Need a better way to loop through a Matrix column and then output the row

I have a matrix (Nodal_Coordinates). I'm trying to loop through column 1 to find Node 1, Node 2... Node n. Then once Node_1 is found, I need it to display the respective row and only column 2 and column 3. The matrix row size can change with respect to the input, so it needs to be flexible on evaluating the matrix up to nth nodes. However, the columns will always be three. Col 1 is the node number, Col 2 is the x coordinate, Col 3 is the y coordinate. Below is the code I've been trying, but it outputs the entire row instead of just the 2nd and 3rd values of the row, and it's not flexible for nth rows... it only works for a fixed number of rows. Thanks for the help.

Nodal_Coordinates =

1 0 0

2 3 0

3 6 0

4 9 0

8 0 3

7 3 3

6 6 3

5 9 3

for i = Nodal_Coordinates(:,1)

for i = 1

Node_1 = Nodal_Coordinates(i,:)

for i = 2

Node_2 = Nodal_Coordinates(i,:)

for i = 3

Node_3 = Nodal_Coordinates(i,:)

for i = 4

Node_4 = Nodal_Coordinates(i,:)

for i = 5

Node_5 = Nodal_Coordinates(i,:)

for i = 6

Node_6 = Nodal_Coordinates(i,:)

for i = 7

Node_7 = Nodal_Coordinates(i,:)

for i = 8

Node_8 = Nodal_Coordinates(i,:)

end

end

end

end

end

end

end

end

end

[Turlough Hughes]

Your code tells me you are quite new to matlab. I would recommend you do the introductory onramp course for matlab.

Answer 1

[Tom Holz]

Something like this

for k = 1:size(Nodal_Coordinates,1)
    
    % Find the index of the kth row
    ind = find(Nodal_Coordinates(:,1) == k, 'first');
    
    % Extract the rest of that row, skipping the first column
    Node_Data = Nodal_Coordiantes(ind,2:end);
    
    % Do what you want with that data
    disp(Node_Data);
end

However, you might be able to simply this process by sorting the data first:

% Sortrows defaults to sorting by the first column in numeric order, 
% and if no two rows have the same index in the first column, 
% this will produce the desired result.
Nodal_Coordinates = sortrows(Nodal_Coordinates);

% Now you can just loop through the matrix and grab the data without using find().
for k = 1:size(Nodal_Coordinates,1)
    ind = Nodal_Coordinates(k,1);
    Node_Data = Nodal_Coordinates(k, 2:end);
end

[Damon Schmidt]

This works for me. Thanks! the sortrows was a moneymaker, thanks.

[Tom Holz]

Good to hear. MATLAB is full of gems like sortrows that can often replace entire loops. It just takes time to discover and become familiar with them.

[Damon Schmidt]

Sorry to ask a side question, but is there a way to label each output that the Node_Data displays. For example, instead of it displaying:

Node_Data =

0 0

Node_Data=

3 0

Can you have it say:

Node_1=

0 0

Node_2=

3 0

Node_3=

6 0

And so on? So basically have those outputs become variables? Thanks again for the help.

[Stephen23]

"Node_1 ... Node_2 ... Node_3 ... So basically have those outputs become variables?"

Putting numbers into variable names indicates that you are doing something wrong.

Accessing variable names dynamically is one way that beginners force themselves into writing slow, complex, obfuscated, buggy code that is hard to debug. Read this to know why:

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

The simple and efficient solution is to use one array and access the data using indexing.

[Tom Holz]

I agree with Stephen. If you want to display the data, look into a function like printf:

for k = 1:size(Nodal_Coordinates,1)
    ind = Nodal_Coordinates(k,1);
    Node_Data = Nodal_Coordinates(k, 2:end);
    fprintf('Node #%d: %s\n', ind, num2str(Node_Data));
end

If you want to reuse the data later, consider putting it into another variable, like a cell array. Cell arrays are quite versatile and are worth learning about.

nodeCount = size(Nodal_Coordinates,1);
nodes = cell(nodeCount,1);

for k = 1:nodeCount
    ind = Nodal_Coordinates(k,1);
    Node_Data = Nodal_Coordinates(k, 2:end);
    nodes{k} = Node_Data;
end