MATLAB find and apply function to values of repeated indices

Question

I have a 352x11 matrix, indexed by column 1 with 10 data points. Some of the index values are repeated. I\'d like to find the repeated indices and calculate the mean data po

Answer 1

Given you input

x = [ ...
    26   77.5700   17.9735   32.7200; ...
    27   40.5887   16.6100   31.5800; ...
    28   60.4734   18.5397   33.6200; ...
    28   35.6484   27.2000   54.8000; ...
    29   95.3448   19.0000   37.7300; ...
    30   82.7273   30.4394   39.1400];

You can create an array of indexes where duplicated vgalues share the same index, using the third output of unique.

%Get index of unique values (1 - N)
[~, ~, ix] = unique(x(:,1))

Then you can use this array to rebuild your matrix, combining duplicated values with the function of your choice.

%Use accumarry to rebuild the matrix one column at a time
result = [...
    accumarray( ix, x(:,1), [], @max )  ...  %Many functions works here, as all inputs are the same.  E.G.  @mean, @max, @min
    accumarray( ix, x(:,2), [], @mean ) ...  %Use mean to combine data, per problem statement.
    accumarray( ix, x(:,3), [], @mean ) ...
    accumarray( ix, x(:,4), [], @mean ) ...
    ]