for loop indexing in temporary arrary problem in MATLAB.

Hello Friends,

I have the following code:

P = [1, 0, 3];
Q = [0, 4, 5];  

AB = [P + Q];  % A vector
CD = [P - Q];  % A vector
EF = [P ./ Q]; % A vector

M = {'AB', 'CD', 'EF'};

for i = 1:length(M)

      P1 = P(M{i}~=0);          %It removes those elements of P which correspond to 0 entries in M.
      t = M{i};                 %Create a temporary variable.
      M = t(M{i}~=0);           %It removes 0 entries from M.

      P = P1(~isnan(M(i)));     %It removes those elements of P which correspond to NaN entries in M.
      M = t(~isnan(M(i)));      %It removes NaN entries from M.

      if strcmp(M, 'AB')
          f = f(P,M);
      elseif strcmp(M, 'CD')
          f = g(Q,M);
      elseif strcmp(M, 'EF')
          f = h(R,M); 
      end
end

This code is not computing P1, t, M, P values properly. For example the following line of code takes M{i} = AB for i = 1, but gives totally wrong answer.

I have tried to change {} to () and [], etc., but nothing works. I will appreciate any advice.

P1 = P(M{i}~=0);

Answer 1

[Geoff Hayes]

Your line of code has me confused

 M = t(M{i}~=0);           %It removes 0 entries from M.

The comment says that you are removing zero entries from M. But M has been defined to be a cell array of strings as

 M = {'AB', 'CD', 'EF'};

What is the intent of the above assignment? Do you really mean for M to be a cell array of strings, or do you mean it to be a cell array of numbers?

 M = {AB, CD, EF};

Also, the line of code

 CD = [P*Q]

will generate an error since P and Q do not have the compatible dimensions for matrix multiplication.

Use the debugger to step through the code and you will probably get a good idea as to what is going on. Always look at the variables to verify that they are (with respect to dimension, type, etc.) what you expect them to be.

[hello_world]

Hi Geoff,

Thank you for your response.

The purpose to show AB = P +Q; CD = P*Q; EF = P ./Q was just illustrate that how they are computed to give a array of vector. They could be any functions giving an output vector. I have edited them in my question above not to confuse.

The main problem is how to pass computed values of vector AB, CD, EF into the for loop to remove 0 and NaN entries. Here I need advice.

[Geoff Hayes]

But you will still have the same problem of your M cell array containing strings and not numbers.

 M = {'AB', 'CD', 'EF'};

so M{1} is the string 'AB' and not the sum of P and Q. I would just do

 M = {AB, CD, EF}

which will allow you to continue, but will cause other problems. The for loop is iterating over each element of M but then you do

 M = t(M{i}~=0);

which removes the zero entries from M{i} and not from M as the comment indicates. I suspect that you may want

 M{i} = t(M{i}~=0);

The subsequent lines using P and P1 are confusing. And then the code goes back to comparing M to strings.

You have to decide what M is - is it a cell array of strings or a cell array of numbers?

[Geoff Hayes]

I see that you have a similar question at http://www.mathworks.com/matlabcentral/answers/284841-how-to-compare-each-content-of-string-which-is-of-type-double-matrix-in-if-loop-recursively. I will assume that this question can be closed.

Answer 2

[Todd Leonhardt]

I don't understand how that code could even get far enough to attempt to compute P1, etc. It should error out on the 4th line of code where you attempt to compute:

    CD = P * Q;

P and Q are both 1 x 3 matrices, so you can not perform matrix multiplication on them as such. You can do any one of the following:

    CD = P' * Q;  % Take transpose of P, P', so it is a 3 x 1 matrix and results is a 3x3
    CD = P * Q';  % Take transpose of Q, Q', so it is a 3 x 1 matrix  and result is a 1x1 scalar
    CD = P .* Q;  % Perform element-wise multiplication so result is a 1 x 3

[hello_world]

Hi Todd,

Thank you for your reply. I have commented above to Geoff as he had the similar questions. Please check my modified question.