# How to use variable names/strings in a For cycle

6 views (last 30 days)
Hugo on 25 Jan 2022
Answered: Image Analyst on 26 Jan 2022
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?
Best regards,
Stephen23 on 25 Jan 2022
Edited: Stephen23 on 25 Jan 2022
"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:
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

KSSV on 25 Jan 2022
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)

Voss on 25 Jan 2022
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 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

Image Analyst on 26 Jan 2022

### Categories

Find more on Loops and Conditional Statements in Help Center and File Exchange

R2021a

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!