How to call index of vector from matrix?

Hello, I have: n-by-3 matrix A, where xi,yi,zi are non-integer(e.g., x=-1.23;y=2.45;z=123.11):

   A contain 10million element
    A=[ x1 y1 z1  % element 1         
        x2 y2 z2  % element 2  
        x3 y3 z3  % element 3  
        x4 y4 z4  % element 4  
        .......
        xn yn zn] % element n , where n=10,000,000

   B=[ x3   y3   z3      % B contain 2 elements       
       x70  y70  z70]

Question: How to call index of matrix B from matrix A?(i don't wanna use "find(ismember(A,B,'rows'));" , since time consuming with large matrix A & B)

result=[3;70]

[Image Analyst]

I don't know what "call index of matrix" or "call index of vector" means. Perhaps you can get a native English speaker to look it over. And what would you want? Would you want a 2-by-1 vector of rows where the numbers are found? Like output = [3; 70]? Or what??? Or do you want null/empty because row 3 followed by row 70 never ever appear in A?

Answer 1

[Rik]

The code below executes in 0.35 seconds. This will scale about linearly with the number of rows in B.

%Generate data
A=rand(10000000,3);
B=A([3 70],:);

tic
%loop through B to find row indices
result=NaN(size(B,1),1);
for n=1:length(result)
    true_row=...
        A(:,1)==B(n,1) & ...
        A(:,2)==B(n,2) & ...
        A(:,3)==B(n,3);
    row_idx=find(true_row);
    if numel(row_idx)==0
        %row not found, keep NaN
    elseif numel(row_idx)>1
        %more than 1 row found, throw warning
        warning('no unique match for row')
        %you can use the line below as well, if you just want to use the
        %first row that matches.
        %result(n)=row_idx(1);
    else
        result(n)=row_idx;
    end
end
toc

[ha ha]

With size of B is small, time execution of your code is acceptable. But , If size of B is large,e.g,. the number of element in B is 1000 elements(Since in my real application the number of element in B is just less then number element in A), your code is time consuming compared with "ismember"

[Rik]

Comparing many elements just is very time consuming. Use the profiler or tic and toc to figure out which method is fastest for you application. I don't think there are many ways to improve beyond ismember or the method I outline here. You might check to see if arrayfun or parfor yield any speedup.

[ha ha]

@Rik Wisselink. Thank so much