problem with eval notation

I have a question about eval in this script

function sort=sortstructure(strucvector, field)

if isfield(strucvector,field)

for i=1:length(strucvector)-1

    low=i

    for j=i+1:length(strucvector)

if eval(['strucvector(' num2str(j) ' ). ' field])<...

eval(['strucvector(' num2str(low) ' ). ' field])

    low=j

end

    end

    temp=strucvector(i)

    strucvector(i)=strucvector(low)

    strucvector(low)=temp

end

else disp('wrong field')

end

sort=strucvector

Why cab't I write the part with eval like this:

if eval(['strucvector(num2str(j)).''field'])<...

eval(['strucvector(num2str(low)).''field'])

I know eval is often considered unhandy but it is part of curriculum

[Daniel Shub]

The eval is only there because the curriculum is dated to before MATLAB handled dynamic field names.

Answer 1

[Sean de Wolski]

I don't see any reason for eval above. If you get rid of it, it should still run. If you're required by law of the professor to use it, have it do something useless and not dangerous at the end.

E.g.

%the rest of the code
eval('disp(''we did it!'')')

[Tor Fredrik Hove]

I tried to erase eval and it stilled worked like this:

if (['strucvector(' num2str(j) ' ). ' field])<...

(['strucvector(' num2str(low) ' ). ' field])

Then i erased a space after the first period and it did not work:

if (['strucvector(' num2str(j) ' ).' field])<...

(['strucvector(' num2str(low) ' ). ' field])

I just get trapped in not understanding where the spaces and '' should be. Is there any system?

[Daniel Shub]

I believe in MATALB x.y, where x is smaller than I care to go back to figure out its actual value, the eval was required.

[Daniel Shub]

@Tor, it is not really working when you remove the eval. You are in fact doing a comparison of two strings. It gives you an error (I am guessing something about matrix dimensions), when you delete a space from the j side and not a space in the low side.

Answer 2

[Daniel Shub]

You cannot do

eval(['strucvector(num2str(j)).''field'])

because

['strucvector(num2str(j)).''field']

expands to a string of strucvector(num2str(j)).'field (notice the odd '). EVen if you eliminate the odd ', you won't get the correct answer.

strucvector(num2str(j)).field

will essentially do

strucvector(double(num2str(j))).field

which is not:

strucvector(j).xxx

where xxx is whatever string is contained in field.

That is why you cannot do what you want and the original version works.

Current versions of MATLAB all for

strucvector(j).(field)

Answer 3

[Wayne King]

Hi Tor, Not sure why you want to use eval() here, but:

    test = struct('data',randn(10,1));
    for nn = 1:9 
    if (eval('test.data(nn) < test.data(nn+1)')) 
    disp('hi'); end
    end