nums = [5, 12, 7, 9, 21, 7, 14]
# 统计数字7出现的次数
count_seven = nums.count(7)
print("数字7出现的次数：",count_seven)
# 判断99是否存在，不存在则添加
if 99 not in nums:
	nums.append(99)
print("添加99后的列表：",nums)
# 删除第一次出现的数字7
nums. remove(7)   # 只删除第一个7
print("删除第一个7后的列表：", nums)
# 取出第2个到第4个元素（索引1到3）
sub_list = nums [1:4]
print("第2~4个元素组成的列表：",sub_list)
 

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