Extract data from all values of a containers.Map collection

Hi all,

Suppose I have a containers.Map key/value collection with n entries (hundreds and more). Suppose all entries have a filed ‘My_Filed’ which is an array of 1x3 doubles.

I want to create a matrix of nx3 from all these values. Of course, I can achieve this by getting all values and use a ‘for loop’ to build the matrix (with pre-allocation for speed).

My question is: is there a faster way to create this matrix?

Thanks,

Alon

Answer 1

[Guillaume]

This doesn't involve any loop, either explicit or via any of the ***fun functions. However all the structures in the map must have identical fields and all the structures must be scalar:

allvalues = yourmap.values;  %extract values, stored as a cell arrays
allvalues = vertcat(allvalues{:});  %convert cell array of N structures into a Nx1 structure array, will fail if at least one structure has different fields
fieldvalues = vertcat(allvalues.My_Filed);  %extract field into a Nxm matrix. Will fail if at least one field is not the same size as the others

[Alon Rozen]

Thanks!!!

It works :)

Answer 2

[Mohammad Sami]

I assume that each of value in the container is a struct with identical fields.

You can just get all the values like this

% M = containers.Map
valueSet = values(M); % this will return a cell array of structs.
My_Filed = cellfun(@(x)x.My_Filed,valueSet,'UniformOutput',false);
My_Filed = vertcat(My_Filed{:});

[Guillaume]

Note that cellfun is not very different from a loop. If anything, it's most likely slower than a loop due to the cost of the anonymous function call.

I would still use cellfun over an explicit loop as it's clearer at what it does.

[Alon Rozen]

Thanks,

It is similar the answer I accepted - but I prefere the one without the cellfunc. These functions tends to be slower then a loop in many cases.