How to use EOD data to get weekly info?

["Greg Q. Liu" <liugq@xxxx>]

I try to write an AFL fomula to scan weekly 
crossing for 4 week periods by using daily data as
 
<FONT face=Arial 
size=2>weekly_avg=(weekly_high+weekly_low+weekly_open+weekly_close)/4;
filter=cross(weekly_avg, ema (weekly_avg,40) 
);
addcolumn( weekly_avg, "weekly_avg" );

addcolumn( ema (weekly_avg,40), "ema 
(weekly_avg,40)" );
 
But I only have daily data, how this can be done? 
Thanks.
 
GL