
Precision question
Posted:
Oct 20, 2011 11:28 PM


I have three algorithms to calculate the inner product of vector A(i,j) and vector V for M x N times. With the line A = round(A), B1 = B2 = B3. Without the line A = round(A);, B1 != B2, B2 = B3. Is this a precision error? Which one is correct?
clc;100 clear all; M = 1000; N = 1000; P = 10;
A = 10*rand(M,N,P); A = round(A);
% A = zeros(M,N,P); % for i = 1:P % A(:,:,i) = i; % end
V = 1:P;
% Algorithm 1 A = shiftdim(A,2); tic; B1 = zeros(M,N); for i = 1:M for j = 1:N B1(i,j) = V*A(:,i,j); end end t1 = toc; A = shiftdim(A,1); %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Algorithm 2 tic; temp = zeros(M,N,P); for i = 1:P temp(:,:,i) = A(:,:,i)*V(i); end
B2 = sum(temp,3); clear temp; t2 = toc;
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Algorithm 3 tic v = repmat(reshape(V, [1 1 P]), [M N]); temp = bsxfun(@times, A, v); B3 = sum(temp,3); clear temp; t3 = toc; %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
isequal(B1,B2,B3) isequal(B1,B2) isequal(B2,B3) isequal(B1,B3) t1 t2 t3

