这是基本的流程控制,实在不明白你就拿纸一步一步的写出来
for 循环第一次:
i = 1, i < 5
do while: i++; a++//i = 2, a = 1
i < 3
do while: i++; a++//i = 3, a = 2
i = 3
结束do while
for 里面的 i++//i = 4
for 循环第二次:
i = 4, i < 5
do while: i++; a++//i = 5, a = 3
i > 3
结束do while
for 里面的 i++//i = 6