How to use variable names/strings in a For cycle

Question

Hugo 2022-1-25

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1635965-how-to-use-variable-names-strings-in-a-for-cycle

回答： Image Analyst 2022-1-26

采纳的回答： KSSV

在 MATLAB Online 中打开

Hi,

I have the following code:

B1=matrix(:,1)
B2=matrix(:,2)
B3=matrix(:,3)
B4=matrix(:,4)
B5=matrix(:,5)

I would like to replace the 5 lines of code above by a "For" cycle.

However, I get an error when I try to compile the following code:

For i=1:5
"B"+i=i
end

The error is:

Incorrect use of '=' operator. Assign a value to a variable using '=' and compare values for equality using '=='.

How can I define variable names/strings correctly?

I thank you in advance,

Best regards,

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Stephen23 2022-1-25

编辑：Stephen23 2022-1-25

在 MATLAB Online 中打开

"How can I define variable names/strings correctly?"

Dynamically naming variables is one way that beginners force themselves into writing slow, complex, inefficient, obfuscated code that is buggy and difficult to debug. Here are some reasons why:

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

The simple and efficient MATLAB approach is to use indexing. Is there a particular reason why you cannot use indexing?

Here is simpler, much more efficient code that actually works (unlike your code):

for k = 1:5
    vec = matrix(:,k);
end

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

KSSV 2022-1-25

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1635965-how-to-use-variable-names-strings-in-a-for-cycle#answer_881610

在 MATLAB Online 中打开

You need not to do that.....it is sheer waste of time and not a good coding practise. You can save the data into a 3D matrix. This is preferred.

B = zeros(2,2,5) ; 
B(:,:,1) = rand(2) ;
B(:,:,2) = rand(2) ;
B(:,:,3) = rand(2) ;
B(:,:,4) = rand(2) ;
B(:,:,5) = rand(2) ;

You can access them by using B(:,:,1),..B(:,:,5)

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

Answer 2

Voss 2022-1-25

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1635965-how-to-use-variable-names-strings-in-a-for-cycle#answer_881615

在 MATLAB Online 中打开

Here is how you can do it:

matrix = magic(5);
for i = 1:5
    eval(['B' num2str(i) '=matrix(:,' num2str(i) ');']);
end
whos()
  Name        Size            Bytes  Class     Attributes

  B1          5x1                40  double              
  B2          5x1                40  double              
  B3          5x1                40  double              
  B4          5x1                40  double              
  B5          5x1                40  double              
  i           1x1                 8  double              
  matrix      5x5               200  double              

Here is why you shouldn't do it: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Here is what you should do instead:

clear variables
matrix = magic(5);
B = cell(1,size(matrix,2));
for i = 1:5
    B{i} = matrix(:,i);
end
whos()
  Name        Size            Bytes  Class     Attributes

  B           1x5               720  cell                
  i           1x1                 8  double              
  matrix      5x5               200  double              
celldisp(B)
 
B{1} =
 
    17
    23
     4
    10
    11

 
 
B{2} =
 
    24
     5
     6
    12
    18

 
 
B{3} =
 
     1
     7
    13
    19
    25

 
 
B{4} =
 
     8
    14
    20
    21
     2

 
 
B{5} =
 
    15
    16
    22
     3
     9

 

Or:

clear variables
matrix = magic(5);
B = num2cell(matrix,1);
whos()
  Name        Size            Bytes  Class     Attributes

  B           1x5               720  cell                
  matrix      5x5               200  double              
celldisp(B)
 
B{1} =
 
    17
    23
     4
    10
    11

 
 
B{2} =
 
    24
     5
     6
    12
    18

 
 
B{3} =
 
     1
     7
    13
    19
    25

 
 
B{4} =
 
     8
    14
    20
    21
     2

 
 
B{5} =
 
    15
    16
    22
     3
     9

 