MATLAB: Faster way of replacing multiple rows with same vector without using a loop

vector replacement

Hi,
I have a matrix a (see below). I would like to replace the 1st and 3rd row with the same row vector [255 100 0]. 
a =
   245   255   255
   254   252   255
   251   250   239
Initially I created an index variable 'idx' 
idx = [1;3]; % indexing row number for vector replacement
However 
a(idx,:) = [255 100 0 ];
would give error messages
'Unable to perform assignment because the size of the left side is 2-by-3 and the size of the right side is 1-by-3'.
How to do it correctly without do it in a loop?

Best Answer

Here are your two main choices for avoiding a loop.
Method one: match the elements. You need to make the LHS and the RHS have the same number of elements (and they need to be in the required order). Here is one easy way to do that:
a(idx,:) = repmat([255,100,0],numel(idx),1)
Tested:
>> idx = [1;3];
>> vec = [255,100,0];
>> a = [245,255,255;254,252,255;251,250,239]
a =
   245   255   255
   254   252   255
   251   250   239
>> a(idx,:) = repmat(vec,numel(idx),1)
a =
   255   100     0
   254   252   255
   255   100     0
Method two: use scalars. Treat each column individually and assign scalar values. This can be a viable solution if you working with a fixed, small number of columns (e.g. RGB channels):
>> a = [245,255,255;254,252,255;251,250,239]
a =
   245   255   255
   254   252   255
   251   250   239
>> idx = [1;3];
>> a(idx,1) = vec(1);
>> a(idx,2) = vec(2);
>> a(idx,3) = vec(3)
a =
   255   100     0
   254   252   255
   255   100     0
This can also be trivially looped ( I am sure that you can see how).
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