# How can I fill a matrix column with dta coming from another matrix according to index

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olivier espeli il 28 Gen 2015
Commentato: olivier espeli il 29 Gen 2015
I need to fill a large matrix with data coming from a smaller matrix. Both matrices contains a first column with an index and a second column with the results of an experiment. I need to combine the results according to their index. For example I have a matrix A:
1 1
2 1
3 1
4 1
5 1
6 1
7 1
8 1
9 1
And a matrix B
1 10
2 10
4 10
7 10
I would like to combine them to obtain the matrix C with 3 columns (column 1 for index, column 2 results from A, column 3 results from B)
1 1 10
2 1 10
3 1 NaN
4 1 10
5 1 NaN
6 1 NaN
7 1 10
8 1 NaN
9 1 NaN
I have to do that on matrices with millions of rows, therefore methods limiting calculation time would be welcome. Thank you very much for any help.
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### Risposta accettata

Matz Johansson Bergström il 28 Gen 2015
Modificato: Matz Johansson Bergström il 28 Gen 2015
Without knowing the size of the matrix, it might be a good idea to use a sparse matrix, instead of filling the non existant elements with NaN. Now, this is not exactly what you did, but it will give you a good hint of what you can accomplish with a sparse structure.
I did this fairly quick, because it is late where I live, but it should work fine
%create the example vectors
A = [(1:9)',ones(9,1)];
B = [1,2,4,7]';
B = [B, 10+0*B];
%putting the data into a sparse matrix
M = sparse([]);
M(A(:,1), 1) = A(:,2);
M(B(:,1), 2) = B(:,2);
Note that I use the first column in A as an index in M and the same goes for B. When I add the elements I add them in the first column of M, then the second column, separating where the values come frome.
I hope this helps.
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olivier espeli il 29 Gen 2015
thank you very much for your help

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### Più risposte (2)

Shoaibur Rahman il 28 Gen 2015
A = [(1:9)' ones(9,1)];
B = [1 10; 2 10; 4 10; 7 10];
C = NaN(size(A,1),1);
C(B(:,1)) = B(:,2);
C = [A C]
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olivier espeli il 29 Gen 2015
Thank you very much for you help

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Image Analyst il 28 Gen 2015
Try this:
A = [...
1 1
2 1
3 1
4 5
5 1
6 1
7 9
8 1
9 1]
B = [...
1 10
2 20
4 70
7 90]
% Find out what the max index could possibly be.
maxRow = max([A(:,1); B(:,1)])
C = zeros(maxRow, 3); % Preallocate C.
% Assign all possible indices to column 1 of C.
C(:,1)=1:maxRow;
% Assign column 2 of A to column 2 of C.
C(A(:,1),2) = A(:, 2);
% Assign column 2 of B to column 3 of C.
C(B(:,1), 3) = B(:, 2)
In the command window:
C =
1 1 10
2 1 20
3 1 0
4 5 70
5 1 0
6 1 0
7 9 90
8 1 0
9 1 0
It's vectorized so it should be pretty fast. Anyway, a million rows is not that many. Here it is with a million rows and it took 0.03 seconds:
numberOfRows = 1000000;
A = int32(randi(99, [numberOfRows, 2]));
B = int32(randi(99, [numberOfRows, 2]));
tic;
% Find out what the max index could possibly be.
maxRow = max([A(:,1); B(:,1)]);
C = zeros(maxRow, 3); % Preallocate C.
% Assign all possible indices to column 1 of C.
C(:,1)=1:maxRow;
% Assign column 2 of A to column 2 of C.
C(A(:,1),2) = A(:, 2);
% Assign column 2 of B to column 3 of C.
C(B(:,1), 3) = B(:, 2);
toc;
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olivier espeli il 29 Gen 2015
Thank you very much, very helpful.

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