How do i create variables from strings for the purpose of writing different variables to text file.

I want to be able to do something like choose scenario a and get 5 text files with the numbers 1,2,3... 2,2,3... 3,2,3... 4,2,3... 5,2,3 and then with the same hardcode but in a new run choose the letter b and now produce 5 text files like 1,1,3... 1,2,3... 1,3,3... 1,4,3... 1,5,3. Any help on this matter is greatly appreciated. My not working attempt is shown below.

My thought process is to try to redefine a vraiable name by reading the string I enter, making a variable called that string (overwriting the predefined ones) and then trying to somehow make that the argument of the for loop.

a = 1
b = 2
c = 3

prompt = 'what would you like to sweep over\r\n'
sweepstr = input(prompt,'s')
sweep = genvarname(sweepstr)

for sweep = 1:1:5

filename = sprintf('test%s%i.txt',sweepstr,sweep);
circuitfile = fopen(filename,'w');
fprintf(circuitfile,' %i \r\n %i \r\n %i',a,b,c);
fclose(circuitfile);

end

Thanks in advance for all and any help

[Stephen23]

"My thought process is to try to redefine a vraiable name by reading the string I enter, making a variable called that string (overwriting the predefined ones) and then trying to somehow make that the argument of the for loop"

That would be a pointlessly difficult, slow, complex, buggy, and hard to debug way to try and solve this problem. Here is why this approach is not recommended:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

A much simpler approach is to put the data into one small vector and use indexing. Then this task is trivial to solve, using neat and highly efficient indexing.

Answer 1

[Stephen23]

This is trivial when you put those values into one small vector. Then you can use indexing very simply:

V = [1,2,3];
S = input('var to sweep (1/2/3): ','s');
X = str2double(S);
for k = 1:5
    V(X) = k;
    fprintf('%d\n%d\n%d\n',V)
end

Tested:

>> temp0
var to sweep (1/2/3): 3
1
2
1
1
2
2
1
2
3
1
2
4
1
2
5

If you want to let the user select a name rather than an index, then just add strcmp, something like this:

S = input('var to sweep (a/b/c): ','s');
X = find(strcmp(S,{'a','b','c'}));

Tested:

>> temp1
var to sweep (a/b/c): c
1
2
1
1
2
2
1
2
3
1
2
4
1
2
5

[Thomas Dixon]

Thanks for this rapid response to the eval problem. Let's say I wanted to retain the name of the variable and the variable value for use in a for loop for example. I have three parameters paramstr = ['current','capacitance','inductance'], paramvar = [current,capacitance,inductance] I ask the user what he wants to sweep over (they all sweep from 1:1:5) while the others remain constant current = 1, capacitance = 2, inductance = 3 and then I create a text file which says filename = ['%s sweep_current=%iamps.txt',(string of thng being swept),(value at this iteration of loop)] is this possible?

Thanks again

[Stephen23]

"is this possible?"

A simple, intuitive approach is to use switch:

a = 1;
b = 2;
c = 3;
S = input('var to sweep (a/b/c): ','s');
for k = 1:5
    switch S
        case 'a'
            a = k;
        case 'b'
            b = k;
        case 'c'
            c = k;
    end
    fprintf('%d\n%d\n%d\n',a,b,c)
end

I am sure that you can see how this will get ungainly quite quickly, if you want more variables. A neat alternative would be to access the fields of a structure:

A.a = 1;
A.b = 2;
A.c = 3;
S = input('var to sweep (a/b/c): ','s');
for k = 1:5
    A.(S) = k;
    fprintf('%d\n%d\n%d\n',A.a,A.b,A.c)
end

Both of these will be must more efficient and less buggy than magically trying to access variable names dynamically.

[Thomas Dixon]

This is great! As you have probably assumed the code has more than 3 variables and as you rightly assume this has got unwieldy very quickly which is where my problem started. I did use the case scenario but I am writing a large circuit file which as passed to a different program and then the output is analysed in MATLAB. It got too much when trying to make one alteration. I like the structure idea and will give it a go. Be ready for my next question. Many thanks :) Tom

[Stephen23]

@Thomas Dixon: I hope that it helps. Structures are highly recommended for handling lots of parameters like this: they make them easy to access, to pass to/from functions, and intuitive to read/understand.

Questions are always welcome, that is why we are here!