c语言我给数组的随机数显示不出来

c语言我给数组赋值的随机数显示不出来，显示无，代码可以运行

int main()
{
char ch;
char str[51] = "";
int i;
time_t start_time, end_time;
while (1)
{
system("cls");
help();
ch = _getch();

    srand((unsigned int)time(NULL));
    for (i = 0; i < 50; i++);
    {
        str[i] = rand() % 26 + 'a';
    }
    str[50] = '\0';
    printf("%s\n", str);
      }

return 0;
}

人才，for();{}你这个分号加的我无语了