4.1. 串的基本运算 设 s =I AM A STUDENT, t =GOOD, q=WORKER求 练习 StrLength(s) StrLength(t) SubString(s, 8, 7)= SubString(t, 2, 1)= index(s, "A)= index(s, t)= 14 4 "STUDENT" "O" 3 0 s中没有t 再问strcat(substr(s,6,2,