Is there the more elegant way to do this?
15 views (last 30 days)
Show older comments
MUKESH KUMAR on 10 Aug 2017
I have 10 matrices(10*24 size each) named A1,A2,,,,A10.Now i want this
thus E1,E2,,,,,E10 each matrices size of 1*24 . so there is another way to write in short code ?
Stephen23 on 10 Aug 2017
Edited: Stephen23 on 15 Aug 2017
"I have 10 matrices(10*24 size each) named A1,A2,,,,A10"
Well, that is a bad way to write code: trying to access those variables dynamically will be slow, buggy, obfuscated, hard to debug, etc.
"Is there the more elegant way to do this?"
Yes, there is! It would be much simpler to put all of your data into one array, then your code will be simple, efficient, neat, easy to debug, etc, etc. Some other beginners will tell you to magically create variables, but instead you might like to read what the MATLAB documentation has to say about that: "A frequent use of the eval function is to create sets of variables such as A1, A2, ..., An, but this approach does not use the array processing power of MATLAB and is not recommended." (this advice also applies to evalin and assignin. source: https://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html
Read this to know more:
See KSSV's much simpler, more efficient answer.
KSSV on 10 Aug 2017
Edited: KSSV on 10 Aug 2017
You make A1,A2,...A10 matrices a 3D matrix while computing or defining them..and the call sum..
Read about sum..you can specify dimension there and you can apply this function on 3D matrix also.
A = rand(10,24,10) ;
B = sum(A) ;
Walter Roberson on 10 Aug 2017
Walter Roberson on 13 Aug 2017
"why is Mathworks keeping command evalin, and other similar commands, in the common toolbox"
Because evalin() is required to implement "syms" and cplex, and to make it possible for various graphical tools such as the curvefitting tool or Simulink to retrieve user-defined values without the user having to pass in everything that might be needed.
Edited: Jan on 11 Aug 2017
Not an answer, but a comment concerning the discussion:
This is another example for an inefficient debate: The group of experienced Matlab programmers provide profound arguments, and one person insists on claiming the opposite and suggesting it to beginners. Neither code examples, nor timings, nor references in Matlab's documentation can force somebody to change his opinion. But some contributors, as me, still spend time to post the important details to warn other beginners not to get trapped by the pitfall of eval.
There is a common sense about good programming practices. Voting for good solutions helps to share this knowledge. Currently there are 9 votes for the good solution, and 0 vote and 1 acceptance for a bad solution. All readers can draw their own conclusions. The only problem is, that the accepted status has such a prominent position and shiny green check mark.
The editors can un-accept an answer, but they use this power very rarely for good reasons. The drawback for the forum would be reduced, if e.g. 4 votes (of maybe MVP members) move an answer on top of the accepted one.
It will not be possible to convince all forum members to suggest good programming practices. Discussing the same point with the same person again, will not help. What can we do instead to support the quality of the forum's contents?
Jiro Doke on 15 Aug 2017
Edited: Jiro Doke on 15 Aug 2017
I'm commenting as a user, not as a MathWorks staff (despite the little indicator by my name). It would be nice that if a thread has an answer with "significantly more" votes than the accepted answer, there is an indication on that page about it. Perhaps a hyperlinked asterisk by the accepted answer that points to that popular answer.
John BG on 10 Aug 2017
Edited: John BG on 11 Aug 2017
Hi Mukesh Kumar
this is John BG <mailto:email@example.com firstname.lastname@example.org>
Elegance like beauty lives in the eyes of the observer, so while some people abhor the command evalin, I consider it's a really powerful and elegant option to simplify the generation of variables.
-2 4 -10
3 -10 -8
-7 -5 7
>> A2=randi([-10 10],3)
4 -10 6
-4 -1 6
9 -2 -7
>> A3=randi([-10 10],3)
0 4 4
-1 5 3
3 -5 -7
2. Code that writes code
str1=['E' num2str(k) '=sum(A' num2str(k) ')'];
-6 -11 -11
9 -13 5
2 4 0
if you find this answer useful would you please be so kind to consider marking my answer as Accepted Answer?
To any other reader, if you find this answer useful please consider clicking on the thumbs-up vote link
thanks in advance
Stephen23 on 12 Aug 2017
Edited: Stephen23 on 13 Aug 2017
"If you all consider that the command evalin has problems"
No one has said that the command itself is buggy, because the problems are simply inherent in what it does. The problems with evalin and its relatives are much more general and are not even specific to MATLAB: the issues are fundamental to how JIT compilers work, about the inefficiency of string evaluation, about how static code checking works, about how objects are identified in workspaces, about interference between threads or workspaces, about the risks of evaluating arbitrary code, and other aspects of high-level computing. Together these cause code that uses evalin (and eval, etc.), to be buggy, slow, unreliable, a security risk, hard to debug, obfuscated, etc., etc..
"have you already reported to Mathworks? If so, what has been the answer?"
Seriously, anyone can read the documentation: "Avoid functions such as eval, evalc, evalin, and feval(fname)". source: https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html
"in any case, why do you choose to ignore the human factor for this command, yet we all know that it must always be considered when attempting to solve MATLAB questions?"
We do consider the human factor: the factor that your own example clearly shows how using evalin turned a bug in a trivially simple one-line piece of code into something that the user could not solve by themselves. And that is why we recommend people to use methods that are simpler, faster, less buggy, easier to debug, more reliable, more secure, where the MLINT and Editor warnings help them, etc., etc. Which just happen to be the same methods that the MATLAB documentation recommends.
"...while some people abhor the command evalin, I consider it's a really powerful and elegant option to simplify the generation of variables."
You still have not explained why writing slower, buggier, inefficient, hard-to-debug, insecure code is "elegant". You have provided us with one example of where a user got completely flummoxed trying to fix code that used exactly that buggy and hard-to-debug coding method that you call "elegant" (code that should have been trivial to fix). I would be interest what magic secrets you know about evalin that no one else knows (including the authors of MATLAB who wrote the documentation advising to avoid evalin).
Find more on Historical Contests in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!Start Hunting!