how to change part of a matrix into zero

i have a matrix A= (m,n) and i want to make the first 10% row elements 0 suppose A= [1 1 1; 2 2 2; 3 3 5; 4 4 4; 5 5 1; 6 1 3; 1 6 5; 7 7 1; 4 8 2; 5 0 9] will become B= [0 0 0; 2 2 2; 3 3 5; 4 4 4; 5 5 1; 6 1 3; 1 6 5; 7 7 1; 4 8 2; 5 0 9]

Answer 1

[Azzi Abdelmalek]

n1=ceil(0.1*size(A,1));
A(1:n1,:)=0

Answer 2

[Andrei Bobrov]

n = round(numel(A)*.1);
B = A';
B(1:n) = 0;
B = B';

[napo]

very nice work, thank you

[djibeyrou ba]

a=4;

b=3;

A=zeros(a,b);

B=size(A)

if a== b

B=A==-1;

else

B=A==0;

end

disp(B)

I need help with this code i want to change some value in the matrix

Answer 3

[djibeyrou ba]

I need help with this code