────────────┐│ ││┌─────┬─┬───────────────┐││ │││1 × 4│+│┌─────┬─┬─────┐│││ │││ │ ││2 × 5│+│3 × 6││││ │││ │ │└─────┴─┴─────┘│││ ││└─────┴─┴───────────────┘││ │└─────────────────────────┘│ └───────────────────────────┘

     x+.×y ⍝ matrix multiplication


32 </source>

The shapes of the arguments must be compatible with each other: The last axis of the left argument must have the same length as the first axis of the right argument, or formally, for <syntaxhighlight lang=apl inline>X f.g Y</source> it must be that <syntaxhighlight lang=apl inline>(¯1↑⍴X)≡(1↑⍴Y)</source>. Although this rule differs from conformability, the arguments may also be subject to scalar or singleton extension. The shape of the result is <syntaxhighlight lang=apl inline>(¯1↓⍴X),(1↓⍴Y)</source>.

For example, when applying inner product on two matrices, the number of columns in the left array must match with number of rows in the right array, otherwise we will get an error. <syntaxhighlight lang=apl>

     ⎕  ← x ← 2 3⍴⍳10


1 2 3 4 5 6

     ⎕ ← y ← 4 2⍴⍳10


1 2 3 4 5 6 7 8

     x+.×y


LENGTH ERROR

     x+.×y
∧
⎕ ← y ← 3 2⍴⍳10 ⍝ reshape y to be compatible with x
x+.×y


22 28 49 64 </source>

## History

Inner product appeared in early Iverson Notation as ${\displaystyle _{g}^{f}}$ and applied even to non-scalar functions, like Compress, Iverson bringing:[1]

{\displaystyle {\begin{aligned}{\text{For example, if}}\\{\boldsymbol {A}}&={\begin{pmatrix}1&3&2&0\\2&1&0&1\\4&0&0&2\\\end{pmatrix}}\qquad {\text{and}}\qquad {\boldsymbol {B}}={\begin{pmatrix}4&1\\0&3\\0&2\\2&0\\\end{pmatrix}}\\{\text{then}}\qquad {\boldsymbol {A}}\;_{\times }^{+}\,{\boldsymbol {B}}&={\begin{pmatrix}4&14\\10&5\\20&4\\\end{pmatrix}},\quad {\boldsymbol {A}}\;_{=}^{\land }\,{\boldsymbol {B}}={\begin{pmatrix}0&1\\0&0\\1&0\\\end{pmatrix}}{\text{,}}\\{\boldsymbol {A}}\;_{\neq }^{\lor }\;{\boldsymbol {B}}&={\begin{pmatrix}1&0\\1&1\\0&1\\\end{pmatrix}},\qquad {\text{and}}\qquad ({\boldsymbol {A}}\neq 0)\;_{\,/}^{+}\,{\boldsymbol {B}}={\begin{pmatrix}4&6\\6&4\\6&1\\\end{pmatrix}}{\text{.}}\end{aligned}}}

When the inner product notation was linearised (made to fit on a single line of code) the glyph <syntaxhighlight lang=apl inline>.</source> was chosed to denote what was previously indicated by positioning the two operands vertically aligned. Thus, the above correspond to the following modern APL: <syntaxhighlight lang=apl> ⍝ For example, if

     A←3 4⍴1 3 2 0 2 1 0 1 4 0 0 2
B←4 2⍴4 1 0 3 0 2 2 0


⍝ then

     A +.× B
4 14


10 5 20 4

     A ∧.= B


0 1 0 0 1 0

     A ∨.≠ B


1 0 1 1 0 1

     (A ≠ 0) +./ B


4 6 6 4 6 1 </source> Note that some dialects implement Compress (<syntaxhighlight lang=apl inline>/</source>) as a monadic operator rather than as a function, which means it cannot be an operand in the inner product. Instead, a cover function is necessary: <syntaxhighlight lang=apl> ∇z←a Compress b

z←a/b


∇ </source>

## Differences between dialects

Implementations differ on the exact behaviour of inner product when the right operand is not a scalar function. It follows from page 121 of the ISO/IEC 13751:2001(E) standard specifies that <syntaxhighlight lang=apl inline>X f.g Y</source> is equivalent to <syntaxhighlight lang=apl inline>f/¨ (⊂[⍴⍴x]x)∘.g ⊂[1]y</source>. This is indeed what APL2, APLX, APL+Win, and ngn/apl follow, while Dyalog APL, NARS2000 and GNU APL differ as described by Roger Hui:[2]

The following dop models inner product in Dyalog APL, with caveats. If you find a case where <syntaxhighlight lang=apl inline>f.g</source> differs from <syntaxhighlight lang=apl inline>f IP g</source>, not covered by the caveats, I'd be interested. <syntaxhighlight lang=apl> IP←{ assert((⊃⌽⍴⍺)≡≢⍵)∨(1=×/⍴⍺)∨1=×/⍴⍵: ⊃⍤0 ⊢ (↓⍺) ∘.(⍺⍺/⍵⍵¨) ↓(¯1⌽⍳⍴⍴⍵)⍉⍵ }

assert←{⍺←'assertion failure' ⋄ 0∊⍵:⍺ ⎕SIGNAL 8 ⋄ shy←0} </source> (Explanation: What's with the <syntaxhighlight lang=apl inline>⊃⍤0</source> in <syntaxhighlight lang=apl inline>IP</source>? It's because <syntaxhighlight lang=apl inline>∘.f</source> has an implicit each, applying <syntaxhighlight lang=apl inline>⊂</source> to each item of its result. But the <syntaxhighlight lang=apl inline>⍺⍺/</source> in <syntaxhighlight lang=apl inline>(⍺⍺/⍵⍵¨)</source> also has an implicit each. So the <syntaxhighlight lang=apl inline>⊃⍤0</source> gets rid of one of those encloses.)

Caveats:

• You can not use the hybrid <syntaxhighlight lang=apl inline>/</source> directly as an operand as it runs afoul of the parser in weird and wonderful ways. Instead, you have to use <syntaxhighlight lang=apl inline>{⍺/⍵}</source>. The same goes for <syntaxhighlight lang=apl inline>\</source> and <syntaxhighlight lang=apl inline>{⍺\⍵}</source> I guess.
• It differs from ISO/IEC 13751:2001(E) in using <syntaxhighlight lang=apl inline>⍵⍵¨</source> instead of just <syntaxhighlight lang=apl inline>⍵⍵</source> in the central key expression (i.e. <syntaxhighlight lang=apl inline>(⍺⍺/⍵⍵¨)</source> instead of <syntaxhighlight lang=apl inline>(⍺⍺/⍵⍵)</source>). So does the primitive <syntaxhighlight lang=apl inline>f.g</source>.
• It differs from ISO/IEC 13751:2001(E) in doing full-blown single extension instead of just scalar and 1-element vector extension (as in APL2). So does the primitive <syntaxhighlight lang=apl inline>f.g</source>. e.g.<syntaxhighlight lang=apl>

(3 4⍴5)+.×1 1 1 1⍴6 ⍝ works in Dyalog, not in ISO or APL2</source>

• It differs from NARS2000 or APL\360 in not permitting unit axis extension. So does the primitive <syntaxhighlight lang=apl inline>f.g</source>. e.g.<syntaxhighlight lang=apl>

(3 4⍴5)+.×1 5⍴6 ⍝ works in NARS2000 or APL\360, not in Dyalog APL</source>