Dot product

Fortran Equivalent

SDOT/DDOT/CDOTC/CDOTU/ZDOTC/ZDOTU

REAL*4 FUNCTION SDOT (N, X,INCX, Y,INCY)

REAL*4 X(*),Y(*)

SDOT = 0.0

IF ( N .LE. 0 ) RETURN

IX = 1

IY = 1

IF ( INCX .LT. 0 ) IX = 1 - (N-1) * INCX

IF ( INCY .LT. 0 ) IY = 1 - (N-1) * INCY

DO 10 I = 1, N

SDOT = SDOT + X(IX) * Y(IY)

IX = IX + INCX

IY = IY + INCY

10CONTINUE RETURN END

Example 1 Compute the REAL*8 dot product

10

s = ∑ xi yi

i = 1

where x and y are vectors 10 elements long stored in one-dimensional arrays X and Y of dimension 20.

INTEGER*4 N,INCX,INCY

REAL*8

S,DDOT,X(20),Y(20)

N =

10

 

INCX = 1

 

INCY = 1

 

S =

DDOT (N,X,INCX,Y,INCY)

Example 2 Compute the REAL*8 dot product

10

s = ∑ xi yi

i = 1

where x is the 4th row of a 10-by-10 matrix stored in a two-dimensional array X of dimension 20-by-21, and y is a vector 10 elements long stored in one-dimensional array Y of dimension 20.

INTEGER*4

N

REAL*8

S,DDOT,X(20,21),Y(20)

N =

10

 

S =

DDOT (N,X(4,1),20,Y,1)

Chapter 2 Basic Vector Operations 87