VB 一个循环搜索的问题

不用循环
Private Sub Command1_Click()
s$ = "dadfdsa1fdafjfkdafdsaaadfdsasdf"
MsgBox InStrRev(s, "a")
End Sub

非要用循环就这样
Private Sub Command1_Click()
s$ = "dadfdsa1fdafjfkdafdsaaadfdsasdf"
For i& = Len(s) To 1 Step -1
If Mid$(s, i, 1) = "a" Then Exit For
Next
MsgBox i
End Sub

'本程序是用right 和 left 两个函数对字符串进行操'作。先取字符串长存在Num中，再从后往前依次取倒数'第1、2、3...直到第Num个字符串，并在取完后对字符'穿首字符进行判断，如果是a则得到a在整个串中的倒'数位置并存于i中，再用整串的长相减便得所需结果
sub commond_click()
dim str as string,nowstr as string
dim Num as integer,i as integer,Lastanum as integer
str="要操作的字符串"
Num=Len(str)
for i=1 to Num
nowstr=right(str,i) '取右边i个字符串
if left(nowstr,1)=="a" then
Lastanum=Num-i+1
MsgBox Lastanum,vbinformation '消息框显示a的位置
exit for ’如果找到推出循环
end if
next i

end sub

Sub