Dot product

Create a function/use an in-built function, to compute the dot product, also known as the scalar product of two vectors. If possible, make the vectors of arbitrary length.

As an example, compute the dot product of the vectors [1, 3, -5] and [4, -2, -1].

If implementing the dot product of two vectors directly, each vector must be the same length; multiply corresponding terms from each vector then sum the results to produce the answer.

Clojure

Works with: Clojure version 1.1

Preconditions are new in 1.1. The actual code also works in older Clojure versions.

<lang clojure>(defn dot-product [c1 c2]

 {:pre [(== (count c1) (count c2))]}
 (reduce + (map * c1 c2))
 )

(println (dot-product [1 3 -5] [4 -2 -1]))</lang>

Factor

The built-in word v. is used to compute the dot product. It doesn't enforce that the vectors are of the same length, so here's a wrapper.

<lang factor>USING: kernel math.vectors sequences ;

dot-product ( u v -- w )

   2dup [ length ] bi@ =
   [ v. ] [ "Vector lengths must be equal" throw ] if ;</lang>

( scratchpad ) { 1 3 -5 } { 4 -2 -1 } dot-product .
3

Forth

<lang forth>: vector create cells allot ;

th cells + ;

3 constant /vector /vector vector a /vector vector b

dotproduct ( a1 a2 -- n)

 0 tuck ?do -rot over i th @ over i th @ * >r rot r> + loop nip nip

vector! cells over + swap ?do i ! 1 cells +loop ;

-5 3 1 a /vector vector! -1 -2 4 b /vector vector!

a b /vector dotproduct . 3 ok</lang>

Fortran

Works with: Fortran version 90 and later

<lang fortran>program test_dot_product

 write (*, '(i0)') dot_product ((/1, 3, -5/), (/4, -2, -1/))

end program test_dot_product</lang> Output: <lang>3</lang>

Haskell

<lang haskell>dotp a b | length a == length b = sum (zipWith (*) a b)

        | otherwise = error "Vector sizes must match"

main = print $ dotp [1, 3, -5] [4, -2, -1] -- prints 3</lang>

J

<lang j> 1 3 _5 +/ . * 4 _2 _1 3

  dotp=: +/ . *                  NB. Or defined as a verb (function)
  1 3 _5  dotp 4 _2 _1

3</lang> The conjunction . applies generally to matricies and arrays of higher dimensions and can be used with verbs (functions) other than sum ( +/ ) and product ( * ).

Logo

<lang logo>to dotprod :a :b

 output apply "sum (map "product :a :b)

end

show dotprod [1 3 -5] [4 -2 -1] ; 3</lang>

Lua

<lang lua>function dotprod(a, b)

 local ret = 0
 for i = 1, #a do
   ret = ret + a[i] * b[i]
 end
 return ret

end

print(dotprod({1, 3, -5}, {4, -2, 1}))</lang>

Oz

Vectors are represented as lists in this example. <lang oz>declare

 fun {DotProduct Xs Ys}
    {Length Xs} = {Length Ys} %% assert
    {List.foldL {List.zip Xs Ys Number.'*'} Number.'+' 0}
 end

in

 {Show {DotProduct [1 3 ~5] [4 ~2 ~1]}}</lang>

Perl

<lang perl>sub dotprod (\@\@) {

       my($vec_a, $vec_b) = @_;
       die "they must have the same size\n" unless @$vec_a == @$vec_b;
       my $sum = 0;
       $sum += $vec_a->[$_]*$vec_b->[$_] for 0..$#$vec_a;
       return $sum;

}

my @vec_a = (1,3,-5); my @vec_b = (4,-2,-1);

print dotprod(@vec_a,@vec_b), "\n"; # 3</lang>

PureBasic

<lang PureBasic>Procedure dotProduct(Array a.i(1), Array b.i(1), length.i)

 Protected result.i, i.i
 
 For i = 0 To length - 1
   result + a(i) * b(i)
 Next
 
 ProcedureReturn result

EndProcedure

Dim a.i(2) a(0) = 1 : a(1) = 3 : a(2) = -5 Dim b.i(2) b(0) = 4 : b(1) = -2 : b(2) = -1

Debug dotProduct(a(), b(), 3) </lang>

Python

<lang python>def dotp(a,b):

   assert len(a) == len(b), 'Vector sizes must match'
   return sum(aterm * bterm for aterm,bterm in zip(a, b))

if __name__ == '__main__':

   a, b = [1, 3, -5], [4, -2, -1]
   assert dotp(a,b) == 3</lang>

R

Here are several ways to do the task.

<lang R> x <- c(1, 3, -5) y <- c(4, -2, -1)

sum(x*y) # compute products, then do the sum x %*% y # inner product

loop implementation

dotp <- function(x, y) { n <- length(x) if(length(y) != n) stop("invalid argument") s <- 0 for(i in 1:n) s <- s + x[i]*y[i] s }

dotp(x, y) </lang>

Ruby

<lang ruby>class Array

 def dot_product(other)
   raise "not the same size!" if self.length != other.length
   self.zip(other).inject(0) {|dp, (a, b)| dp += a*b}
 end

end

p [1, 3, -5].dot_product [4, -2, -1] # => 3</lang>

Scheme

Works with: Scheme version R

^{5}

RS

<lang scheme>(define (dot-product a b)

 (apply + (map * a b)))

(display (dot-product (list 1 3 -5) (list 4 -2 -1))) (newline)</lang> Output: <lang>3</lang>

SNOBOL4

<lang snobol4> define("dotp(a,b)sum,i") :(dotp_end) dotp i = 1; sum = 0 loop sum = sum + (a * b)

i = i + 1 ?a :s(loop) dotp = sum :(return)
dotp_end
a = array(3); a<1> = 1; a<2> = 3; a<3> = -5; b = array(3); b<1> = 4; b<2> = -2; b<3> = -1; output = dotp(a,b)
end</lang>
Tcl
Library: tcllib
<lang tcl>package require math::linearalgebra
set a {1 3 -5} set b {4 -2 -1} set dotp [::math::linearalgebra::dotproduct $a $b] proc pp vec {return \[[join $vec ,]\]} puts "[pp $a] \u2219 [pp $b] = $dotp"</lang> Output:
[1,3,-5] ∙ [4,-2,-1] = 3.0
TI-89 BASIC
dotP([1, 3, –5], [4, –2, –1]) 3
Ursala
A standard library function for dot products of floating point numbers exists, but a new one can be defined for integers as shown using the map operator (*) with the zip suffix (p) to construct a "zipwith" operator, which operates on the integer product function. A catchable exception is thrown if the list lengths are unequal. This function is then composed (+) with a cumulative summation function, which is constructed from the binary sum function, and the reduction operator (:-) with 0 specified for the vacuous sum. <lang Ursala>#import int
dot = sum:-0+ product*p
cast %z
test = dot(<1,3,-5>,<4,-2,-1>)</lang> output:
3