Matlab - select all rows starting with a particular number

Say I have a mat开发者_JAVA技巧rix A, and I want to construct a matrix B that contains all rows of B that start with a particular number. How? Thanks

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Select all rows of B into A, where the first colum of B has value n:

A = B(B(:,1) == n,:); 

In contrast to that, the following selects all rows of B into A, starting from row index n:

A = B(n:end,:);