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Construct a reference to a matrix where the matrix name is the value of a variable

Asked by William on 9 Apr 2013

I want to construct a matrix reference from information contained in a set of variables. Example: say I want to set a matrix with these values:

fData = saaData(:, 16)

However, the statement is part of a for loop, and both the column and the particular matrix being referenced change on each pass. They are specified by a pair of variables:

fctSource = factors{i, 2};
fctIdx = factors{i, 3};

where on this pass, factors{i, 2} = saaData and factors{i, 3} = 16.

How can I construct a command, using fctSource and fctIdx, that will return the column vector given by saaData(:, 16)?

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William

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2 Answers

Answer by Teja Muppirala on 9 Apr 2013
Accepted answer

If you need to reference a variable with a dynamic name based on a string, you'd generally call EVAL. That being said, doing these sorts of manipulations with EVAL can often get confusing, and is usually not a efficient way to solve a problem.

saaData = rand(3,16)
factors = {'something' 'saaData' 16}
if exist(factors{2},'var') %Make sure it's actually a variable...
  string = [factors{2} '(:,' num2str(factors{3}) ')'];
  fData = eval( string )
end

1 Comment

William on 9 Apr 2013

That's done it, thanks for the answer.

Teja Muppirala
Answer by Walter Roberson on 9 Apr 2013

2 Comments

William on 9 Apr 2013

I think I'm missing something on how to use that. This seems to be advice about creating a set of variables. In my case, I'm trying to construct a matrix from columns in a number of other matrices. (The example I gave above is a touch simplified; where it says fData = saaData(:, 16), what I properly want is fData(:, 1) = saaData(:, 16). The other columns will be set in the same way, referencing different matrices, in the subsequent passes of the for loop.)

Am I missing a connection between creating variables and creating matrices, or should I try a different approach?

Walter Roberson on 9 Apr 2013

The techniques for creating a set of variables are closely related to the techniques for using a set of variables.

And you missed the big point of the beginning: DON'T DO THAT! Store your variables in indexable forms instead of using different variable names.

Walter Roberson

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