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FAQ: How can I create variables A1, A2,...,A10 in a loop?

조회 수: 2,550(최근 30일)
Jan
Jan 2012년 12월 26일
편집: Stephen 2020년 1월 1일
How can I create variables A1, A2,..., A10 in a loop?
  댓글 수: 6
Jan
Jan 2019년 3월 15일
[MOVED from flags] Alexandre THIBEAULT wrote at Stephen's comment:
"cmon, we will not read all of the content of all of the pages... We are looking for solutions."
@Alexandre: You find solutions, when you scroll down and read the answer. Stephen's exhaustive explanations have been required after hundreds of discussions in the last years.

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채택된 답변

Jan
Jan 2012년 12월 26일
편집: Jan 2017년 4월 24일
Please don't do this! You will find that MATLAB arrays (either numeric or cell) will let you do the same thing in a much faster, much more readable way. For example, if A1 through A10 contain scalars, use:
A = zeros(1,10); % Not necessary, just much faster
for i=1:10
A(i) = % some equation
end
Now refer to A(i) whenever you mean Ai. In case each Ai contains a vector or matrix, each with a different size, you want to use cell arrays, which are intended exactly for this:
for i=1:10
A{i} = 1:i;
end
Note that each A{i} contains a different size matrix. And be careful to use the curly braces for the subscript!
Another way to have your cake and eat it too is to use structures instead of cell arrays. The fields of the structure can be the variable names you want. And you can index into them with dynamic field references. For example:
names = {'fred' 'sam' 'al'};
for ind = 1:length(names)
s.(names{ind}) = magic(length(names{ind}));
end
In this case, you end up with the variable s, a structure, containing fields specified by the strings stored in the cell array names.
Now, if you still really want to create variables with dynamically generated names, you need to use EVAL. With EVAL, you use MATLAB commands to generate the string that will perform the operation you intend. For example, eval('A=10') has the same effect as A=10, and eval(['A' 'B' '=10']) has the same effect as AB=10, only the EVAL method executes much more slowly. So in a loop, you could use:
for i=1:10
eval(sprintf('A%d = [1:i]', i));
end
Notice how much more obfuscated this is. In addition, this can cause difficult-to-troubleshoot problems in your code, particularly if you try to dynamically create a variable with the same name as a function:
function y = mysin(x)
eval('sin = 5;');
y = sin(x);
Calling this function with "y = mysin(1)" will not return y = 5 (the first element of the sin variable created by EVAL) -- it will return the sine of 1, because when the function was parsed there was no variable named sin and so the usage of sin on the last line was parsed as a call to the built-in SIN function. The fact that a variable named sin existed at runtime is irrelevant; the parsetime "decision" takes precedence.
Repeat: don't create variables at runtime using EVAL unless you have a very good reason, such as someone gives you a MAT file with 2000 variables named A1428, for example. Even in that case, you can avoid EVAL:
% Assume the MAT-file example1.mat contains 2000 variables, A1 through A2000
S = load('example1.mat');
% S is now a struct array with 2000 fields, S.A1 through S.A2000.
% To access A1428, use:
x1 = S.A1428;
% If the "index" of the variable you want to access is stored in a variable:
k = 1428;
x2 = S.(sprintf('A%d', k));
x3 = S.(['A', num2str(k)]);
  댓글 수: 1
Edward Byers
Edward Byers 2016년 7월 19일
Hi Is there a good way to store multiple iterated LinearModel objects? i.e. with an index notation that makes it easy to loop and call specific instances, I have tried:
mdl{x} = fitlm(tbl,modelspec);
But that doesn't work, as: "Assignment using () is not allowed for a FitObject."

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추가 답변(2개)

Robert Cumming
Robert Cumming 2014년 9월 10일
편집: Robert Cumming 2014년 9월 10일
I 100% agree with Jan that creating new variables on the fly is something that should be avoided - but if you must then please consider this alternative method:
function generateVariableOnFly
% lets tic/toc to compare the use of eval and assignin
tic
eval ( 'a = zeros(10,10);' )
toc
% an alternate method is to use a
% sub function which assigns vars in the caller function:
tic
variableCreator ( 'b', zeros(10,10) )
toc
% validate that a and b both exist and are the same:
isequal ( a, b )
end
% Use a sub function which assigns variable in the caller function:
function variableCreator ( newVar, variable )
assignin ( 'caller', newVar, variable );
end
To complete Jans example you could use this in the following way:
for ii=1:10
variableCreator ( sprintf ( 'A%i', ii ), ii )
end
That would create variables A1, A2.... A10.
  댓글 수: 3
Stephen
Stephen 2020년 1월 1일
@Oussama HAYANE: use indexing. Indexing is much simpler and more efficient than what you are trying to do:
A(5) = 8;
A(5) = 19;
Indexing is explained quite well in the introductory tutorials:
Is there a particular reason why you need to write such slow, complex, obfuscated code?

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