求大侠帮忙，以下这段代码如何实现让其循环运行50次？
USE ALL.
do if $casenum=1.
compute #s_$_1=1900.
compute #s_$_2=2000.
end if.
do if  #s_$_2 > 0.
compute filter_$=uniform(1)* #s_$_2 < #s_$_1.
compute #s_$_1=#s_$_1 - filter_$.
compute #s_$_2=#s_$_2 - 1.
else.
compute filter_$=0.
end if.
VARIABLE LABEL filter_$ '从第一 2000 个案中的 200 (SAMPLE)'.
FORMAT filter_$ (f1.0).
FILTER  BY filter_$.
EXECUTE.
RELIABILITY
  /VARIABLES=V2 V52 V54 V60 V7 V36 V37 V33 V19 V42
  /SCALE('ALL VARIABLES') ALL
  /MODEL=ALPHA.