Turning raw_input into python expression

Try doing:

x = float(raw_input('-->'))

I'm assuming prob is an integer. raw_input returns the variable as a string, so you want to convert it to an integer/float. You get the error because you are trying to "add" a string and an integer; either you add two integers or you concatenate two strings, but they can't be done to one another. Hence, since x has to be an integer (or a float), use float(obj) or int(obj)--- you don't know whether or not x will be a float or an integer, so just use float.

To parse through the inequality given, do this:

#assuming no spaces in the inequality are included; if the input isn't in the correct form, the program will NOT work correctly
ineq = input("What is the inequality? Input in the form: ax+by<=c: ")
a, b, c, sign, index = 0, 0, 0, '', 0 #multiple assignment
while ineq[index] != 'x': #until it reaches x, it will keep going
   a += int(ineq[index])
   if ineq[index] == 'x':
      break
   index += 1  # index = index + 1

index += 2 #to go through the 'x' and '+'

while ineq[index] != 'y':
   b += int(ineq[index])
   if ineq[index] == 'y':
      break
   index += 1

index += 1 #to go through the 'y'

if ineq[index] == '<' and ineq[index + 1] == '=':
   sign = '<='
elif ineq[index] == '>' and ineq[iddex + 1] == '=':
   sign = '>='
elif ineq[index] == '>':
   sign = '>'
else:
   sign = '<'

if sign == '<=' or sign == '>=':
   index += 2 #to go through the '>' or '<' and the '='
else:
   index += 1 #to go through the '>' or '<'

c = int(ineq[index:])
#code to solve the inequality

An easier and faster way is to just prompt the user for each value of 'a', 'b', 'c', and the 'sign'.

So, there's a fundamental difference between your user input, and your example of in-code python:

User input, in this example, will be a string literal "2*a + 3*b <=4" You're not able to append a string literal to prob, because it's expecting a constraint or similar object -- this is where your error comes from.

prob += x 

When you evaulate the same expression in python, you'll notice it lacks the quotes which I helpfully placed around the statement up there.
This is not a string literal, it's a python expression. Some part of pulp evaluates this into a constraint object, and then adds that constraint to the problem.

prob += 2*a +3*b <= 4

Now, completely ignoring the potential for arbitrary code execution, the easiest way to solve your problem is to evaluate the user input as python code and let the language do the rest:

exec("prob +="+x);

It should be noted that unless this script is entirely for your personal use, this is a terrible idea.

The correct solution from a general programming standpoint is to construct an LPConstraint object from the string, and then add your newly constructed object to prob.

In an ideal world, you'd have a constructor for LPConstraint that takes a string, or a helper method somewhere in the libraries which does this for you.

I've done (admittedly all of 5 minutes...) some reading through the documentation and can't find any such methods. Assuming nobody else turns up any luck, you're left with parsing the string yourself (and all the hassle that entails) and building a constraint object from the results.

With a problem like this the question is really one of scope:

• Where are you installing this script?
• Who is going to be using it?
• What potential damage can they do?

If you're creating the python script to run in the computer labs of a high school or university, for the love of god do not use exec.

If you're creating the python script to run on your (and possibly your friends') machine, for your own purposes, then writing a parser is a blatant waste of your time and effort -- you are not going to execute malicious code on your own box.