(P4) 数组，String s的元素表达方式，[m,n)区间元素可表示为s[m:n]
For now, think of a slice as a dynamically size d sequence s of array elements where individual elements can be accessed as s[i] and a contiguous subsequence as s[m:n]. The number of elements is given by len(s).
(P4) 区间元素集表示，可以省略index,默认位置变为0（前位）和len(s)后位
If m or n is omitted, it defaults to 0 or len(s) respectively, so we can abbreviate the desired slice as os.Args[1:].
(P5) i++,i--但是没有--i, ++i
the y are postfix only, so --i is not legal
(P6) for循环条件语句不需要（）包裹
The for loop is the only loop statement in Go. It has a number of for ms, one of which is illustrated here:
for initialization; condition; post {
// zero or more statements
}
P6）for语句的条件语句有缺省部分，对应的分号可以省略
Any of these parts may be omitted. If there is no initialization and no post, the semicolons may also be omitted:
// a traditional "while" loop
for condition {
// ...
}
If the con dition is omitted entirely in any of these for ms, for example in
// a traditional infinite loop
for {
// ...
}
