MATLAB: Copying variables from table to struct based on certain criteria - matlab

Question

I have a table
column1data = [11; 22; 33];
column2data = [44; 55; 66];
column3data = [77; 88; 99];
rows = {'name1', 'name2', 'name3'};
T = table(column1data, column2data, column3data);
T.Properties.RowNames = rows
column1data column2data column3data
name1 11 44 77
name2 22 55 88
name3 33 66 99
and a struct array
S(1).rownamefield = 'name3';
S(2).rownamefield = 'name1';
S(3).rownamefield = 'name2';
S(1).columnnumberfield = 1;
S(2).columnnumberfield = 3;
S(3).columnnumberfield = 2;
S(1).field3 = [];
S(2).field3 = [];
S(3).field3 = [];
rownamefield columnnumberfield field3
1 'name3' 1 []
2 'name1' 3 []
3 'name2' 2 []
The struct array S contains criteria needed to pick the variable from table T. Once the variable is picked, it has to be copied from table T to an empty field in struct S.
S(1).rownamefield contains the name of the row in table T where the target variable resides. S(1).columnnumberfield contains the number of the column in table T with the target variable. So S(1).rownamefield plus S(1).columnnumberfield are practically the coordinates of the target variable in table T. I need to copy the target variable from table T to field3 in the struct array: S(1).field3. This has to be done for all structs so it might need to be in a for loop, but I am not sure.
The output should look like this:
rownamefield columnnumberfield field3
1 'name3' 1 33
2 'name1' 3 77
3 'name2' 2 55
I have no idea how to approach this task. This is, of course, a simplified version of the problem. My real data table is 200x200 and the struct array has over 2000 structs. I will greatly appreciate any help with this.

Answer 1

You could do something like the following.
First convert the rownamefield and columnnumberfield fields to cells and arrays to use as indices for the table.
rows = {S.rownamefield};
cols = [S.columnnumberfield];
subtable = T(rows, cols);
This gives you a square table which you can then convert to a cell and take the diagonal elements which are the ones you care about.
values = table2cell(subtable);
values = values(logical(eye(numel(rows))));
Then this gives a cell array of the values corresponding to the entries in S. We can then assign them
[S.field3] = deal(values{:});
disp([S.field3])
33 77 55
This would be much easier if table had an equivalent to sub2ind.

Answer 2

% Extract table data and linearly index it
tdata = T{:,:};
[~,row] = ismember({S.rownamefield}, T.Properties.RowNames);
col = [S.columnnumberfield];
pos = sub2ind(size(tdata),rowpos, col);
val = tdata(pos);
% Assign to struct
for ii = 1:numel(S)
S(ii).field3 = val(ii);
end
Instead of the for-loop, you can use Suever's solution with the deal() to assign values in one go (have to num2cell(val) first). Whatever is faster and more intuitive.