Why eval() and evalin() are not recommended?

I have read so many times that functions eval() and evalin() are not recommended. What is the reason?

I have a situation where i want to evaluate the value of a parameter whose name is stored in string.

I obtained parameter name as

CostName=get_param(cblk,'Name');

Now i want value of this parameter from workspace so i wrote

CostVal=eval(strcat(CostName,'.Value'));

Is there any alternative for above line instead of using eval();

Answer 1

[Daniel Shub]

See the FAQ and TMW for reasons to avoid eval and evalin

http://matlab.wikia.com/wiki/FAQ#Why_is_it_advised_to_avoid_using_the_.22eval.22_function.3F

http://www.mathworks.co.uk/support/tech-notes/1100/1103.html

As your code is written now, you probably cannot avoid eval. One possible work around would be if all of your "parameters" (i.e., anything that could be returned by get_param) were saved in a structure. Then you could do

param.(CostName).Value

You really need to show us how the variable that the string in CostName points to was created in order for us to give you ideas about how to avoid eval.

[TAB]

I am using simulink. I created the parameter in base workspace by

SensorStatus=Simulink.Parameter;

I am using this parameter in a constant block in model. Name of const. block is same i.e 'SensorStatus'.

Now i am writing a script to read the const. block name from model using 'get_param()' and evaluate its value from workspace. Finally put all data in text file.

'CostName=get_param(cblk,'Name')' returns the const. block name as string only. It can not be collected in struct.

[Daniel Shub]

If I follow you correctly, CostName is the string "SensorStatus" and your workspace also has the variable SensorStatus. You want to evaluate the expression

CostVal=SensorStatus.Value

I am not suggesting collection the strings in a struct (although I don't see why you couldn't). Rather, I think the following should work:

param.SensorStatus=Simulink.Parameter;

CostName=get_param(cblk,'Name');

CostVal=param.(CostName).Value;

[Jan]

+1, I agree with Daniel.

After "CostName=get_param(cblk,'Name');" you need an EVAL, e.g. in:

CostVar = eval(CostName); CostVal = CostVar.Value;

But there is no good reason to pass variables by their names.

[TAB]

@Daniel:

You got my question correctly. I tried code you suggested, but it is not working.

I tried following lines on command window

SensorStatus=Simulink.Parameter;

SensorStatus.Value=100;

param.SensorStatus=Simulink.Parameter;

CostName='SensorStatus';

CostVal=param.(CostName).Value;

>> CostVal =

[]

I think here 'param.SensorStatus=Simulink.Parameter' will create a separate structure 'param'.

[Daniel Shub]

Try this:

param.SensorStatus=Simulink.Parameter;

param.SensorStatus.Value=100;

CostName='SensorStatus';

CostVal=param.(CostName).Value;

[TAB]

I think, there is no alternative of eval() in this case.

[K E]

Simulink coding seems to rely on many practices that Matlab experts recommend against, such as frequent use of eval and evalin; or populating the workspace with many variables without tracking where they came from. See #6, 7 and 9 on http://blogs.mathworks.com/loren/2012/01/13/best-practices-for-programming-matlab/ . Perhaps these two departments of Mathworks could benefit from talking more often.