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

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

[Azzi Abdelmalek]

What do you mean by the first field?

[Kevin Bachovchin]

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]

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?

Answer 1

[Kevin Bachovchin]

MyFieldNames = fieldnames(MyStruct);

for i=1:3

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

end

Answer 2

[Michael Kane]

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)))

[Stephen23]

Simpler:

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

Answer 3

[Jan]

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);

[Kevin Bachovchin]

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.

Answer 4

[Azzi Abdelmalek]

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

[Kevin Bachovchin]

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?

[Azzi Abdelmalek]

Look at edited answer

[Kevin Bachovchin]

I'd prefer to get a 1x3 matrix for values in fields 1 to 3. Is there a way to do that all at once rather than extracting the values one at a time in a for loop like shown below?

MyFieldNames = fieldnames(MyStruct);
for i=1:3
MyValues(i,1) = getfield(MyStruct,MyFieldNames{i});
end

[Azzi Abdelmalek]

I am not following you. Have you tried the edited answer?

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

[Azzi Abdelmalek]

Once you get your three fields in the variable out use

result=reshape(cell2mat(struct2cell(out)),3,[])

[Kevin Bachovchin]

Yes, I did. Your code produces a 1x1 struct with 3 fields as the answer. I'm looking to obtain a 1x3 for the values only, which is what my code using the for loop does. If there's no simpler way of doing that besides the for loop, that's fine I guess. I just would have thought MATLAB would have some built-in way of extracting values for more than 1 field at a time.

[Azzi Abdelmalek]

s=fieldnames(a)
out=a
out=rmfield(out,s(4:end)) 
result=reshape(cell2mat(struct2cell(out)),3,[])

[Kevin Bachovchin]

The last line produces an error for me. "Error using cell2mat (line 53) Cannot support cell arrays containing cell arrays or objects."

I think I'm just going to stick with extracting the values using the for loop unless there is some way to use the "getfield" command with multiple fields at a time.

[Azzi Abdelmalek]

Look at this example

out=struct('v',{4},'y',{6},'z',{45})
result=reshape(cell2mat(struct2cell(out)),3,[])

[Kevin Bachovchin]

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]

result=struct2cell(out)

Answer 5

[Bart McCoy]

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