Define a matrix elementwise

I would like to define the matrix Q based on the given matrix A and B (both very large). So can I define like this?

Q(i,j) = A(i,j);
if B(i,j)=0
0; elseif A(i,j)=0    
else 1/abs(A(i,j));
end

[James Tursa]

Your code does not make sense. Can you fix it so that we can understand the algorithm you are trying to implement?

[MatLab]

I want to define the following matrix Q, such that Q_ij=A_ij when B_ij=0 Q_ij=0 if A_ij=0, otherwise set Q_ij=1/|A_ij| where A and B are defined in the algorithm

[dpb]

Which is what the Answer provided does...

>> A=randn(4)  % sample data
A =
  0.9642   -0.7982    1.3514   -0.8479
  0.5201    1.0187   -0.2248   -1.1201
 -0.0200   -0.1332   -0.5890    2.5260
 -0.0348   -0.7145   -0.2938    1.6555
>> B=ones(4);B(randperm(numel(A),round(0.2*numel(A))))=0  % hit-n-miss zeros in B
B =
   1     0     1     1
   1     1     1     0
   1     0     1     1
   1     1     1     1
>> Q=1./abs(A).*(B~=0); Q(isnan(Q))==0;
Q =
  1.0371         0    0.7400    1.1793
  1.9229    0.9817    4.4490         0
 49.9305         0    1.6977    0.3959
 28.7595    1.3995    3.4042    0.6040
>>

[MatLab]

Thanks, appreciated

[MatLab]

So what to do if the Q is not square matrix, can you define a rectangular matrix with all 1?

[dpb]

Makes no difference as long as A,B same size...and Q is size(A). Did you even try it?

[MatLab]

Sorry I m new to Matlab, I tried but I m wondering how to define Q as a variable of A,B first. It always say, functions definition are permitted this way

[dpb]

Sorry, don't understand the question...you have an A and B? If so, just type in what I did...Matlab allocates automatically on assignment, there's nothing needed a priori.

[MatLab]

I understand your code, however, I want to define the function Q(A,B) and employ it in other parts of my algorithm. So I want to define an actual function instead of running it under command menu. Hopefully, this clarifies

[Walter Roberson]

function result = Q(A,B)
  result = 1./abs(A).*(B~=0);
  result(isnan(result))==0;

and store it in Q.m

[MatLab]

Thanks so much, this really helps. Appreciated! I wrote function Q(A,B)=result instead, it didn't run

[dpb]

Ah, sorry I didn't follow where the hangup was...note now that if you follow Walter's lead and name the function Q if you subsequently write

Q=Q(A,B);

that by Matlab parsing rules you will have aliased the definition of the function Q by a resulting array Q and won't be able to use the function again until clear Q or change context such that the array isn't in scope or the like.

Answer 1

[dpb]

Q=1./abs(A).*(B~=0);  %  1/abs(A) and zero B locations
Q(isnan(Q))==0;       %  fixup 0 locations from A  (logical*Inf-->NaN)

ADDENDUM

Or, if one doesn't like the cleanup after the fact, one can do it first...

A(A==0)=inf;          % so 1/Inf-->0
Q=1./abs(A).*(B~=0);  % 1/abs(A) and zero B locations