MATLAB: Accessing values of fieldnames in a structure without using the fieldnames function

fieldnamesstructures

Hi all,

I've got a structure which has 20 different field names, e.g. 'fieldname01','fieldname02' etc.

Each fieldname has a numeric value, such as:

fieldname01: 0.56 fieldname02: 0.93 fieldname03: -0.34

Up to fieldname20

I need to access the numeric values of certain field names, for example the 2nd, 5th and 15th. Just wondering what would be the right way about doing this without using the fieldnames function if that's possible at all?

Thank you.

Best Answer

Hi Jess,

Use parentheses:

S = struct('field1',1,'field2',2)
fieldStr = ['fields' num2str(1)]
S.(fieldStr)
ans = 
   1

Or, with your particular example:

for i = 1:20
  val = S.(sprintf('fieldname%02d',i))
end

Please note however that the "right" way to store data from 1 to 20 isn't in a structure with the number built into the name. Instead it sounds like you should be using an array (or cell array if your datatypes are different).

For example:

myData = [12 10 3];

is much cleaner than:

myData.element1 = 12
myData.element2 = 10
myData.element3 = 3

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Your code is a little confusing because it doesn't appear that you are assigning anything to the structure. Here is code that does assign values to the structure:

names = {'danny';'edgar';'larry'};
field = {'one';'two';'three'};
for ii =1:length(names)
    Newstruct.(names{ii}).(field{ii}) = ii;
end

This is how I am interpreting your code. It could be that you mean this instead (or switch names and field...):

names = {'danny';'edgar';'larry'};
field = {'one';'two';'three'};
for ii =1:length(names)
    Newstruct.(names{ii}) = field{ii};
end

If these aren't what you mean, please create one top level of the new structure by hand and show the code. Then perhaps we can figure out how to make it from names and values.

Best Answer

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