Possible error with dot product

조회 수: 4(최근 30일)
William Kett
William Kett 2021년 9월 22일
댓글: William Kett 2021년 10월 4일
There appears to be an issue with the dot function which computes the dot product between 2 vectors. Either that, or I am using it incorrectly.
xa = -4.2162; % x-component of vector a
ya = -7.1335;% y-component of vector a
xb = -1.4941e-08; % x-component of vector b
yb = 1.7922e-08; % y-component of vector b
% dot product = a_i*b_i + a_j*b_j =|a|*|b|*cos(theta)
%% Get values for above formulae
dotprod1 = xa * xb + ya * yb
dotprod1 = -6.4852e-08
theta = rad2deg(acos((xa*xb+ya*yb)/(sqrt(xa^2+ya^2)*sqrt(xb^2+yb^2))));
dotprod2 = sqrt(xa^2+ya^2)*sqrt(xb^2+yb^2) * cos(deg2rad(theta))
dotprod2 = -6.4852e-08
dotprod1 == dotprod2 % Should give 1
ans = logical
1
%% Using the dot function
% a = a_i + a_j
% b = b_i + b_j
dotprod3 = dot(xa+ya,xb+yb) % dot of full vectors a and b
dotprod3 = -3.3833e-08
dotprod1 == dotprod3
ans = logical
0
Why isn't dot producing the same value?

채택된 답변

Geoff Hayes
Geoff Hayes 2021년 9월 22일
William - from dot, this function returns the scalar dot product of the two input vectors. So in your case, the
dot(xa+ya,xb+yb)
is equivalent to
(xa + ya) * (xb + yb)
which is not the same as what you calculated previously as
dotprod1 = (xa * xb) + (ya * yb) % I added the brackets
Note also that your code and comment
dotprod1 == dotprod2 % Should give 1
is not necessarily true for floating point numbers. See Compare Floating-Point Numbers for details.
  댓글 수: 1
William Kett
William Kett 2021년 10월 4일
Thank you for your response. I needed to use this for computing advection and was uncertain if dot worked.

댓글을 달려면 로그인하십시오.

추가 답변(0개)

제품


릴리스

R2021a

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by