Save a non scalar struct

I've the following function

function fun(i)
var =  true
if var
    ss(i).bv = rand(10,1)
    
    if i  == 4
        pwd
        save(fullfile(pwd, 'ss.mat'), 'ss');    
    end
end

The above function is called as below

for i = 1:4
i
fun(i)
end

The result is loaded,

l = load('ss.mat')
l.ss.bv

ans =

     []


ans =

     []


ans =

     []


ans =

    0.8217
    0.4299
    0.8878
    0.3912
    0.7691
    0.3968
    0.8085
    0.7551
    0.3774
    0.2160

I am not sure why the first there indices are empty. I want to store the results for all i.

Any suggestions?

Answer 1

[per isakson]

The variable ss isn't saved between the calls of fun(). For the struct saved, only ss(4).bv is defined.

That's the way Matlab works:

>> clearvars
>> ss(4).bv=17
ss = 
  1×4 struct array with fields:
    bv
>> ss(1).bv
ans =
     []
     

I failed to find the behaviour described in the documentation. The closest is in Memory Requirements for Structure Array. I replaced [] by 1, 2 and 3.

>> newStruct(25,50) = struct('a',1,'b',2,'c',3);
>> newStruct(1,50) 
ans = 
  struct with fields:
    a: []
    b: []
    c: []
>> 

[Stephen23]

"Any unspecified fields for new structs in the array contain empty arrays."

source: https://www.mathworks.com/help/matlab/matlab_prog/create-a-structure-array.html

[per isakson]

Maybe it goes without saying that the code below creates patient(1) and patient(2) with empty fields However, it's not spelled out explicitely for structs (as far as I can see).

clearvars patient
patient(3).name = 'New Name';
patient(3)

Answer 2

[Bhaskar R]

Here the MATLAB variable scope and life time plays.

For each iteration of for loop calling in the function fun variable ss is new, so last iteration will be saved as per isakson answer. If you want to work variable as you required then declare variable ss as persistent.

And in your code you have applied if condition i == 4 that also causes the problem, so remove that

function fun(i)
persistent ss
var =  true
if var
    ss(i).bv = rand(10,1);
    save(fullfile(pwd, 'ss.mat'), 'ss');
end
end

This can get you what you want

[Deepa Maheshvare]

Hi, Many thanks. This helps. And in your code you have applied if condition i == 4 that also causes the problem, so remove that

Could you please explain why this causes problem? Instead of writing to a file for each iteration , I want to write at the final iteration.