Append results into an array in a for loop as in python

I created an array to store results from a for loop, looks like it seems to break

vertices = zeros(length(Data.X3),1);

for i = 1:length(Data.X3)

[vertices(i), ~]= Vertex([Data.X3(i); Data.Y3(i); Data.Z3(i)])

end

But I am not getting the desured results

[VBBV]

Can you elaborate what desired results mean?

[Reuben Addison]

I want the results stored in the array labelled "vertices"

[Cris LaPierre]

What is Vertex? A function? An array?

[VBBV]

Please attach complete code and/or data

[dpb]

Besides @Cris LaPierre's Q?, what is the content of Data.X, ...? Is it actually the data itself or an index into the data? As written, the expression [Data.X3(i);Data.Y3(i);Data.Z3(i)] creates a column 3-vector and making the assumption that Vertex() is an array, then each call will return a 3-vector of the values at those indices, if they are indeed valid indices into the array. If they're actually values instead, then "Boom!"; either likely will be indices outside the range of the array or invalid floating point values attempted to be used an indices.

On the LHS, the "~" tells MATLAB to throw away any second returned value from a function call and the single index (i) on the preallocated array is attempting to store three elements into a single location, so that is bad syntax from multiple points of view.

IF the Data structure elements are indeed valid indices into the Vertex array, then the output vector size must be 3X the size of each of those if the idea is to concatenate all into one long column array; given that it is preallocated only to that size vertically and by the use of a second element in the LHS expression, I'm guessing the intent was to produce a 2D array of the X,Y,Z locations. If that is the case, and also guessing that the "3" on the stuctuure names implies a 3D array, then "the MATLAB way" using vectorized operations would simply be

vertices=[Vertex(Data.X3, Data.Y3,Data.Z];

The above makes lots of assumptions, but without the details of what really have, it's about best can make a stab at -- other than the syntax issues.

[Reuben Addison]

So Vertex is a function that returns the vertex number and vertex coordinates (I am trying to get rid of the vertex number hence the tilde and RHS goes through a data table to pick out some numbers to compute the vertex number and coordinates whereas the LHS is suppose to store the results

Answer 1

[Cris LaPierre]

Not sure what the desired results are, but here's a sample of your code. It works as I'd expect. Perhaps you can be more clear on what is not working for you.

% Make up some data
X3 = rand(10,1);
Y3 = rand(10,1);
Z3 = rand(10,1);
Data = table(X3,Y3,Z3)

Data = 10×3 table
       X3         Y3          Z3   
    ________    _______    ________

     0.26829    0.41537     0.57013
     0.54631    0.64146     0.97118
     0.24915    0.58069     0.31743
     0.83112    0.80822     0.42471
    0.035294    0.52805     0.59974
     0.46136    0.57216     0.47514
     0.31929    0.89843     0.11462
     0.22365    0.53646     0.36506
     0.94193     0.6462     0.87688
     0.11137    0.65684    0.056632

% your original code
vertices = zeros(length(Data.X3),1);
for i = 1:length(Data.X3)
    [vertices(i), ~]= Vertex([Data.X3(i); Data.Y3(i);	Data.Z3(i)]);
end

% View results
vertices

vertices = 10×1

3 3 2 1 3 2 2 2 1 2

% made up function for demonstration purposes
function [vertex_number, vertex_coord] = Vertex(D);
[vertex_coord, vertex_number] = max(D);
end