MATLAB: Allocate values avoiding loop

accumarrayallocate matrixavoid loopsreshape matrix

I have the following matrix [t k p]
    1.0000    1.0000   -1.1471
    1.0000    2.0000   -1.0689
    2.0000    1.0000   -0.8095
    2.0000    2.0000   -2.9443
    3.0000    1.0000    1.4384
    3.0000    2.0000    0.3252
and I want an additional column with the mean of p for every t, hence
    1.0000    1.0000   -1.1471   -1.1080
    1.0000    2.0000   -1.0689   -1.1080
    2.0000    1.0000   -0.8095   -1.8769
    2.0000    2.0000   -2.9443   -1.8769
    3.0000    1.0000    1.4384    0.8818
    3.0000    2.0000    0.3252    0.8818
I can do it with the following code
if true
%Calulate the mean
A=[t p_tk];
p_t= accumarray(A(:,[1]), A(:,2), [], @nanmean, NaN);
% allocate it to long form
p_t_long= NaN(size(t));
for d = 1:max(t)
  ind= t ==d; 
  p_t_long(ind)= p_t(d);
end
end
However, I want to avoid loops since I have a big dataset. Can anybody help?

Best Answer

Some indexing using the first column does what you want, more efficiently than a loop or unique:
>> M = [1,1,-1.1471;1,2,-1.0689;2,1,-0.8095;2,2,-2.9443;3,1,1.4384;3,2,0.3252]
M =
   1.00000   1.00000  -1.14710
   1.00000   2.00000  -1.06890
   2.00000   1.00000  -0.80950
   2.00000   2.00000  -2.94430
   3.00000   1.00000   1.43840
   3.00000   2.00000   0.32520
>> V = accumarray(M(:,1),M(:,3),[],@mean)
V =
  -1.10800
  -1.87690
   0.88180
>> M(:,4) = V(M(:,1))
M =
   1.00000   1.00000  -1.14710  -1.10800
   1.00000   2.00000  -1.06890  -1.10800
   2.00000   1.00000  -0.80950  -1.87690
   2.00000   2.00000  -2.94430  -1.87690
   3.00000   1.00000   1.43840   0.88180
   3.00000   2.00000   0.32520   0.88180
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