How to use genvarname variable after creation?

let's take an ex. from here:

https://www.mathworks.com/help/matlab/ref/genvarname.html

"Example 2

Read a column header hdr from worksheet trial2 in Excel® spreadsheet myproj_apr23:

[data hdr] = xlsread('myproj_apr23.xls', 'trial2');

Make a variable name from the text of the column header that will not conflict with other names:

v = genvarname(['Column ' hdr{1,3}]);

Assign data taken from the spreadsheet to the variable in the MATLAB workspace:

eval([v '= data(1:7, 3);']);"

How do I use the variable that is created?

[Stephen23]

"How do I use the variable that is created?"

By using more slow, complex, inefficient code just like the code that you used to create that variable.

Is there a particular reason why you cannot use simpler and more efficient code (e.g. indexing, tables)?

Answer 1

[Rik]

This is a terrible idea. You are forced to use eval every time.

The only time you wouldn't need eval is if you are using it to access fields of a struct. I would say that is the primary use case of this function.

[Steven Lord]

Using it to access or create fields of a struct array is one use case. You could also use it to access or create variables in a table or timetable array.

r = randperm(5).';
t = table(r);
s = genvarname('hocus pocus')

s = 'hocusPocus'

t.(s) = r.^2

t = 5×2 table
    r    hocusPocus
    _    __________

    2         4    
    1         1    
    4        16    
    3         9    
    5        25    

[Rik]

I keep forgetting table exists. Maybe because I don't use them and they look to me like syntactic sugar for a struct vector. I suspect I'm just missing the point.

[Steven Lord]

Storing the data from that sample table I created in a struct would require deciding which type of access to the data you're going to do more often and so want to make easier.

r = randperm(5).';
hocusPocus = r.^2;
% Scalar struct with array fields
s1 = struct('r', r, 'hocusPocus', hocusPocus);
% Or array of structs with scalar fields
s2 = struct('r', num2cell(r), 'hocusPocus', num2cell(hocusPocus));

It's easy to retrieve all the elements of one of the fields using s1 but more difficult to get a specific element from both r and hocusPocus.

allOfR = s1.r

allOfR = 5×1

3 5 1 4 2

secondElements.r = s1.r(2);
secondElements.hocusPocus = s1.hocusPocus(2)

secondElements = struct with fields:
             r: 5
    hocusPocus: 25

It's harder to get all of the elements of one of the fields using s2 but it's easier to get a specific element from both r and hocusPocus. The first one looks easy but that's because I took advantage of my knowledge that r contains scalars and so can be concatenated into a column vector. In the general case this is trickier.

allOfR = vertcat(s2.r)

allOfR = 5×1

3 5 1 4 2

secondElements = s2(2)

secondElements = struct with fields:
             r: 5
    hocusPocus: 25

In a table array, you can slice across rows or down variables basically equally easily.

t = table(r, hocusPocus);
allOfR = t.r % or

allOfR = 5×1

3 5 1 4 2

allOfR = t(:, 'r') % or t(:, 2)

allOfR = 5×1 table
    r
    _

    3
    5
    1
    4
    2

secondElements = t(2, :)

secondElements = 1×2 table
    r    hocusPocus
    _    __________

    5        25