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Summary:
Dynamically accessing variable names can negatively impact the readability of your code and can cause it to run slower by preventing MATLAB from optimizing it as well as it could if you used alternate techniques. The most common alternative is to use simple and efficient indexing.
Explanation:
Sometimes beginners (and some self-taught professors) think it would be a good idea to dynamically create or access variable names, the variables are often named something like these:
- matrix1, matrix2, matrix3, matrix4, ...
- test_20kmh, test_50kmh, test_80kmh, ...
- nameA, nameB, nameC, nameD,...
Good reasons why dynamic variable names should be avoided:
- Slow
- Buggy
- Security Risk
- Difficult to Work With
- Obfuscated Code Intent
- Confuses Data with Code
- Code Helper Tools do not Work
- Magically Making Variables Appear in a Workspace is Risky
There are much better alternatives to accessing dynamic variable names:
- Indexing into Cell Array or ND-Array
- Non-scalar Structures (with Indexing)
- Dynamic Field-Names in a Structure
- load into a Structure, not into the Workspace
- save the Fields of a Scalar Structure
- Use a table or timetable Array
- Use more Efficient Ways to Pass Variables Between Workspaces
Note that avoiding eval (and assignin, etc.) is not some esoteric MATLAB restriction, it also applies to many other programming languages as well:
MATLAB Documentation:
If you are not interested in reading the answers below then at least read MATLAB's own documentation on this topic Alternatives to the eval Function, which states "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. The preferred method is to store related data in a single array." Data in a single array can be accessed very efficiently using indexing.
Note that all of these problems and disadvantages also apply to functions load (without an output variable), assignin, evalin, and evalc, and the MATLAB documentation explicitly recommends to "Avoid functions such as eval, evalc, evalin, and feval(fname)".
The official MATLAB blogs explain why eval should be avoided, the better alternatives to eval, and clearly recommend against magically creating variables. Using eval comes out at position number one on this list of Top 10 MATLAB Code Practices That Make Me Cry. Experienced MATLAB users recommend avoiding using eval for trivial code, and have written extensively on this topic.
What should you post where?
Next Gen threads (#1): features that would break compatibility with previous versions, but would be nice to have
@anyone posting a new thread when the last one gets too large (about 50 answers seems a reasonable limit per thread), please update this list in all last threads. (if you don't have editing privileges, just post a comment asking someone to do the edit)
This topic is for features you would like to see for the MATLAB Answers facility itself, and also for bug reports about the MATLAB Answers facility.
This topic is the follow on to the first Wish-list for MATLAB Answer sections and second MATLAB Answers Wish-list #2 (and bug reports). Those grew large enough to become unwieldy; and Mathworks has made enough changes to make a number of the past points no longer of relevance. More recently there was the limited purpose New design of the forum - grey on white which turned into a bug and wish list; I have renamed that for continuity.
I suggest one wish (or bug report) per answer, so that people can vote their wishes.
The community is very helpful, yet I feel really powerless that I cannot find the appropriate way to code, nor find the problems with the codes I have written. I have read numerous books on MATLAB, mostly related with science and engineering applications. Any advice to improve would be greatly appreciated. Thanks.
I've opened MATLAB Answers this morning and found the new design.
The field for typing the "Body" does not consider the font settings of my browser anymore, such that my preference of sans-serif fonts is ignored. In addition the text color is a medium gray, which is hard to read for me due to the too light contrast.
Blank lines in the code let two separate code boxes appear. This makes almost all code, I've posted in the forum, invalid. It has been discussed repeatedly, that blank lines in the code confuse the indentation of the display in the forum and that this is a really bad idea. But instead of improving this, it is made severely worse now.
The new design contains even more white space, such that standard questions cannot be answered without extensive vertical scrolling. It is a very bad drawback, that I cannot see the question while I type the answer.
There is still no suggestion to use a proper code formatting, such that I have to spend 20% of my forum time typing corresponding comments as before.
But I'm coming back to the most important problem for me: It is a physical problem for me to read the low contrast grey on white text. Does anybody know a tweak or CSS trick to increase the readability?
TMW, please take into account that this new design is physically hard to read for people without young and 100% perfect eyes. This is very annoying for me.
Splitting code blocks at white lines is simply a bug. I cannot imagine why this error has not been detected before the new design has been published. The argument, that TMW is extremely conservative with changes in the forum to ensure a stability does not convince me anymore.
[EDITED] The box around the thread, the preview box, the boxes for preformatted text and code have a grey background now. So some text is even medium grey on light grey.
I'd be glad if the designers refocus on the purpose of the forum. Whatever this purpose might be, the optical reception of the characters is fundamental.
What MATLAB tools or functions have you been thinking about making but haven't quite gotten around to it?
It can be something specific to your work if you'd like, but it's preferably:
- the kind of tool or task that we don't really expect in the next MATLAB release (ie., not a frequent entry in a MATLAB wishlist)
- something that would be useful to you and (hopefully) others if it existed.
- something that could conceivably be made by people(s) in the user community
Think of it maybe as a file exchange wishlist.
Votes for good ideas . I guess you'll get more votes if it's an idea others want to see made as well.
Probably no single accepted answer, but it would be nice to see what people have thought about and maybe we can bring a few ideas to fruition (or find that someone's already done it)
For example:
- I've been wanting to write an object-oriented replacement for the NURBS toolbox which is a great toolbox but is very unwieldy to use.
Some of Matlab's toolbox functions are affected by magic strings or magic numbers, which are strings or numbers with a deeper meaning besides the normal value. Both are considered as bad programming patters, because they provoke confusions, when the magic keys appear with the normal meaning by accident. See http://en.wikipedia.org/wiki/Anti-pattern
Example 1:
clear('myVariable')
clear('variables')
While the 1st clears the variable myVariable, the later clears all variables. Here 'variables' has a meta-meaning. The problem appears, when 'variables' is an existing variable:
a = 1;
variables = 2;
clear('variables')
disp(a) % >> 1
Only variables is cleared, which cannot be understood directly when its definition is 1000 lines before.
Example 2:
uicontrol('String', 'default')
This creates a button with the empty string '' instead of the expected 'default', because this is the magic string to invoke the default value get(0, 'DefaultUIControlString'). The same concerns properties of other graphic objects also, e.g. the 'name' property of figure or the string of uimenu. There is a workaround which allows the user to display 'default': Simply use '\default'. Unfortunately this is doubled magic, because in consequence it is impossible to display the string '\default'. Obviously a bad idea.
Example 3:
Graphic handles are doubles (although gobject of the new R2013a seems, like this is subject to changes? [EDITED: Yes, it changed with HG2 in R2014a]). But then a handle can be confused with data:
a = axes; % e.g. 0.0048828125
plot(a, 2, '+')
But you cannot draw the point [0.0048828125, 2] by this way, because the 1st input is considered as handle of the parent. Here all possible values of handles are magic. Collisions are very unlikely, but there is no way to avoid them reliably - as long as handles have the type double.
Question:
Which functions are concerned by magic values? What are the pitfalls and workarounds?
How do I can delete my account?
I would like to receive your feedback:
- would you find useful to have spellcheck built into the MATLAB Editor?
(Consider also an associated toggle, positioned somewhere in the Editor, that would enable/disable the spellcheck for selective use)
Lately, I am writing many quick analyses for publish() and I find that first publishing to MS Word (to check the spelling) and then re-publishing to .html, after manual corrections, disrupts the working.
IMHO, the idea to include spellcheck into the editor would be consistent with the renewed visibility of the PUBLISH tab.
I am wondering what others use for those little short-cuts or niceties in MATLAB. I have in mind something you wrote or something somebody else wrote or an underused MW function.
Here are my two favorites.
This is a simple script I use. Here is the entire contents of CLC.m (yes, it is capitalized):
clear all,close all,clc
Very simple, but I use it all the time. Here is another one I use so often that I forget not every machine has it (though every machine should, IMO):
Here is an underused MW function that I occasionally employ when working on someone else's machine. The usual response is, "Wait, what did you just do?"
home
What are some of yours?
This topic is for features you would like to see for the MATLAB Answers facility itself, and also for bug reports about the MATLAB Answers facility.
This topic is the follow on to the earlier Wish-list for MATLAB Answer sections. That topic grew large enough to become unwieldy; and Mathworks has made enough changes to make a number of the past points no longer of relevance. There was also a more limited purpose <http://uk.mathworks.com/matlabcentral/answers/216662-new-design-of-the-forum-grey-on-white-wish-list-3-bug-reports
I suggest one wish (or bug report) per answer, so that people can vote their wishes.
As I'm becoming more and more familiar with MATLAB, I'm starting to fall in love with it. I was wondering what are the coolest things that you all know MATLAB can do? As for me so far, the auto-code generation into another language is the coolest thing.
It is not uncommon for students to be assigned questions which they are required to complete "without using any built-in functions". There is not a great deal that can be programmed in MATLAB without using any built-in functions, but a little can be done -- but what, exactly is possible?
What a "built-in function" is, exactly, is open to interpretation. In the below, I refer instead to "publicly visible routines". Keywords (see below) are not publicly visible routines (they are "statements" or components of statements.) Any documented operation or call that invokes a MATLAB-supplied .m or .p or mex file or built-in library to do its work is a publicly visible routines. If you can use documented methods override the normal meaning of a statement or expression in practice by supplying alternate code, then the code probably involves publicly visible routines. If the language design is such that you could use documented methods to override the normal meaning of a statement or expression in theory (such as the behavior of adding two double, the code for which is in practice bundled into an internal MATLAB library), then I would still consider that a call to a publicly visible routine.
A MATLAB-supplied routine that is not documented, which is used for internal MATLAB purposes, could perhaps be held not to be a publicly visible routine, but it certainly would still be a "built-in function".
I exclude from the list any routine which there is no direct way to access, and is only used for internal purposes, such as the memory allocation routines.
This is what I have come up with:
- the names defined as "keywords" do not in themselves involve function calls to publicly visible routines. These keywords currently include 'break', 'case', 'catch', 'classdef', 'continue', 'else', 'elseif', 'end', 'for', 'function', 'global', 'if', 'otherwise', 'parfor', 'persistent', 'return', 'spmd', 'switch', 'try', 'while'. There is no functional form of any of these: for example, one cannot use global(s) to declare the name contained in the variable "s" to be global. (However, you can define an "end" method; https://www.mathworks.com/help/matlab/matlab_oop/object-end-indexing.html )
- scalar numeric double precision real-valued constants are handled at parse time, including unary plus and unary minus in front of them
- scalar numeric double precision constants followed immediately by "i" or "j" create a complex-value constant at parse time, including unary plus and unary minus in front of them
- whether a complete complex constant with real and imaginary part is handled at parse time is unknown
- literal character vectors and string objects are handled at parse time
- in sufficiently new versions, int64() and uint64() around an integer constant is handled at parse time. This was a change from previous versions which handled it at run time (after the integer had been converted to double precision...)
- whether any other casts such as uint16() or logical() are now handled at parse time is unknown
- assignment of a compete variable (no indexing, no substructure references, etc.) to a plain variable (no indexing, no substructure references, etc.) does not involve any function calls to publicly visible routines (unless I have overlooked a case involving objects)
- "if" or "while" applied to a scalar logical constant or to a scalar logical variable does not involve any function calls to publicly visible routines. However, it is not known whether there is any method to construct a logical value without calling a MATLAB routine: "true" and "false" are MATLAB routines, not constants, and logical() of a numeric constant might be handled at run time
- "for" in which the range is named as a scalar constant or scalar variable do not involve any function calls to publicly visible routines; for example, "for K = 5"
- defining an anonymous function does not involve any function calls to publicly visible routines
I may have overlooked something due to shortage of chocolate in my bloodstream.
The language described above is not Turing complete, and is not "sufficiently powerful" for the purposes of the Church-Rosser Theorem of general-purpose computability. It is also not possible to do any arithmetic in it, as arithmetic must be reducible to the Peano Postulates, and those require at the very least the ability to compare a value for equality with 0, which in MATLAB would require a call to the MATLAB routine "eq".
Hello all,
Please explain good MATLAB programming practice methods. It will help to the guys who are new to programming like me.
Previously I used
for i=1:10
after following some suggestions from this answers pages I learnt to use
for i1=1:100
This is the good way to write programs.
Like this, as a professional programmer, please mention some good programming practice techniques.
It will useful to all!
Capital letters are obtained by capitalizing the LaTeX command for the lowercase version. Capital letters in grey are exceptions which have no LaTeX commands. For example, to produce a capital chi simply type X (this also applies for the lowercase omicron).
When two versions of the lowercase letter are available, a var prefix can be added to obtain the second version. For example, the two versions of epsilon are \epsilon and \varepsilon.
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The code used to generate the table:
greeks = ...
{'ALPHA' 'A' '\alpha'
'BETA' 'B' '\beta'
'GAMMA' '\Gamma' '\gamma'
'DELTA' '\Delta' '\delta'
'EPSILON' 'E' {'\epsilon','\varepsilon'}
'ZETA' 'Z' '\zeta'
'ETA' 'H' '\eta'
'THETA' '\Theta' {'\theta','\vartheta'}
'IOTA' 'I' '\iota'
'KAPPA' 'K' '\kappa'
'LAMBDA' '\Lambda' '\lambda'
'MU' 'M' '\mu'
'NU' 'N' '\nu'
'XI' '\Xi' '\xi'
'OMICRON' 'O' 'o'
'PI' '\Pi' {'\pi','\varpi'}
'RHO' 'P' {'\rho','\varrho'}
'SIGMA' '\Sigma' {'\sigma','\varsigma'}
'TAU' 'T' '\tau'
'UPSILON' '\Upsilon' '\upsilon'
'PHI' '\Phi' {'\phi','\varphi'}
'CHI' 'X' '\chi'
'PSI' '\Psi' '\psi'
'OMEGA' '\Omega' '\omega'};
h = figure('units','pixels','pos',[300,100,620,620],'Color','w');
axes('units','pixels','pos',[10,10,600,600],'Xcol','w','Ycol','w',...
'Xtick',[],'Ytick',[],'Xlim',[0 6],'Ylim',[0,4]);
% Loop by column and row
for r = 1:4
for c = 1:6
el = (r-1)*6 + c;
% Title
text(c-0.5,5-r,greeks{el,1},'Fonts',14,'FontN','FixedWidth',...
'Hor','center','Ver','cap')
% Color cap latter in grey or black
if strcmp(greeks{el,2}(1),'\')
clr = [0, 0, 0];
else
clr = [0.65, 0.65, 0.65];
end
% Cap letter
text(c-0.5,4.87-r,['$\rm{' greeks{el,2} '}$'],'Fonts',40,...
'Hor','center','Ver','cap','Interp','Latex','Color',clr)
% Lowercase letter/s (if two variants)
if iscell(greeks{el,3})
text(c-0.75,4.48-r,['$' greeks{el,3}{1} '$'],'Fonts',20,...
'Hor','center','Interp','Latex')
text(c-0.25,4.48-r,['$' greeks{el,3}{2} '$'],'Fonts',20,...
'Hor','center','Interp','Latex')
% Latex command
text(c-0.5,4.3-r,['\' greeks{el,3}{1}],'Fonts',12,'FontN','FixedWidth',...
'Hor','center','Ver','base')
else
text(c-0.5,4.48-r,['$' greeks{el,3} '$'],'Fonts',20,...
'Hor','center','Interp','Latex')
text(c-0.5,4.3-r,['\' greeks{el,3}],'Fonts',12,'FontN','FixedWidth',...
'Hor','center','Ver','base')
end
end
end
% Print to pdf
export_fig greeks.pdf
And here is the link to the pdf on scribd: http://www.scribd.com/doc/159011120/Greek-alphabet-in-latex
We all know that MATLAB is probably the best software for engineering purposes, I think it's a little expensive unless you have it for free on your school or place you work, please share your opinion about MATLAB cost, including toolboxes, student versions... is it that expensive?
[INDEX]
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[MOTIVATION]
Why should we use markups in the body of our questions?
The answer is a question: which of the two versions is more likely to be understood in a glimpse and has more chances to be answered by our readers?
.
< Consider the following question >
I have a vector of weights W=[10,20,30,50,23434,1,2.4,2] and a matrix A=rand(100,8) and I would like to find the row-wise weighted sum of A. I am proceeding in the following way: B=zeros(size(A)); for c=1:numel(W) B(:,c)=A(:,c)*W(c); end B=sum(B,2); Somehow I get huge numbers can you please help?
.
< Now, consider its formatted version >
I have a vector of weights W = [10,20,30,50,23434,1,2.4,2] and a matrix A = rand(100,8) and I would like to find the row-wise weighted sum of A.
I am proceeding in the following way:
B = zeros(size(A));
for c = 1:numel(W)
B(:,c) = A(:,c)*W(c);
end
B = sum(B,2);
Somehow I get huge numbers can you please help?
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[AKNOWLEDGMENTS]
In alphabetical order by nickname, thanks for their suggestions to:
Walter Roberson
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[LOG]
- 06 Aug 2011, 13:17 BST - created and added boldface.gif
- 06 Aug 2011, 14:59 BST - added italic.gif
- 06 Aug 2011, 18:58 BST - added index section
- 07 Aug 2011, 00:03 BST - added code.gif and tutorial series section
- 07 Aug 2011, 01:50 BST - added monospaced.gif, numlist.gif, bullist.gif and hyperlink.gif
- 13 Aug 2011, 14:27 BST - added motivation section
- 18 Aug 2011, 01:44 BST - added aknowledgments section and link to wish-list
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[TUTORIAL Series]
Vote on Wish-list for MATLAB Answer sections my post if you think that a tutorial section on top of Answers could be useful.
I think a lot about how to more effectively teach MATLAB, internally to new hires, but also through my blog.
How did you learn MATLAB, what knowledge of programming did you have coming into learning MATLAB? When did you learn it is important also as the resources available have expanded radically in the last 15 years.
Answers from new users who are just beginning down this path are of particular interest, as those are who we can help the most.
Now, I am still a novice when it comes to programming. I believe MATLAB is definitely a great programming tool, one that I can play with, particularly, when I have free time.
I would love to hear from all answerers, what are the ways that can make one proficient in this field?
Don't be shy, what was your matlab learning curve, how many years or months, what were the difficulties to begin with.
I think that the answers would be most valuable for new users, maybe you can also tell us the tricks that allowed you to master some parts or all matlab.
Now it's your turn...
Which Matlab related forums and newsgroups do you use beside MATLAB Answers? Which languages do they use? Which advantages and unique features do they have?
Do you think that these forums complement or compete against MathWorks and its communication platform?
Actually all answers are accepted.
また、以下のリストから Web サイトを選択することもできます。
南北アメリカ
- América Latina (Español)
- Canada (English)
- United States (English)
ヨーロッパ
- Belgium (English)
- Denmark (English)
- Deutschland (Deutsch)
- España (Español)
- Finland (English)
- France (Français)
- Ireland (English)
- Italia (Italiano)
- Luxembourg (English)
- Netherlands (English)
- Norway (English)
- Österreich (Deutsch)
- Portugal (English)
- Sweden (English)
- Switzerland
- United Kingdom(English)
アジア太平洋地域
- Australia (English)
- India (English)
- New Zealand (English)
- 中国
- 日本Japanese (日本語)
- 한국Korean (한국어)