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Executing eval in function handle ?

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holistic
holistic il 18 Mar 2018
Commentato: Jan il 19 Mar 2018
I have the following code which does not work and I don't understand why:
foo='name'
name='C23';
handle=@(var) eval(var)
handle(foo)
I get the following error:
Error using eval
Undefined function or variable 'name'.
Error in @(var)eval(var)
Here,
handle(foo)
should return
'C23'
Can someone tell me what I did wrong and how to make this work?
Thanks in advance
  3 Commenti
holistic
holistic il 19 Mar 2018
Modificato: holistic il 19 Mar 2018
Thanks all for the answers so far,
I will describe a simplified example of what I wanted to achieve with this code. A string should be build that contains the name and surname, but the order should be defined by the user.
nameOrder={'surname','name'}; %Specified by user
handle=@(var) strcat(eval(var{1}),'_',eval(var{2})); %function handle that builds string
surname='test'; %read from data automatically
name='guy'; %read from data automatically
handle(nameOrder) %output to user should be test_guy
In this case nameOrder is set by the user which chose surname and then name. Later, there is a routine that reads in name and surname from some data and should output a string that is the name and surname or surname and name, i.e. the order specified by the user.
I could not think of any other way to do this, so any help appreciated!
Stephen23
Stephen23 il 19 Mar 2018
"I could not think of any other way to do this"
Using eval is rarely the solution that beginners think it is: it will just make your code slow, complex, and (as you are finding out now) much buggier and harder to debug. Read this to know more:

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Stephen23
Stephen23 il 19 Mar 2018
Modificato: Stephen23 il 19 Mar 2018
This is a classic example of how the decision to use eval to access variables just makes code more complex and introduces more problems than it solves. The name MATLAB comes from "MATrix LABoratory": when you put your data into matrices/arrays then this problem is easy to solve. Thus the first step is to simply put the input data into one cell array (which they should be in anyway, rather then in separate variables):
>> nameOrder = {'sur','given'};
>> sname = 'Smith';
>> gname = 'Jane';
>> tmp = {gname,sname}; % first step: put into one array!
>> [~,idx] = ismember(nameOrder,{'given','sur'});
>> sprintf('%s_%s',tmp{idx})
ans = Smith_Jane
Simple, reasonably efficient, and it avoids all of the pointless problems of eval. I changed the variable names and char vectors to make it clear that the nameOrder contents are not the same as the variable names. You could simplify it even more by storing the names in a standard format, e.g. perhaps a structure:
S.surname = 'Smith';
S.givenname = 'Jane';
tmp = {S.givenname,S.surname};
Note that using a cell array has the advantage that you could write code which takes into account cultures which have multiple or only one name, all of which could be handled trivially with indexing (and not an eval in sight!).
Summary: keep data together as much as possible, rather than splitting it apart. Keeping data together makes it easier to work with.
  2 Commenti
holistic
holistic il 19 Mar 2018
Ah I see, that's a much better solution :). Thank you!
Jan
Jan il 19 Mar 2018
Very good. +1

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Più risposte (1)

Greg
Greg il 19 Mar 2018
Most importantly! respond to Rik's comment above. The use of eval is very nearly always a horrible idea.
However, to your question: it does not work because the scope of name is where the anonymous function is defined. When executing the anonymous function, name is out of scope in that workspace.
  1 Commento
Greg
Greg il 19 Mar 2018
I hesitate to mention it could work if you used evalin('caller',...); but again, bad idea!

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