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# Creating a Structure for Two Elements

## Extend to two elements

Let's now consider the case where we would like to create a structure to store the strain and stress values for two triangular elements. The structure from Step 1 can be modified as per below to accommodate the second element. We'll create the above structure by modifying the code from Step 1. Recall that the statements for creating the structure were

`eps_val = [2 7 4]; %Nonsense values`
`R = 2*eye(3); %Dummy R`
`sig_val = (R*eps_val')'; %Derived nonsense`
`sigepsstruct = struct('eps', eps_val, ...`
`'sig', sig_val);`

We have to create a second row for the eps and sig fields, corresponding to values for the second element. We can do this by looping over the elements, adding one row at a time.

`numels = 2;%Total number of elements`
`for iele = 1:numels`

`     stuff`

{{}}
`end`
`sigepsstruct = struct();`

We will keep the R matrix calculation out of the loop (since it doesn't need to be recalculated each time anew). This leaves us with the `eps_val, sig_val` and `sigepsstruct` statements inside the loop. In a real calculation, the strain values for each element , or equivalently, the rows of the eps field, are going to be different. To make the rows of the epsfield different, we'll blithely multiply each row by the loop counter. This is again done to explain programming concepts; it makes no sense from a solid mechanics sense.

`numels = 2;%Total number of elements`
`R = 2*eye(3); %Dummy R`
`for iele = 1:numels`
`   eps_val = iele*[2 7 4]; %Nonsense values`
`   sig_val = (R*eps_val')'; %Derived nonsense`
`end`

Another thing: `eps_val` and `sig_val` above are row vectors. But the epsfield is a matrix. We'll create this matrix as follows: after each row of the matrix is calculated, we will "push" it into the appropriate row of the matrix. Let's call this matrix geps_val, the g prefix standing for global. The following assignment will push the current eps_val vector into the second row of the matrix.

`geps_val(2, : ) = eps_val;`

Chew over this and make sure you understand it. If need be, look up the help on the ":" operator. The matrix for the sig field can be created in a similar fashion. When using the struct function to create sigepsstruct, we now need to use the geps_val and gsig_val to assign values to the fields. The following code brings this all together. Run your code and check the values in the Workspace. Once your code is working correctly, test extracting data from the structure.Note that you can look up the names of each field within the structure using the fieldnames() command.

`%Extract data from structure`
`myeps = sigepsstruct.eps`
`sigxy2 = sigepsstruct.sig(2,3)`
`fieldnames(sigepsstruct)`

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