shift matrix from a specific row

I have a matrix that I need to shift some rows according to a special number i, for example if i=3 I have to shift all rows from row 3 down because I have to add new elements to that row, so I should shift all the rows after this one (rows 1 and 2 have to stay the same.) for doing this I added a new row to my matrix with all elements of zero, and now I have to shift this row of zeros (which is the last row) to the i'th row. for example:

i=3
a=[1,2,3;4,5,6;7,8,9,10,11,12]

and I want to add b=[8,8,8] to the third row, so rows: 7,8,9 and 10,11,12 have to shift down. after adding row of zeros:

   a=[1,2,3;4,5,6;7,8,9,10,11,12;0,0,0]

and then row 0,0,0 should be shifted to the third row which is 7,8,9 and row 7,8,9 should shift to 4th row and so on.. how can I do this in matlab?

Answer 1

[Geoff Hayes]

negin - if you want to insert b as the third row of a then you could just do

 a = [1,2,3;4,5,6;7,8,9;10,11,12];
 b = [8 8 8];
 a = [a(1:2,:) ; b ; a(3:end,:)];

In the above, we just concatenate the first two rows of a (that is, a(1:2,:)) with b and then with the remaining rows of a to get

 a =
     1     2     3
     4     5     6
     8     8     8
     7     8     9
    10    11    12

[negin tebyani]

thanks

Answer 2

[KL]

Pretty much the same answer as Geoff Hayes' but just with a handle,

add_row = @(ind,a) [a(1:ind-1,:); zeros(1,size(a,2)); a(ind:end,:)];
ind=3
a=[1,2,3;4,5,6;7,8,9;10,11,12]
b = add_row(ind,a)

[negin tebyani]

thank you for the answer

[KL]

You're very welcome!