print values in one matrix to another

Hi,

I have very large matricies but will abbreviate them as best as possible here.

I have a matrix called "connectivity" which contains 5884 rows (total_elements = 5884) and 5 columns. The first column is the element number, the second column is node 1 and the third column is node 2:

connectivity(total_elements, 5); 
connectivity(:,1) = 1:total_elements;
connectivity(:,2) = N1;
connectivity(:,3) = N2;

making this matrix look like

1   900    175
2   175    200
3   200    145
4   145     300...

I have a second matrix "C" which has the x and y coordinates for each of the nodes.

NodeNumber Xcomponent Ycomponent

...

I need to write a loop that will search matrix "C" for the correct x and y coordinates for each node and create a new matrix "length," which looks like the following:

length = zeros(total_elements, 7);
    length(:,1) = 1:total_elements
    length(:,2) = N1;
    length(:,3) = N2;
    length(:,4) = x1; %x coordinate of node 1
    length(:,5)=x2; %x coordinate of node 2
    length(:,6)=y1; %y coordinate of node 1
    legnth(:,7)=y2; %y coordinate of node 2

being:

element# N1 N2 x1 x2 y1 y2

[Walter Roberson]

Is it mandatory to use a for loop? This is something that is easily vectorized.

[k]

No, a for loop isn't necessary. I essentially just need to match values from one matrix to another and print them into a final matrix.

Ex:

matrix A

node # xcoordinate ycoordinate

1 0.23 0.5

2 2.3 6.2

3 2.1 3.2

matrix B

element# N1 N2

1 2 3

2 3 1

3 2 1

matrix C, which I'm trying to create

element# N1 xcoordinate ycoordinate N2 xcoordinate yycoordinate

1 2 2.3 6.2 3 2.1 3.2

2 3 2.1 3.2 1 0.23 0.5

3 2 2.3 6.2 1 0.23 0.5

Hopefully this makes more sense! Again, I need to scale this to where the total number of elements is 5884 and the number of nodes is 2000.

Answer 1

[Tommy]

Let me know if this works:

length = [connectivity, zeros(total_elements, 4)];
for i = 1:total_elements
    length(i, [4 6]) = C(C(:,1)==connectivity(i,2),[2 3]);
    length(i, [5 7]) = C(C(:,1)==connectivity(i,3),[2 3]);
end

(edit) If you want to avoid the loop, and to match the format you've given in the comments:

[~,i] = ismember(connectivity(:,[2 3]),C(:,1));
length = [connectivity(:,[1 2]), C(i(:,1),[2 3]), connectivity(:,3), C(i(:,2),[2 3])];

Either way should work for any number of elements and nodes, provided your connectivity and C matrices are formatted as you've described.

[k]

Thank you! Do you mind explaining a little more what is going on here? I need to scale it up to 5884 rows

[Tommy]

Absolutely! It should work for any value of total_elements, I just used 100 to test it.

length = [connectivity, zeros(total_elements, 4)];

This line creates the matrix length, sets the first 3 columns equal to connectivity, and allocates space for 4 more columns.

for i = 1:total_elements

For each row in connectivity...

connectivity(i,2)

determine the 1st node in that row,

C(:,1)==connectivity(i,2)

determine the row within C corresponding to that node,

C(C(:,1)==connectivity(i,2),[2 3])

and pull the 2nd and 3rd values from that row of C (which are the x and y coordinates of the node).

length(i, [4 6]) = C(C(:,1)==connectivity(i,2),[2 3]);

Put those values into the corresponding row of length (in the 4th and 6th columns, where the x and y coordinates of node 1 should go).

length(i, [5 7]) = C(C(:,1)==connectivity(i,3),[2 3]);

Repeat for node 2.

[k]

It partially worked! The nodes are already arranged in a random order and range from 1 to 2,000, so I don't think I need to use rand here.

[Tommy]

Sorry, I might have caused some confusion with those first few lines. I was only creating sample input matrices so I could test the code. I edited it to only include the lines used in finding length.

Hmm, what about it worked and what didn't?

[k]

The "length" matrix was created with the correct number of columns. Unfortunatley, the node numbers were overwritten with rand, ranging from 1 to 5, but the correct values from the C matrix for nodes 1 to 5 printed in the length matrix.

[k]

I tried the code that you gave without the for loop and got this error:

Subscript indices must either be real positive integers or

logicals.

Error in fibers (line 46)

length = [nodes(:,[1 2]), C(i(:,1),[2 3]), nodes(:,3),

C(i(:,2),[2 3])];

[k]

It worked!! Thank you so so much!!

[Tommy]

Oh awesome! Happy to help!