pb字符串加1或者0,该怎么处理

pb字符串加1或者0
string ls_qx = "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
ls_qx共256位，通过数据窗口dw_1中的单选按钮列xzbz，可以设置每一位为1或者0。
do while li_num <= dw_1.rowcount()
ll_xh = dw_1.getitemnumber(li_num,"RoleCode")
if dw_1.getitemnumber(li_num,"xzbz") = 1 then
replace(ls_qx,ll_xh,1,'1') 
else
replace(ls_qx,ll_xh,1,'0') 
end if
li_num = li_num + 1
loop
执行上述代码后，ls_qx返回值仍然是256个0,请朋友指导,谢谢！

------解决方案--------------------
忘了说了，ls_qx最好定义实例变量
------解决方案--------------------
if dw_1.getitemnumber(li_num,"xzbz") = 1 then
ls_qx=replace(ls_qx,ll_xh,1,'1')
else
ls_qx=replace(ls_qx,ll_xh,1,'0')
end if