CODEWITHSUNDEEP
pythonglobal-variablesbacktrackingrecursive-backtracking
crazyy_photonn
crazyy_photonn

Reputation: 171

Global variable for recursion in Python

I am facing some difficulties in backtracking.

  1. How do I define a global list to be used in the problems of backtracking? I saw a couple of answers and they have all suggested using 'global' keyword in front of the variable name, inside the function to use as global. However, it gives me an error here.

  2. Is there any good general method that can be used to get the result, instead of a global variable?

The code below is trying to solve the backtrack problem wherein, a list of numbers is given, and we have to find unique pairs (no permutations allowed) of numbers that add to the target. 

For example, given candidate set [2, 3, 6, 7] and target 7, 

A solution set is: 
[
  [7],
  [2, 2, 3]
] 


       ///////////////////////////////CODE/////////////////////////////


       seen = []
        res = []

        def func(candidates, k, anc_choice):     #k == target

            #global res -- gives me an error --  global name 'res' is not defined

            if sum(anc_choice) == k:
                temp = set(anc_choice)
                flag = 0

                for s in seen:
                    if s == temp:
                        flag = 1
                if flag == 0:
                    seen.append(temp)
                    print(anc_choice)  #this gives me the correct answer
                    res.append(anc_choice)  #this doesn't give me the correct answer?
                    print(res)

            else:
                for c in candidates:
                    if c <= k:
                        anc_choice.append(c) #choose and append
                        if sum(anc_choice) <= k:
                            func(candidates, k, anc_choice) #explore
                        anc_choice.pop() #unchoose

        func(candidates, k, [])

Can someone please provide me answers/suggestions?

Upvotes: 0

Views: 4136

Answers (2)

Olivier Melan&#231;on
Olivier Melan&#231;on

Reputation: 22324

There are plenty of reasons why you should not use global variables.

If you want a function that updates a list in the above scope, simply pass the list as argument. Lists are mutable, so it will have been updated after the function call.

Here is a simplified example.

res = []

def func(candidate, res):
    res.append(candidate)

func(1, res)

res # [1]

Upvotes: 1

johnashu
johnashu

Reputation: 2211

To utilise the global keyword, you first need to declare it global before instantiating it..

global res
res = []

Though from looking at your code. Because the res = [] is outside the function, it is already available globally.

Upvotes: 1

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