problem with code

I am trying a script that does not work. First I define a structure in matlab:

>> package=struct('item_no',123,'cost',19.99,'price',39.95,'code','g')

package =

    item_no: 123
       cost: 19.9900
      price: 39.9500
       code: 'g'

then I have made a script who will try to find out the value of a field in the structure:

 inputfield=input('Which field would you like to see: ','s')

 if isfield(package, inputfield)

     fprintf('The value of the %s field is: %c\n',inputfield,...
     eval(['package.'inputfield]))

 else
     fprintf('Error: %s is not valid field\n', inputfield)

 end

but i get error message:

>> structurevalue ??? Error: File: structurevalue.m Line: 6 Column: 22 Unexpected MATLAB expression.

Answer 1

[Mitch Martelli]

the problem is in the eval,try to put a blank after 'package.' :

eval(['package.'  inputfield])

[Tor Fredrik Hove]

Thanks for both answers!

I have added a space and it works except one problem. When I want value for item_no it does not give right information:

>> package=struct('item_no',123,'cost',19.99,'price',39.95,'code','g')

package =

item_no: 123

cost: 19.9900

price: 39.9500

code: 'g'

>> structurevalue

Which field would you like to see: cost

inputfield =

cost

The value of the cost field is: 1.999000e+001

>> structurevalue

Which field would you like to see: item_no

inputfield =

item_no

The value of the item_no field is: {

>>

the script again with only an extra space added:

inputfield=input('Which field would you like to see: ','s')

if isfield(package, inputfield)

fprintf('The value of the %s field is: %c\n',inputfield,...

eval(['package.' inputfield]))

else

fprintf('Error: %s is not valid field\n', inputfield)

end

[Mitch Martelli]

you have right...

eval(package.item_no)

return the corresponding ASCII symbol because the argoument is integer

[Image Analyst]

I'm not sure why you accepted this answer instead of my answer, or why Mitch even offered this answer, since my answer was posted previously here. I believe my answer is the preferred method, as I often see other experts recommend for dynamic field names and against eval. If you still plan on using eval, you should read the FAQ: http://matlab.wikia.com/wiki/FAQ#Why_is_it_advised_to_avoid_using_the_.22eval.22_function.3F

[Tor Fredrik Hove]

Hi Image Analyst! I am a bit new with matlab so convenient choices is not my strong side. Eval is also a part of the curriculum so I have to know something about it either way

[Jan]

I do not understand what "is part of the curriculum" means. Using EVAL is a bad programming practize, because it is prone to errors and results in slow code.

"package.(inputfield)" is much safer and faster than "eval(['package.' inputfield])".

Answer 2

[Image Analyst]

This is what the line should look like:

       fprintf('The value of the %s field is: %c\n',inputfield, package.(inputfield));

Look up "dynamic field names" in the help for more info.

[Jan]

+1: Best answer. Perhaps ignoring your answers is a running joke. However, I'm missing the punchline.