How to avoid data overwrite problem in looping

I have some data overwrite problem in looping

%%%%%%%read data%%%%%%%%%%%%%%%
origindata=importdata('rawdata.txt');
f=origindata(:,1);
a=origindata(:,2);
b=origindata(:,3);
c=origindata(:,4);
d=origindata(:,5);

fstep=0.17;

y=0;
for t=(0.1:0.1:20)

e=a-j*b;
u=c-j*d;
rl=u.*e.*f*t/3;

%%%save results in y1, for each t, it save three columns 3*y-2, 3*y-1 and 3*y%%%
y=y+1;
y1(:,3*y-2)=f;
y1(:,3*y-1)=rl;
y1(:,3*y)=t;

%%%select data from three columns 3*y-2, 3*y-1 and 3*y and save in x1, only select those with rl value less than 0%%%

x1=y1(y1(:,3*y-1)<0, :); hold on;

%%%count the number of row in column 3*y-1%%%
n=size(x1,3*y-1);
totalwidth = fstep*(n+4);

%%%write the data in columns 3*y-2, 3*y-1 and 3*y%%%
x1(n+4, 3*y-2:3*y)=[totalwidth 0 t];

hold on;
end;

xlswrite('y1',y1);
xlswrite('x1',x1);

Problems: in x1, the last three column overwrite all previous columns.

Can anyone help me out? Thank you.