int(rnd*31-5)的范围

Rnd 函数返回小于 1 但大于或等于 0 的值。
故0<=rnd*31<31
-5<=rnd*31-5<26
int(rnd*31-5) 的值域就是[-5,26)中的整数，即
{-5,-4,-3,-2,
-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24，25}

0<rnd*31<30
所以
-5<rnd*31-5<25
于是
int(rnd*31-5)属于{-5,-4,-3,-2,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24}