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

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?

[David Braden]

Turn out my other code doesn't actually need it, but i'm still curious.

[dpb]

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.

Answer 1

[Philip Borghesani]

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);

[dpb]

Good on you, Philip...wasn't thinking of all possibilities...

[David Braden]

That seems to work nicely and should expand with the number of fields. Thank you.

Answer 2

[Bruno Luong]

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{:})