Error on Filter function

[Christian D'Aquino]

Hi, I'm trying to run a butterworth filter, from a code example I found on the documentation package. This is the code I'm using: 

sf = 800; sf2 = sf/2;
 data=""
 [b,a]=butter ( 1, 50 / sf2 );
 filtered = filter(b,a,data);

 clf
 subplot ( columns ( filtered ), 1, 1)
 plot(filtered(:,1),";Impulse response;")
 subplot ( columns ( filtered ), 1, 2 )
 plot(filtered(:,2),";25Hz response;")
 subplot ( columns ( filtered ), 1, 3 )
 plot(filtered(:,3),";50Hz response;")
 subplot ( columns ( filtered ), 1, 4 )
 plot(filtered(:,4),";100Hz response;")

And this is the error I keep getting:

>> filter
error: invalid use of script C:\Users\Ingo\filter.m in index _expression_
error: called from
    filter at line 4 column 11

Can anyone help me understand what's wrong here?

Thanks,

Christian