MATLAB: Quickest way to get points in a matrix given two 1-D arrays

array manipulation

Let's say that A is an NxN matrix, and B and C are 1-D arrays of the same size, containing numbers between 1 and N.

Is there a better way than going through a "for loopvar=1:length(B)" loop to get the values of A that correspond to the (Bi, Ci)

Example: A is a 5×5 matrix B contains [1 3 5 2 4 5] C contains [2 1 4 3 5 2]

What would be the quickest way to get:

A(1,2) (Corrseponding to A(B(1), C(1)))
A(3,1) (Corrseponding to A(B(2), C(2)))
A(5,4)
A(2,3)
A(4,5)
A(5,2) (Corrseponding to A(B(6), C(6)))

I'd like the quickest way to get this, since I need to run this calculation an enormous number of times with matrices much larger than a 5×5.

Best Answer

A(sub2ind(size(A),B(:),C(:)))

Related Solutions

MATLAB: How to split a vector into unequal sections

>> V = [1;1;2;2;2;2;3;4;4;5];
>> D = diff(find([1;diff(V)~=0;1]));
>> C = mat2cell(V,D,1);
>> C{:}
ans =
   1
   1
ans =
   2
   2
   2
   2
ans =
   3
ans =
   4
   4
ans =
   5

You can use cell array indexing to access the numeric vectors in C.

MATLAB: How to intersect a matrix with a set and keep the rows of the matrix

If you only want to get the min and max of each row, and really want to avoid for loops, here's a solution. This gives the min and max for each row of A, ignoring values not in B, which is I think what you are looking for.

A = [...
  2  12 6  4  8
  1  3  5  7  9
];
B = [ 1 2 3 4 5 6 ];
tf = ismember(A, B);
Amax = A;
Amin = A;
Amax(~tf) = -Inf;
Amin(~tf) = Inf;
maxval = max(Amax, [], 2);
minval = min(Amin, [], 2);

results

>> maxval
maxval =
       6
       5
>> minval
minval =
       2
       1

Best Answer

Related Solutions

MATLAB: How to split a vector into unequal sections

MATLAB: How to intersect a matrix with a set and keep the rows of the matrix

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