python - 使用贪心python算法解决背包问题

Question

我正在尝试使用 Python 解决背包问题，实现贪心算法。我回来的结果对我来说毫无意义。

背包：

第一行给出了物品的数量，在本例中为 20。最后一行给出了背包的容量，在本例中为 524。其余行给出了每个项目的索引、价值和重量。

20
    1    91    29
    2    60    65
    3    61    71
    4     9    60
    5    79    45
    6    46    71
    7    19    22
    8    57    97
    9     8     6
   10    84    91
   11    20    57
   12    72    60
   13    32    49
   14    31    89
   15    28     2
   16    81    30
   17    55    90
   18    43    25
   19   100    82
   20    27    19
524

Python代码：

import os 

def constructive():     
    knapsack = []
    Weight = 0
    while(Weight <= cap):
        best = max(values)
        i = values.index(best)
        knapsack.append(i)
        Weight = Weight + weights[i]
        del values[i]
        del weights[i]
    return knapsack, Weight


def read_kfile(fname):
    with open(fname, 'rU') as kfile:
        lines = kfile.readlines()     # reads the whole file
    n = int(lines[0])
    c = int(lines[n+1])
    vs = []
    ws = []
    lines = lines[1:n+1]   # Removes the first and last line
    for l in lines:
        numbers = l.split()   # Converts the string into a list
        vs.append(int(numbers[1]))  # Appends value, need to convert to int
        ws.append(int(numbers[2]))  # Appends weigth, need to convert to int
    return n, c, vs, ws

dir_path = os.path.dirname(os.path.realpath(__file__))  # Get the directory where the file is located
os.chdir(dir_path)  # Change the working directory so we can read the file

knapfile = 'knap20.txt'
nitems, cap, values, weights = read_kfile(knapfile)
val1,val2 =constructive()
print ('knapsack',val1)
print('weight', val2)
print('cap', cap)

结果：

knapsack [18, 0, 8, 13, 3, 8, 1, 0, 3]
weight 570
cap 524

标签： pythonknapsack-problem

Answer 1

欢迎。您的程序给出的权重超过上限的原因是因为在您放入背包的最后一件物品上，您没有检查它是否可以放入其中。为此，只需添加一个 if 语句，您还应该检查值列表是否为空。请注意，我已附加 (i+1)，因为您的文本文件的索引从 1 开始，但 Python 从 0 开始它的列表索引：

def constructive():
    knapsack = []
    Weight = 0

    while(Weight <= cap and values):
        best = max(values)
        i = values.index(best)
        if weights[i] <= cap-Weight:
            knapsack.append(i+1)
            Weight = Weight + weights[i]
        del values[i]
        del weights[i]

    return knapsack, Weight