how to compare string array with the names of structure's fields. (2024)

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Sara on 2 Jul 2018

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Commented: Sara on 3 Jul 2018

Accepted Answer: Paolo

I have a structure with 12 fields and a string array named variables containing 4 values. I want to compare all values in my string arrays with the names of my structure fields, and if the fields' name and the string value were the same, return values of that field as an output. I try strcmp command to compare my string array with the fields' name in a for loop.(for i = 1:numel(variables)) then I use switch and case but I have no idea how to return the field values as an output when the statement is true. (I think I should use getfield command but I don't know how). could you please help me in this regard. thank you.

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Adam on 2 Jul 2018

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You can use dynamic strings to get data out of a struct field e.g.

str = 'SomeStructFieldName';

value = myStruct.( str );

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Accepted Answer

Paolo on 2 Jul 2018

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S.a = {'First field'};

S.b = {'Second field, entry 1','Second field,entry 2'};

S.c = {1,2,3};

mystring = {'a','game','matlab','c'};

[~,indx] = ismember(fieldnames(S),mystring);

all = arrayfun(@(x) getfield(S,mystring{x}),indx(indx>0),'un',0);

all{:} =

{'First field'}

{[1]} {[2]} {[3]}

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Sara on 3 Jul 2018

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Thanks for our answer it is totally true. However I have two questions: Firstly I don't understand the last line very well (I am not expert about matlab); what does the function of @(x) ?

secondly what the code should be written if I want to save the fields' values as well as the fields names in the form of a table (now 'all' just contains the values of the fields not their names). I mean considering the field names as a header of a table.

I appreciate your help.

Stephen23 on 3 Jul 2018

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Edited: Stephen23 on 3 Jul 2018

@Sahar: the @(x) defines an anonymous function in x:

Paolo on 3 Jul 2018

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T = table(all{:});

T.Properties.VariableNames = mystring(indx(indx>0));

T =

1×2 table

a c

_____________ _________________

'First field' [1] [2] [3]

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Stephen23 on 3 Jul 2018

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Edited: Stephen23 on 3 Jul 2018

I would use struct2cell to get a cell array, then used basic MATLAB indexing to obtain the "fields" (rows of the cell array) that you want. Using a cell array makes it easy to use cell2table to generate a table from the cell array and the fieldnames:

>> S.a = '1st';

>> S.b = {'2ndOne','2ndTwo'};

>> S.c = 3;

>> F = fieldnames(S);

>> C = struct2cell(S);

>> lookfor = {'a','hello','world','c'};

>> [idx,idy] = ismember(lookfor,F);

>> idz = idy(idx)

>> T = cell2table(C(idz,:),'VariableNames',F(idz))

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Sara on 3 Jul 2018

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I write my code completely according yours; but I got this error

Error using cell2table (line 57) The VariableNames property must contain one name for each variable in the table.

my code is something like this:

battery.Cycle =[1,2];

battery.Voltage = [3,4];

battery.Current = [4,5];

battery.DischargeCapacity = [5,6];

variables = {'Cycle','Voltage','Current','DischargeCapacity'};

Fname = fieldnames(battery );

Cell = struct2cell(battery );

[indx,indy] = ismember(variables,Fname );

indz = indy(indx )

Table = cell2table(Cell(indz,:),'VariableNames',Fname(indz));

Stephen23 on 3 Jul 2018

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Edited: Stephen23 on 3 Jul 2018

@Sahar: you are right, I think the first input needs to be transposed:

Table = cell2table(Cell(indz,:).','VariableNames',Fname(indz));

According to the cell2table help "Each column of C provides data for a table variable", whereas struct2cell returns one row for each field, so a transpose is required.

Sara on 3 Jul 2018

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Many Thanks for your help.

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