MATLAB: Selecting Value in matrix already indexed

matrix indexing

Hi, I'm trying to something relatively simple. Is there a way to select certain value in an already indexed matrix. Here is a simplified exemple of what I want to do.
A = rand(3);
B = @(x) A + x.^2; %Where x is a vector 3x1. Result is thus a 3x3 matrix.
C = @(x) B(x)(:,1:2); %I want C to be the first two columns of the result of B(x)
Obsiously, B(x)(:,1:2) isn't a valid way to write things. Is there a way to do this without finding a work around?
Thanks for the help.
RMT

Best Answer

Some options:
1. avoid the situation by defining a function in an Mfile. This is the simplest and probably most efficient solution.
2. call subsref directly to do the indexing:
>> A = rand(3);
>> Bf = @(x) bsxfun(@plus,A,x.^2);
>> Cf = @(x) subsref(Bf(x),substruct('()',{':',1:2}));
>> Bf(1:3)
ans =
   1.1974   4.4016   9.1932
   1.6292   4.8217   9.0481
   1.6670   4.1590   9.4776
>> Cf(1:3)
ans =
   1.1974   4.4016
   1.6292   4.8217
   1.6670   4.1590
3. Define an intermediate function to do the indexing:
>> A = rand(3);
>> Bf = @(x) bsxfun(@plus,A,x.^2);
>> If = @(M) M(:,1:2);
>> Cf = @(x) If(Bf(x));
>> Bf(1:3)
ans =
   1.5517   4.7931   9.7715
   1.0754   4.6154   9.4393
   1.6711   4.5820   9.2449
>> Cf(1:3)
ans =
   1.5517   4.7931
   1.0754   4.6154
   1.6711   4.5820
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