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元素组成的列表",sub_list)
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数字7出现的次数: 2
添加99后的列表: [5, 12, 7, 9, 21, 7, 14, 99]
删除第一个7后的列表: [5, 12, 9, 21, 7, 14, 99]
第2-4元素组成的列表 [12, 9, 21]