Executing eval in function handle ?
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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 个评论
Rik
2018-3-18
I can't see any direct reason why this doesn't work, however, why do you want to do this? There is almost always a very good replacement for eval. Usually eval solves problems caused by bad data management. Instead of learning how a mop works, it's usually beter to close the faucet.
In other words: what is your goal? Maybe we can help you solve that instead of this question.
Stephen23
2018-3-19
"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:
采纳的回答
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.
更多回答（1 个）
Greg
2018-3-19
1 个投票
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 个评论
Greg
2018-3-19
I hesitate to mention it could work if you used evalin('caller',...); but again, bad idea!
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