How to save struct data in for loop

Hi,

I am working in a for loop where for each iteration I get a new structure data with 3 fields in all of them. How do I store all of these data points so that I could use them together? I tried working with the following format but I am getting the error "Subscripted assignment between dissimilar structures."

The m.get_data() returns a struct of three fields each containg 1024 values.

for i=1:10
    data_struct(i)=m.get_data();
    
end

[Walter Roberson]

You have a left-over value for data_struct in your workspace. Clear it, or better yet use a different variable name.

[Suman Chatterjee]

Walter Roberson could you please explain what is the meaning of the error "Subscripted assignment between dissimilar structures" .And how to avoid it.

[Walter Roberson]

Also note that in order to assign an entire struct into an existing struct array, the structure fields must be in exactly the same order between the two.

[Suman Chatterjee]

Can i save them in any other way. My objective is to save the individual struct for eact loop.

[Walter Roberson]

Use dynamic structure names so your data_struct is a struct with multiple field names. Then use save() with the -struct option; that will create one variable name in the .mat file for each field name in the struct.

http://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Answer 1

[Nitin Kapgate]

You can overcome the problem by pre-allocating the array of structures as follows:

clear all; % Clear the workspace
N = 10;
% Pre-allocate the array of structures
structArray1(N) = struct('field1',[], 'field2',[], 'field3',[]);
  
for i=1:N
    field1 = "field1";
    field2 = "field2";
    field3 = "field3";
    
    structArray1(i)=getStruct(field1, field2,field3);  
end

Alternatively, instead of pre-allocating before the loop, you can also loop backwards so that all the memory allocation is done in the first iteration itself.

N = 10;
for i=N:-1:1
    field1 = "field1";
    field2 = "field2";
    field3 = "field3";
    
    structArray2(i)=getStruct(field1, field2,field3);  
end

structArray2 = fliplr(structArray2);  % Get back the array of structures in correct order

The getStruct function is as follows:

function outStruct = getStruct(field1, field2,field3)
% returns a struct with 3 fields
     outStruct.field1 = field1;
     outStruct.field2 = field2; 
     outStruct.field3 = field3; 
end