Unclear as to why I'm getting a dimension mismatch?

This is the bit of code that is giving me trouble:

 >> strcat('spectra',num2str(j)) = zeros(1340,4);
 ??? Subscripted assignment dimension mismatch.

Only thing I can think of is that matlab believes I'm attempting to assign a matrix into a string. Where as what I'm trying to do is assign the string as the variable for a matrix. For reference j is just an integer counter.

edit Basically trying to create an increasing variable creation so that for each loop of the for fxn I get a new matrix with the assigned variable that then has memory preallocated.

edit2

 eval(['spectra' int2str(j) '=zeros(1340,4);'])

Is what I finally settled on... the full loop:

 for j=1:size(filelist,1)
     eval(['spectra' int2str(j) '=zeros(1340,4);'])
     eval(['spectra' int2str(j)  '(1:1340,3:4)=xlsread(filelist(j,1:end));'])
    for k=1:1340
        eval(['spectra' int2str(j) '(k,1)= k*correction(1,1)+correction(1,2);'])
        eval(['spectra' int2str(j) '(k,2)=(10^7)/532-spectra' int2str(j) '(k,1);'])
        eval(['spectra' int2str(j) '(k,3)=1/spectra' int2str(j) '(k,2)*(10^7);'])
     end
 end

I've seen some mentions of increased speed doing other methods. While this isn't really a time intensive script, I wouldn't mind speeding it up a bit so its a bit more responsive when I'm working.

Thanks for the responses all!

Answer 1

[the cyclist]

This is how to do what you are asking to do:

 eval([['spectra',num2str(j)],' = zeros(1340,4)']);

However, Walter is absolutely correct that cell arrays are the way to go on this. Use his syntax, and you will have variables effectively named "spectra{1}", "spectra{2}", etc instead of spectra1, spectra2, etc. Your code will be more readable and efficient, too.

[Karl]

Hey Cyclist,

Thanks for the response. I'd love to see a suggested bit of code using that method. I spent a good amount of time figuring out the eval method only to come back and see you having already posted it! Man...

But if your willing, how would I do this using the cell arrays?

[the cyclist]

Heading out the door so can't write it up, but basically wherever you have "spectra int2str(j)" just use spectra{j} instead and you don't need the eval. Note the curly brackets instead of parentheses.

Answer 2

[Walter Roberson]

spectra{j} = zeros(1340,4);

See this FAQ

[Karl]

Thx for the reply.

I'm using the strcat('spectra',num2str(j)) line to give me a changing variable name for the matrix based on each iteration of the counter. Ie. first time around I get Spectra1 = zeros(1340,4);, second time spectra2, third time spectra3. Essentially I'm using it because I don't know how many spectra I'm going to use.

Most likely I'm going to have to create the variables in advance some other way.

[Karl]

Btw, under what term can I find documentation for using the "{}" brackets to modify arrays/matrices?

[Daniel Shub]

The "{}" create cell arrays.

[Karl]

Not really sure how I'd use a cell array in this situation so I'm posting hoping this will get your attention and you'll be so kind as to give me a hint(aka some code). ;)

[Walter Roberson]

for j=1:size(filelist,1)

spectra{j}=zeros(1340,4);

spectra{j}(1:1340,3:4)=xlsread(filelist(j,1:end));

and so on.

Answer 3

[the cyclist]

I believe this is the correct transliteration of your code into cell array syntax. I have to admit I have not tried executing it, though. Too lazy to figure out the Excel file, etc.

lengthFileList = size(filelist,1);

spectra = cell(lengthFileList,1);

for j=1:lengthFileList
    spectra{j}=zeros(1340,4);
    spectra{j}(1:1340,3:4)=xlsread(filelist(j,1:end));
    for k=1:1340
        spectra{j}(k,1)= k*correction(1,1)+correction(1,2);
        spectra{j}(k,2)=(10^7)/532-spectra1(k,1);
        spectra{j}(k,3)=1/spectra1(k,2)*(10^7);
    end
end

Answer 4

[Daniel Shub]

First, Walter is correct that you likely don't want to do what you are trying to do. Second, I don't think you are doing what you think you are doing. Third, I am a little surprised by the error you get. I believe what you are really doing is equivalent to:

clear x;
x(double('spectra'),double(num2str(1))) = zeros(1340, 4);
strcat = x;

since

double('spectra')

equals

ans =
  115   112   101    99   116   114    97

there is a dimension mismatch.