I am not able to use eval for a string of variables in order to evaluate these variables in a multidimensional array.

I am not able to use eval for a string of variables in order to evaluate these variables in a multidimensional array. In fact, I want to use one of these index variables as ':'. My code is as following:{

X is a multidimensional array 
%%Initialisation of 15 variables
for i=1:length(X) % length(X)=15
  eval(sprintf('i%d = 1', i));
double(strcat('i',num2str(i)));
end
for i=1:length(X) 
||||| node(i) = X(eval(sprintf('i%d,',[1:length(X)]))); % this doesnt work.||||||| 
}%%

Also I want to use one of these variables as ':' to take all the dimension of the considered index. if the statement above work, then i will be able to initiate one of these variable as ':'

[Geoff Hayes]

Elias - have you considered not using eval (since using it can lead to buggy code, problems like the above, etc.)? See Alternatives to the eval Function for details which probably suggests using an array to manage the data instead of individual variables...

[Stephen23]

length should definitely be avoided: use size instead, and specify the dimension.

In any case, what are you actually trying to do? Given that each variable has the value 1, what output do you expect to get in node ? Please give a simple example.

As far as I can tell, your attempt at indexing using sprintf will generate a string like this:

1,1,1,1,1,1,1,1,...

and you want to use that as an index. However each iteration of the second loop is identical, so node just contains the same value repeated. Apart from the various syntax and design issues, what are you hoping to achieve with that?

Your code is very confusing as you use length (which measures one dimension of the array) to generate a list of 15 indices which you then try to apply to all dimensions of the array. This does not make much sense. What is the actual output that you need?

[Elias Safatly]

Exactly Stephen, sprintf generates fivteen variables i1,i2,...,i15 In a first time they are equal to 1 as you wrote. Now, how can I use alternative functions to use

 X(eval(sprintf('i%d,',[1:length(X)]))).

In fact i am coding on tensors and i want to write a function to extract fibers without writing the whole X(1,1,1,1,1,1,1,1,1,1,1,1,1,1,:)

[Stephen23]

"In fact i am coding on tensors and i want to write a function to extract fibers without writing the whole X(1,1,1,1,1,1,1,1,1,1,1,1,1,1,:)"

Then use the method I showed in my answer, which is much simpler than what you are trying to do.

And in future, remember that eval forces you into writing slow, complex, buggy code that is hard to debug. Avoid using eval:

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

Answer 1

[Stephen23]

Just use a cell array of indices, which is much simpler and much more efficient. And no loop is required either:

X = randi(9,5,4,3,2) % some ND array
C = repmat({1},ndims(X)); % indices in cell array
...
X(C{:})

This is exactly equivalent to writing

X(C{1},C{2},C{3},...,C{end})

thus giving you the indexing that you require. You can easily change the (index) values in C, it is very straightforward, e.g.:

C{2} = 3;

It also makes it easy to specify ranges of a dimension, e.g.:

C{2} = 1:6;

or to get all of any dimension, e.g.:

C{2} = ':';

"i want to write a function to extract fibers without writing the whole X(1,1,1,1,1,1,1,1,1,1,1,1,1,1,:)"

Now you know how easy this is:

C = repmat({1},1,ndims(X));
C{end} = ':';
X(C{:})

Read these to know how it works:

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

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

https://www.mathworks.com/matlabcentral/answers/320713-how-to-operate-on-comma-separated-lists

[Elias Safatly]

Ok, but i need that indices be i1, i2,...,i15 and not always 1! My purpose is to generate a function which extract fibers. For example, in a matrix A, i want extract A(1,:) without writing A(1,:) i want to generate a string '1,:' and evaluate it in A.

In C = repmat({1},1,ndims(X)); C{end} = ':' X(C{:})

in {1} how can put a variable as an index instead of 1! this will solve the problem.

In other words, C= C =

1×15 cell array

    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}    {[1]}

in the first cell i have to put i1, in the second i have to put i2, in the third cell i have to put i3, ... in the last i15 ...

[Stephen23]

"Ok, but i need that indices be i1, i2,...,i15 and not always 1!"

I have no idea why you think that my answer always gives the same index. You can:

• create the cell array with any index values you want.
• use indexing to change any of the cells to any index values you want.

Cell arrays are not limited to only holding the value 1, and neither is my answer.

In my answer I showed various ways to change the cell values, to include one value, ranges, or the entire range. I suggest that you read my answer again, as you seem to have missed those examples.

"For example, in a matrix A, i want extract A(1,:) without writing A(1,:) i want to generate a string '1,:' and evaluate it in A."

This is the wrong approach. Do not evaluate strings like this, unless you want to force yourself into writing slow, complex, buggy code (like the code in your question). In any case, I already showed you how to do this using much simpler, more efficient code:

C = {1,':'}
A(C{:})

Notice how much simpler that is, compared to what you are trying to do. Of course you can change the cell contents to whatever indices you want, to apply those different indices to A.

"in the first cell i have to put i1, in the second i have to put i2, in the third cell i have to put i3, ... in the last i15 ..."

That is exactly what you should avoid doing, and my answer shows you how.

Magically accessing variable names is slow, complex, and (as your question already proves) pointlessly buggy. In contrast my answer is simpler, much more efficient, less buggy, and easier to debug.

Your approach of string evaluation is like shooting yourself in the knee before running a race. The MATLAB documentation specifically recommends against what you are doing: "A frequent use of the eval function is to create sets of variables such as A1, A2, ..., An, but this approach does not use the array processing power of MATLAB and is not recommended. The preferred method is to store related data in a single array."

source: https://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html

I put your indices into a single array, just as the documentation recommends. It makes your code faster, simpler, and much more efficient. Do you have a particular reason why you need to write slow, complex, buggy code?

[Elias Safatly]

Thank you a lot for your detailed answer. Yes I will give you the reason, my principle code for a multidimensional array with three modes is like that:

function  test_3jrs(A1,A2,A3)
  for i=1:size(A1,1)
  for j=1:size(A2,2)
  for k=1:size(A3,3)
      if((A1(i,j,k)==max(double(A1(i,:,k))))&&(A1(i,j,k)==max(double(A1(i,j,:))))&&...
              (A2(i,j,k)==max(double(A2(:,j,k))))&&(A2(i,j,k)==max(double(A2(i,j,:))))&&...
              (A3(i,j,k)==max(double(A3(:,j,k))))&&(A3(i,j,k)==max(double(A3(i,:,k)))))
Y=['Dans la Position  (',num2str(i),',',num2str(j),',',num2str(k),')', ' Le payoff','(',num2str(A1(i,j,k)), ',' ...
              ,num2str(A2(i,j,k)),',' ,num2str(A3(i,j,k)),')', 'est un equilibre USE']; 
          display(Y)
      end

    end
end

Here, A1, A2 and A3 are multidimensional array 2x2x2.

This algorithm is easy to be wrriten with 3 modes dimensions.

But, if now I have 15 modes dimensions, it will be so hard to write each time with the 15 loops for, ( for each i%d) X(i1,...,i15) and deplacing the ':'

In the language of tensors (multidimensional array) i want to extract at each time a fiber. and in the toolbox of tensors, functions to extract fibers doesnt exist.

[Stephen23]

"But, if now I have 15 modes dimensions, it will be so hard to write each time with the 15 loops for, ( for each i%d) X(i1,...,i15) and deplacing the ':'"

I totally agree, that would be hard to write out.

Which is exactly why my answer does not write anything out 15 times, but instead gives you a simpler, easier method to use, which uses just one cell array, and some indexing into that cell array. It is easy to create a cell array for any number of dimensions (I showed you how in my answer), and to change its values (I showed you how in my answer).

Here is a simple example, maybe you might like to try this:

A = rand(4,3,2,2,2,2,2); % as many dimensions as you want
C = repmat({':'},1,ndims(A));
for ii = 1:size(A,2)
    C{2} = ii; % update index of 2nd dimension
    for jj = 1:size(A,1)
        C{1} = jj; % update index of 1st dimension
        A(C{:})
    end
end

In this example I get all of the subarrays for each of the first two dimensions, which thus correspond to these:

A(1,1,:,:,:,...)
A(2,1,:,:,:,...)
...
A(end,1,:,:,:,...)
A(1,2,:,:,:,...)
...
A(end,end,:,:,:,...)

I did not write out lots of indices. I did not evaluate any strings. I was not limited to only the index 1.

You might well have problems due to your algorithm having so many nested loops, but that is an unrelated issue that you will have to figure out when and if it turns out to be a problem. Possibly you could avoid some/most/all of the loops by writing vectorized code.

[Elias Safatly]

Thanks a lot, yes I see. Your answer can resolve my problem!

Best Regards

[Stephen23]

@Elias Safatly: I hope that it works well for you. Please let me know how it goes! Come back and ask if you need more explanation or examples :)

[Elias Safatly]

@Stephen Cobeldick Yes it works. Really I am so thankful. I coded also a function to extract fibers and subarrays...Wonderful!