Python: using while or for loop to iterate through an lists

Question

I am trying to connect values in tuples in a list without using a dictionary. Specifically, I have this list:

adjList = [('0', '3'), ('1', '0'), ('3', '2'), ('4', '2'), ('5', '4'), ('7', '9'), 
('8', '7'), ('9', '6'), ('2', '1'), ('2', '6'), ('6', '5'), ('6', '8')]

and I would like to create a list with the values from a random tuple:

newList = ['1', '0']

then append the second value of a tuple from adjList if the first value of that tuple is the same as the last value of newList, thus:

newList = ['1', '0', '3']

Then delete ('1', '0') and ('0', '3') from adjList.

THEN I want to repeat this action until the last value in newList NO LONGER corresponds to the first value of a tuple from adjList. I am having a lot of trouble figuring out a logical combination of while or for loops that can do this, and any help would be appreciated.

My code so far:

adjList = [('0', '3'), ('1', '0'), ('3', '2'), ('4', '2'), ('5', '4'), ('7', '9'), 
('8', '7'), ('9', '6'), ('2', '1'), ('2', '6'), ('6', '5'), ('6', '8')]

firstNode = random.choice(adjList)
newList = []
newList.append(firstNode[0])
newList.append(firstNode[1])
adjList.remove(firstNode)

## I need to repeat the following block of code:

for ax,bx in adjList:
    if newList[-1] == ax:
        adjList.remove((ax,bx))
        newList.append(bx)
        break

Everything works the way it should, but of course I am only getting 3 values in newList at the end. I can't quite work out how to repeat that final block of code until I run out of tuples in adjList.

Thanks in advance for any help.

What about the case where there are many possible tuples to select from adjList? Let's say that randomly selected tuple is ('9', '6') what would be the correct value of newList? — niemmi
– niemmi, Commented Oct 23, 2016 at 2:09
The correct value of newList would then be ['9', '6', '5']. It is fine to just take the first tuple with the matching value. — Andrade
– Andrade, Commented Oct 23, 2016 at 2:11
I guess it would be easier to debug if you would not choose a random node. — Lord_PedantenStein
– Lord_PedantenStein, Commented Oct 23, 2016 at 2:15
The random node has nothing to do with the block of code that I need to repeat, though. Think of it just as the two starting lists if that's easier: adjList as posted, and newList = ['1', '0'], then I need to go from there, matching values and adding them to newList. — Andrade
– Andrade, Commented Oct 23, 2016 at 2:18

niemmi · Accepted Answer · 2016-10-23 02:31:04Z

1

You could just run the outer while loop while there are items still on the adjList. The inner loop could pick the first suitable item from adjList and append the result to newList. In case the inner loop can't find suitable item the outer loop should be terminated.

Here's a sample of the above:

import random

adjList = [('0', '3'), ('1', '0'), ('3', '2'), ('4', '2'), ('5', '4'), ('7', '9'),
('8', '7'), ('9', '6'), ('2', '1'), ('2', '6'), ('6', '5'), ('6', '8')]

newList = list(adjList.pop(random.randint(0, len(adjList) - 1)))

while adjList:
    for i, (src, dest) in enumerate(adjList):
        if src == newList[-1]:
            del adjList[i]
            newList.append(dest)
            break
    else:
        break

print('Result: {}'.format(newList))
print('Remaining: {}'.format(adjList))

Output:

Result: ['4', '2', '1', '0', '3', '2', '6', '5', '4']
Remaining: [('7', '9'), ('8', '7'), ('9', '6'), ('6', '8')]

answered Oct 23, 2016 at 2:31

niemmi

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1 Comment

Andrade Over a year ago

This is also a great solution to my problem! Thanks so much, I appreciate the help.

Savir · Accepted Answer · 2016-10-23 02:20:32Z

I'm not very sure whether the following code will apply to your needs, but I think you should be able do what you want with very few changes to your code.

I've added a while loop that runs every time there's a change in the structure (basically, every time that a tuple whose first item matches the last item in newList):

#!/usr/bin/env python
import random

adjList = [('0', '3'), ('1', '0'), ('3', '2'), ('4', '2'), ('5', '4'), ('7', '9'),
           ('8', '7'), ('9', '6'), ('2', '1'), ('2', '6'), ('6', '5'), ('6', '8')]

firstNode = random.choice(adjList)
newList = []
newList.append(firstNode[0])
newList.append(firstNode[1])

changes_made = True
while changes_made:
    changes_made = False
    for item in adjList:
        if item[0] == newList[-1]:
            newList.append(item[-1])
            adjList.remove(item)
            changes_made = True
            break

print newList

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