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

19 views (last 30 days)
Damon Schmidt on 2 Mar 2020
Commented: Tom Holz on 2 Mar 2020
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 on 2 Mar 2020
Your code tells me you are quite new to matlab. I would recommend you do the introductory onramp course for matlab.

Tom Holz on 2 Mar 2020
Edited: Tom Holz on 2 Mar 2020
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
Tom Holz on 2 Mar 2020
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

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