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preallocating a structure with fieldnames in the form of a string from another structure

Asked by David Braden on 17 Jul 2018
Latest activity Commented on by David Braden on 18 Jul 2018
say i have a nested structure called tire
tire =
struct with fields:
lat: [1×1 struct]
cmb: [1×1 struct]
tire.lat=
struct with fields:
p8: [1×1 struct]
p10: [1×1 struct]
p12: [1×1 struct]
p14: [1×1 struct]
p12i: [1×1 struct]
I had previously manually entered:
fy_offsets=struct('p8',[],'p10',[],'p12',[],'p14',[],'p12i',[])
fy_offsets =
struct with fields:
p8: []
p10: []
p12: []
p14: []
p12i: []
In order to create an empty struct to fill with data from later processing but want it automated and self forming. I used the following code:
ps=fieldnames(tire.lat); %which pressures do we have in fields tire.lat
p=ps'; %designated pressures in cell vector for new struct fieldnames
p_str=strjoin(ps,"',[],'") %adds ,[], between every entry in string
str_begin=("'") %places ' at the leading position
str_end=("',[]") %places ,[] at the end of the string
p_input=strcat(str_begin,p_str,str_end) %concatnates the strings into one string with one entry "'p8',[],'p10',[],'p12',[],'p14',[],'p12i',[]"
fy_offsets=struct(p_input) %tries to create struct with empty fields for filling
Outputs are as follows:
ps =
5×1 cell array
{'p8' }
{'p10' }
{'p12' }
{'p14' }
{'p12i'}
p =
1×5 cell array
{'p8'} {'p10'} {'p12'} {'p14'} {'p12i'}
p_input =
"'p8',[],'p10',[],'p12',[],'p14',[],'p12i',[]"
fy_offsets=struct(p_input)
Error using struct
Conversion to struct from string is not possible.
Is there a way to get the struct function to accept this input?

  2 Comments

Turn out my other code doesn't actually need it, but i'm still curious.
Not completely directly that I see, no. You can use dynamic field names as character string, but the value entry has to be numeric rather than string. That is,
>> fn
fn =
5×1 cell array
{'p8' }
{'p10' }
{'p12' }
{'p14' }
{'p12i'}
>> struct(fn{1},[]) % works
ans =
struct with fields:
p8: []
but when try to incorporate the brackets as text as in
>> instr=[fn{1} ',[]'] % doesn't
instr =
'p8,[]'
>> struct(instr)
Error using struct
Conversion to struct from char is not possible.
>>
You could make it work by resorting to eval, but that's difficult to code and even more difficult to debug.

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2 Answers

Answer by Philip Borghesani on 17 Jul 2018
Edited by Philip Borghesani on 17 Jul 2018
 Accepted Answer

Don't use strings and don't think about eval. This could be done with colon expansion and a cell array to produce a comma separated list for the call to struct (exercise for reader or I can follow up later if your stuck) or with cell2struct:
ps=fieldnames(tire.lat);
newstruct=cell2struct(cell(numel(ps),1),ps);

  2 Comments

Good on you, Philip...wasn't thinking of all possibilities...
That seems to work nicely and should expand with the number of fields. Thank you.

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Answer by Bruno Luong
on 18 Jul 2018

stemplate=struct('p8',1,'p10',2,'p12',3,'p14',4,'p12i',5)
structarg = fieldnames(stemplate)';
structarg(2,:)={[]};
sempty=struct(structarg{:})
structarg(2,:)={cell(1,5)};
semptyarray=struct(structarg{:})

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