I can't take several non-linear inputs and assign them names for using later in the code.

I am trying to do the following operation:

      a=input('Enter the no. of equations');
      counter=1;
      while counter<=a
      f_counter=input('enter the equation','s');
      F(counter)=f_counter;
      counter=counter+1;
      end

F is a row vector ,supposed to be,changing size and storing the non-linear equations entered by the user. The problem is 'f_counter' is being considered as a single name instead of f_1 , f_2 etc. I am writing a code for Newton's method for non-linear equations(if that helps).

Answer 1

[Matt J]

The problem is 'f_counter' is being considered as a single name instead of f_1 , f_2 etc.

Even if 'f_counter' did what you were hoping, it would (a) be bad coding practice and (b) serve no purpose because you are putting the input in F anyway. There's no reason the inputs have to be held in 2 places, as far as I can see. However, F needs to be a cell array as below

     a=input('Enter the no. of equations');
      counter=1;
      while counter<=a

        F{counter}=input('enter the equation','s');

      counter=counter+1;
      end

[Ahsun Ali]

'Cell contents assignment to a non-cell array object.' This is the error I get while using your idea.Also, If i use a 'F(counter)'vector to store the functions I get 'In an assignment A(I) = B, the number of elements in B and I must be the same.' I am running out of options here please help.

[Matt J]

Pre-initialize F as type cell

     a=input('Enter the no. of equations');

      F=cell(a,1);

[Ahsun Ali]

Thank you very much for your help Matt J. Any ideas to make either the entries or the whole cell array an inline function ? because i'll need to take the jacobian of the equations in this array and also evaluate the function and jacobian at a set of coordinates i.e.X=[points equal to no of unknown variables in the equations]

[Matt J]

I don't have the Symbolic Math Toolbox and there may be better ways of doing this. However, assume you have pre-existing independent sym variables

syms x y

And you now have read in the cell array, F, containing functions of these variables in string form, e.g.,

    >> F={'x+y';'x*y'}

    F = 

        'x+y'
        'x*y'

You can modify it is follows, so that it is a vector-valued function in string form,

    >> F(:,2)={'; '}; F{end,2}=' '; F=F.'; F=strcat('[',F{:},']')

    F =

    [x+y;x*y]

And now, finally I believe you can use symfun() to convert F to a symbolic function of x and y. Then, you can use jacobian, gradient, etc... to your heart's content.

[Ahsun Ali]

Thank you for your prompt response but i'm afraid i can't comprehend the modification you have used. I'm a beginner of MATLAB and have to make this code as an assignment. I would greatly appreciate if you could explain what these steps mean.

[Matt J]

Well, for a full understanding, you'll probably need to read up on cell array manipulation,

http://www.mathworks.com/help/matlab/matlab_prog/create-a-cell-array.html

However, you can also gain an understanding of what my commands are doing by running them individually at the command line and displaying their output. For example, below you can see that F(:,2)={'; '} is just adding a column of semicolons to the original F,

    >> F={'x+y';'x*y'}, F(:,2)={'; '},

     F = 

        'x+y'
        'x*y'

      F = 

          'x+y'    '; '
          'x*y'    '; '

[Ahsun Ali]

I am grateful to you for your great help Matt J. I appreciate your assistance every step of the way. Thank you very much.