In a struct, how can you extract values for a vector of fieldnames

Question

Kevin Bachovchin 2013-2-22

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回答： Bart McCoy 2021-7-2

采纳的回答： Kevin Bachovchin

Hello,

I have a struct called MyStruct. I can extract the field names with the command

MyFieldNames = fieldnames(MyStruct);

If I'm interested in extracting the values for first n of MyFieldNames, is there a way I can do this using the "getfield" command? If I try

Temp = getfield(StateSpace, MyFieldNames(1:n))

or

Temp = Temp = getfield(StateSpace, MyFieldNames)

I get an error saying, "Index exceeds matrix dimensions."

Thank you, Kevin

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Kevin Bachovchin 2013-2-22

For example, if MyStruct has 10 fields, and I was only interested in the first 3 of fields. MyFieldNames(1:3) would produce the field names associated with the first 3 fields. Is there a way to extract the values for the first 3 fields?

Kevin Bachovchin 2013-2-22

If it wasn't clear what I was asking, for example, assume 'x1' is one of the field names.

The command getfield(StateSpace,'x1') works fine and produces the desired result, but if I want to return values for multiple field names, is there a way to do this? Or do I have to extract the values one at a time using a for loop?

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Answer 1

Kevin Bachovchin 2013-2-22

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MyFieldNames = fieldnames(MyStruct);

for i=1:3

MyValues(i,1) = getfield(MyStruct,MyFieldNames{i});

end

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Answer 2

Michael Kane 2019-4-16

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在 MATLAB Online 中打开

Kevin's answer can be condensed to oneline of code using cellfun

cellfun(@(x)(MyStruct.(x)),fieldnames(MyStruct))

You could even make this into an anonymous function fieldvalues

fieldvalues = @(MyStruct)(cellfun(@(fieldName)(MyStruct.(fieldName)),fieldnames(MyStruct)))

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Stephen23 2019-4-16

Simpler:

https://www.mathworks.com/help/matlab/ref/struct2cell.html

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Answer 3

Azzi Abdelmalek 2013-2-22

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编辑：Azzi Abdelmalek 2013-2-22

在 MATLAB Online 中打开

Edit

if a is your struct array

s=fieldnames(a)
out=a
rmfield(out,s(4:end))   % will remove fields from 4 to the end

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Kevin Bachovchin 2013-2-22

For your structure, I don't get an error, but for the structure I'm using, I do. Maybe it's because for my structure the values of the fields are 1x1 syms.

Azzi Abdelmalek 2013-2-22

编辑：Azzi Abdelmalek 2013-2-22

在 MATLAB Online 中打开

result=struct2cell(out)

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Answer 4

Jan 2013-2-22

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You could use a dedicated function like:

function Value = GetFields(S, Name)
if ischar(Name)
  try
    Value = S.(Name);
  catch
    Value = [];  % Or an error message
  end
elseif iscellstr(Name)
  Value = cell(size(Name));
  for iName = 1:numel(Name)
    try
      Value{iName} = S.(Name{iName});
    catch
      % Nothing or an error message?!
    end
  end
else
  error('Input type not handled')
end

For large numbers of fieldnames is this more efficient:

function Value = GetFields(S, Name)
[dummy, iName, iS] = intersect(Name, fieldnames(S));
Value = cell(size(Name));
Data  = struct2cell(S); 
Value(iName) = Data(iS);

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Kevin Bachovchin 2013-2-22

It's much easier to just use a for loop. It's weird that MATLAB only allows the getfield command to take a single field as an argument, but that problem can be overcome by using a for loop.

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Answer 5

Bart McCoy 2021-7-2

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There's just such an assymetry here. You have this great little function fieldnames() that accepts a struct and returns a cell array of names Where's the complementary fieldvalues() that accepts a struct and returns all struct values?

struct2cell() works great, but this question has been asked over & over for many years

Guess I'm writing my own.... just because: :)

function values = fieldvalues(struc)
     % values = fieldvalues(struc)
     % 
     % DESCRIPTION:
     % Extracts all values from a struct and returns them as a Kx1 cell array,
     % mirroring the functionality of MATLAB's fieldnames(struc)
     % 
     % 
     % SEE ALSO:    
     %     fieldnames(struc)     Complementary function to fieldvalues
     
     assert( isstruct(struc), sprintf("\n\nfieldvalues(struc)\n\tArgument 'struc' must be a structure\n\n"));
     values = struct2cell(struc);
end