Creating variable from other variables in MATLAB: code with reproduction

Question

I have a question regarding this type of code:

First = ["A","B","C"];
Second = ["D","E","F"];
Group = ["First", "Second"];
for gr = Group
    current = gr;        
    for number = 1:numel(current)                
        my_variable(number) = current(number);     
    end
end

That's the reproduction of my problem. In this case my_variable is equal to "First" for example. But I wanted it to be "A" then "B" then "C" from variable named "First".

For my understanding this code should do the following:

1st step: for gr = Group means gr = First to Second
2nd step: current = gr; means current = First
3rd step: for number = 1:numel(current) means number = 1:3 (number of elements in "First")
4th step: my_variable(number) = current(number); means my_variable = First(1) = "A"

Instead of that I get my variable equal to "First" or "Second".

I hope you understand what I mean.

P.S. the string arrays I'm using with double quotes (" ") like First = ["A","B","C"]; are only available in Matlab 2016b or later.

You have misinterpreted step 3. numel(current) = numel('First') = 5 as in there are 5 elements in the string First. You could do numel(eval(current)) which would evaluate the variable from the string, but please don't - using eval is bad practice, especially when easily avoidable like here. Instead try using structs or cell arrays — Wolfie
– Wolfie, Commented Oct 3, 2017 at 12:02
Thank you for your comment! Now I see where my understanding is different from what Matlab wants to do. Unfortunely, changing those variables to cell arrays/structure arrays, doesn't change the bad output (still get the name of the variable "First" or "Second" instead of corresponding value, for example "A" or "B". I will try this method with eval instead :(. — BloodyMary
– BloodyMary, Commented Oct 3, 2017 at 13:15
Also, method with "eval" certainly works for counting (step 3), but that's not the solution to my real problem described above. Do you have any other ideas? Thanks! — BloodyMary
– BloodyMary, Commented Oct 3, 2017 at 13:18
What do you actually expect the output to look like here? Because even if this did what you expected, you would over-write my_variable with the values in Second, and those extracted from First are lost... — Wolfie
– Wolfie, Commented Oct 3, 2017 at 13:44
Yes, you're right, but I can repair it later. The real thing is that I want to store two variables in another one, and I want to use that other one in a loop. What I need for my solution is the loop where firstly it takes every element of the first variable (i.e. from variable First, it takes "A", "B" then "C"; and then in a loop it takes every element from variable Second - "D", "E", "F"). I made an error with creating this variable "Group" for loop, because in code it's always a string or structure or cell, but it never creates the "link" to real existing variable "First" or "Second". — BloodyMary
– BloodyMary, Commented Oct 3, 2017 at 13:52

Daniel1000 · Accepted Answer · 2017-10-03 14:48:42Z

2

You should use cell arrays to do this:

first = {'A','B','C'};
second = {'D','E','F'};
group = {first, second};
for group_ind = 1:numel(group)
    current = group{group_ind};
    my_variable = cell(1,numel(current));
    for number = 1:numel(current)
        my_variable{number} = current{number};
        disp(my_variable)
    end
end

For first and second, you can also use the string arrays:

first = ["A","B","C"];
second = ["D","E","F"];
group = {first, second};
for group_ind = 1:numel(group)
    current = group{group_ind};
    my_variable = strings(1,numel(current));
    for number = 1:numel(current)
        my_variable(number) = current(number);
        disp(my_variable)
    end
end

edited Oct 3, 2017 at 14:48

answered Oct 3, 2017 at 14:18

Daniel1000

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