How can I find 2-tuples with a specific first element in the set of 2-tuples?

Question

I have a set of pairs with types (str, int). I shall find the tuples that contain specific string and then increment the corresponding integer by 1. I know how to find tuples in a set with asking whether myTuple in mySet . But I do not know the way to handle such situations. I will appreciate any help.

Here is an example set :

from sets import Set
up = 2
down = 3
right = 1
left = 2
mySet = Set([("up",up),("down",down),("right",right),("left",left)])

Say from mySet I want to increment the second of the pair where its first element is "up" by 1 so I need something like

if ("up",ref) in mySet:
    ref += 1 

By doing this, I want to increment both the value of the original variable up and second element of the tuple.

Answer 1

I would suggest using a dictionary for your purpose. That way, the re-assignment of the new count would be more clean and easier to interpret. For instance:

>>> my_set = { 'up': 0, 'down': 0, 'left': 0, 'right': 0 }
>>> my_set['up'] += 1
>>> my_set
{'up': 1, 'down': 0, 'left': 0, 'right': 0}

As Brian describes, tuples are non-mutable so each time you update the count it has to be created a new one with the updated count. This can then be used to replace the old tuple.

Answer 2

you can use the filter method to find all entries in the set where your condition, in this case the first entry being "up" is true.

subset = list(filter(lambda x: x[0] == "up", mySet))
for entry in subset:
    mySet.remove(entry)
    newEntry = (entry[0],(entry[1]+1))
    mySet.add(newEntry)
    up +=1 

you cannot use entry+=1 because tuples are non-mutable in python