for-in循环遍历项目集合，例如：数字范围，数组中的项目或字符串中的字符 -
语法
Swift 4编程语言中for-in循环的语法是 -
for index in var {
statement(s)
}
执行流程图 -
示例代码
var someInts:[Int] = [10, 20, 30]
for index in someInts {
print( "Value of index is \(index)")
}
执行上面示例代码，得到以下结果 -
Value of index is 10
Value of index is 20
Value of index is 30
