# ========== 列表操作 ==========
nums = [5, 12, 7, 9, 21, 7, 14]
count_seven = nums.count(7)
print("数字7出现的次数：", count_seven)
if 99 not in nums:
    nums.append(99)
print("添加99后的列表：", nums)

nums.remove(7)
print("删除第一个7后的列表：", nums)
sub_list = nums[1:4]
print("第2~4个元素组成的列表：", sub_list)

# ========== 字符串操作 ==========
msg = "hello 123 WORLD"
#统计长度和小写字母个数
length = len(msg)
lower_count = sum(1 for ch in msg if ch.islower())
print("长度：", length)
print("小写字母个数：", lower_count)
#替换并转小写
new_msg = msg.replace("WORLD", "Python")
result = new_msg.lower()
print("结果：", result)