CODEWITHSUNDEEP
matlabmatrixrows
Geoff
Geoff

Reputation: 65

Expand matrix based on first row value (MATLAB)

My input is the following:

X = [1 1; 1 2; 1 3; 1 4; 2 5; 1 6; 2 7; 1 8];

X =

 1     1
 1     2
 1     3
 1     4
 2     5
 1     6
 2     7
 1     8

I am looking to output a new matrix based on the value of the first column. If the value is equal to 1 -- the output will remain the same, when the value is equal to 2 then I would like to output two of the values contained in the second row. Like this:

 Y =

     1
     2
     3
     4
     5
     5
     6
     7
     7
     8

Where 5 is output two times because the value in the first column is 2 and the same for 7

Upvotes: 3

Views: 343

Answers (2)

Bitwise
Bitwise

Reputation: 7807

Here it is (vectorized):

C = cumsum(X(:,1))
A(C) = X(:,2)
D = hankel(A)
D(D==0) = inf
Y = min(D)

Edit:

Had a small bug, now it works.

Upvotes: 2

arr_sea
arr_sea

Reputation: 853

% untested code:
Y = []; % would be better to pre-allocate 
for ii = 1:size(X,1)
   Y = [Y; X(ii,2)*ones(X(ii,1),1)];
end

Upvotes: 0

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