这是一个判断输入的字符串有大写字母多少个，小写字母多少个，数字字符多少个，其它字符多少个的代码，是我看的教材上的，我不太明白，请那位大虾给逐句注释一下，不胜感谢！

dim a(4) as integer,ch as srting,instring as integer
dim i as integer
dim srt as string()={"大写字母","小写字母","数字字符","其它字符"}
instring=txtinput.text
for i=0 to instring.length-1
ch=instring.chars(i)
select case ch
case "A"to"Z"
a(0)=a(0)+1
case "a"to"z"
a(1)=a(1)+1
case "0"to"9"
a(2)=a(2)+1
case else
a(3)=a(3)+1
end select
next i
for i=0 to 3
txtoutput.text +=srt(i)+a(i).tostring+vbcrlf
next

谢谢梓风，有几句不明白，请指点一二，谢谢！
Dim srt As String() = {"大写字母", "小写字母", "数字字符", "其它字符"}
instring = TextBox2.Text
For i = 0 To instring.ToString.Length - 1
ch = instring.ToString.Chars(i)
这几句不太明白啊！

多谢梓风的指点，我还是不太明白 a(0) = a(0) + 1以及后面类似的几句的意思，请您指点啊，谢谢！