range(len(alist) - 1, 0, -1)
时间: 2023-10-07 07:14:17 浏览: 35
这是一个Python中range函数的用法,用于生成一个逆序的整数序列,它的三个参数分别是起始值、终止值和步长。
具体来说,range(len(alist) - 1, 0, -1)会生成一个逆序的整数序列,起始值为len(alist) - 1,即列表alist的最后一个元素的下标;终止值为0,因为range不包含终止值,所以这里是0而不是-1;步长为-1,表示每次迭代递减1。
这个逆序的整数序列可以用于遍历一个列表(或其他序列)的所有元素,通常用于倒序遍历,比如从列表的末尾开始向前遍历。
相关问题
for i in range(len(pts)-1):
This is a Python code snippet that iterates through a range of values from 0 to the length of a list called "pts" minus 1 (meaning it goes through each index of the list except for the last one). It is typically used to perform some operation on adjacent elements in the list.
0-1背包问题贪心算法python·
0-1背包问题是一个经典的组合优化问题,贪心算法是其中一种解决方法。下面是一个基于单位价值量的贪心算法的Python实现:
```python
def fractional_knapsack(value, weight, capacity):
"""Return maximum value of items and their fractional amounts.
(max_value, fractions) is returned where max_value is the maximum value of
items with total weight not more than capacity.
fractions is a list where fractions[i] is the fraction that should be taken
of item i, where 0 <= i < total number of items.
value[i] is the value of item i and weight[i] is the weight of item i
for 0 <= i < n where n is the number of items.
capacity is the maximum weight.
"""
index = list(range(len(value)))
# item index list
ratio = [v/w for v, w in zip(value, weight)]
# calculate unit value of each item
index.sort(key=lambda i: ratio[i], reverse=True)
# sort item index in descending order of unit value
max_value = 0
fractions = [0]*len(value)
for i in index:
if weight[i] <= capacity:
fractions[i] = 1
max_value += value[i]
capacity -= weight[i]
else:
fractions[i] = capacity/weight[i]
max_value += value[i]*capacity/weight[i]
break
return max_value, fractions
```
其中,value和weight分别是物品的价值和重量,capacity是背包的容量。函数返回最大价值和每个物品的分数,以便计算每个物品的实际数量。