PuLP不适用于一个列表，但适用于另一个列表两者有什么区别？

问题描述

我是python的新手，由于进行了优化而选择了它，我想学习一些新知识。我已经使用PyQt5和PuLP制作了一个lp解算器程序。这个概念很简单：用户在QTextEdit小部件中输入Lp问题，然后单击“解决”按钮，然后在QTextBrowser中得到结果。

我正在使用并尝试复制的示例练习：

prob = LpProblem("LP problem",LpMaximize)
x1 = LpVariable("x1",lowBound=0,cat='Integer') # Integer variable x1 >= 0
x2 = LpVariable("x2",cat='Integer') # Integer variable x2 >= 0

prob += x2<=4
prob += 4*x1 + 2*x2 <= 20
prob += 1*x1 + 4*x2 <= 12
prob += 4*x1 + 4*x2

prob.solve()

这种方式就像一种魅力。该按钮的功能：

def textToVar(self):
    prob = LpProblem("LP problem",LpMaximize)
    x1 = LpVariable("x1",cat='Integer') # Integer variable x1 >= 0
    x2 = LpVariable("x2",cat='Integer') # Integer variable x2 >= 0
    mytext = self.lpInput.toPlainText()
    split = mytext.splitlines()

    for ele in range(0,len(split)):
        prob += split[ele]

    prob.solve()

    for v in prob.variables():
        self.lpOutput.append(str(v.name) + ' = ' + str(v.varValue))


    vmax = (value(prob.objective))
    self.lpOutput.append('max = ' + str(vmax))

它不起作用，我认为这是因为split = mytext.splitlines()生成了['x2<=4','4*x1+2*x2<=20','1*x1+4*x2<=12','4*x1+4*x2']而不是[x2<=4,4*x1+2*x2<=20,1*x1+4*x2<=12,4*x1+4*x2]。如何将列表从第一个列表转换为第二个列表？也许我可以使用另一种方法将输入存储在列表或变量中，而不是splitlines()？

提前谢谢！

解决方法

您可以使用exec()，就像@AirSquid在其评论中指出的那样，但这确实引起了安全问题。另一种方法是解析字符串，因为您知道它们将包含什么。然后，如果出现意外情况，则可以轻松引发错误。

import re
import pulp as pl

x1 = pl.LpVariable("x1",lowBound=0,cat='Integer') # Integer variable x1 >= 0
x2 = pl.LpVariable("x2",cat='Integer') # Integer variable x2 >= 0

def parse_equation(string):
    string_parts = re.split("(<=|=|>=)",string)
    if len(string_parts) == 1:
        # Objective function
        return parse_equation_part(string_parts[0])
    if len(string_parts) != 3:
        raise Exception(f"Unexpected number of parts in {string_parts}")
    lhs,comparator,rhs = (
        parse_equation_part(string_parts[0]),string_parts[1],parse_equation_part(string_parts[2])
    )

    if comparator == "<=":
        return lhs <= rhs
    if comparator == ">=":
        return lhs >= rhs
    return lhs == rhs

def parse_equation_part(string):
    addition_parts = re.split("(\+|-)",string)
    result = parse_addition_part(addition_parts.pop(0))

    while addition_parts:
        symbol,addition_part,addition_parts =\
            addition_parts[0],addition_parts[1],addition_parts[2:]
        part_result = parse_addition_part(addition_part)

        if symbol not in ('+','-'):
            raise Exception(f"Unexpected value {symbol}")
        if symbol == '-':
            result -= part_result
        else:
            result += part_result
    return result

def parse_addition_part(string):
    parts = string.split("*")
    result = 1

    for part in parts:
        if part == 'x1':
            result *= x1
        elif part == 'x2':
            result *= x2
        elif part.isnumeric():
            result *= float(part)
        else:
            raise Exception(f"Unexpected part {part},expected number or x1/x2")

    return result

for s in ['x2>=4','4*x1+2*x2=20','1*x1-4*x2<=12','4*x1+4*x2']:
    print(s.ljust(20,' '),'->',parse_equation(s))

收益

x2>=4                -> x2 >= 4.0
4*x1+2*x2=20         -> 4.0*x1 + 2.0*x2 = 20.0
1*x1-4*x2<=12        -> x1 - 4.0*x2 <= 12.0
4*x1+4*x2            -> 4.0*x1 + 4.0*x2