Have an error with dot, while searching value in struct

Hello guys.

Have a little problem with searching parameter value in structure.

If I do like this:

m = data.CH4.td_function_diapason_1.entropy

Everything is okay

m =

    '-4.6413037599999996'

but if I do like this

parametr = ({'td_function_diapason_1.entropy'});
parametr{1}
m = data.CH4.(parametr{1})

I get an error

ans =

    'td_function_diapason_1.entropy'

Reference to non-existent field 'td_function_diapason_1.entropy'.

Error in parsing (line 13)
m = data.CH4.(parametr{1})

What's the problem?

This code works if it doesn't include a dot in parametr

parametr = ({'molar_mass'});
parametr{1}
m = data.CH4.(parametr{1})

ans =

    'molar_mass'


m =

    '0.016042999999999998'

Thank you

Answer 1

[Walter Roberson]

Dynamic field names cannot have multiple field names. You can only dynamically create one field name at a time.

data.CH4.td_function_diapason_1.entropy =  '-4.6413037599999996'

data = struct with fields:
    CH4: [1×1 struct]

parametr = {'td_function_diapason_1' 'entropy'}

parametr = 1×2 cell array
    {'td_function_diapason_1'}    {'entropy'}

subsref(data.CH4, struct('type', '.', 'subs', parametr))

ans = '-4.6413037599999996'

[Ivan Ivan]

Thank you!

[Ivan Ivan]

Did like this:

s = size(parametr,2); %number of parametr columns
    if s == 2 % 2 columns 
    m = data.(components{i}).(parametr{1}).(parametr{2})
    else 
    m = data.(components{i}).(parametr{1})  
    end

Answer 2

[Steven Lord]

Dynamic field names works if the entry inside the parentheses is a field name. "a.b" is not a field name, it represents multiple levels of indexing.

data.CH4.td_function_diapason_1.entropy = 42;

You could use multiple levels of dynamic field names:

q = data.CH4.('td_function_diapason_1').('entropy')

q = 42

or you could use getfield.

s = 'td_function_diapason_1.entropy';
fn = split(s, '.'); % fn is a cell array
q2 = getfield(data.CH4, fn{:}) % use a comma-separated list

q2 = 42

[Ivan Ivan]

Thank you!

Answer 3

[Voss]

Dynamic field names do not support nested fields like that. But you can do it in two separate dynamic field name references:

parametr = {'td_function_diapason_1'};
parametr2 = {'entropy'};
m = data.CH4.(parametr{1}).(parametr2{1})

[Ivan Ivan]

Thank you!