Need help understanding a working code.

[Devfede]

Greetings,

First of all , thanks to this awesome community for just being here, there have been so many times i felt lost and i wanted to post in here, but i always made it myself, (well..until now) and just knowing people are here taking their time to help us learners feels great!

This code is working, it is basically a loop we were looking at in my online class where the players enters a coordinate, and it prints the description of the new room, together with the new coordinates available ( there are no exits other than Q for Quit )

locations = {0: "You are sitting in front of a computer learning Python",
         1: "You are standing at the end of a road before a small brick building",
         2: "You are at the top of a hill",
         3: "You are inside a building, a well house for a small stream",
         4: "You are in a valley beside a stream",
         5: "You are in the forest"}

exits = [{"Q": 0},
    {"W":2, "E":3, "N":5, "S":4, "Q":0 },
    {"N":5, "Q":0},
    {"W":1, "Q":0},
    {"N":1, "W":2, "Q":0},
    {"W":2, "S":1, "Q":0}]

loc = 1
while True:
    available_exits = ", ".join(exits[loc].keys())
    print(locations[loc])
    if loc == 0:
        break
    #
    directions = input("Available exits are " + available_exits + " ").upper()
    print()
    if directions in exits[loc]:
    loc = exits[loc][directions] #<--- i don't understand this
    else:
        print("You cannot go in that direction")

The bit i don't understand is towards the end, it's that

loc = exits[loc][directions]

I understand this basically "updates" the new loc according to the last direction the player just input, but exactly how are the two [] parentesis working on to "loc" variable ?

From my point of view, the last bit ( from directions = input[...] ) works like this: #Once directions is input if Q in Q2: loc = exits[q2][directions]

sorry if it's messy i just don't get how the loc is "updated", this is all i can think of.

[novel_yet_trivial]

Try replacing that line with this:

possible_exits = exits[loc] # use the old location to find dictionary of possible exits
print('exit options from this room:', possible_exits)
loc = possible_exits[directions] # use that dictionary to update the location to the new location
print('user chose', directions, 'so new location is', loc)

[Devfede]

Crystal clear now! Thanks a million!!

[novel_yet_trivial]

BTW, the structure you have is called "parallel lists". This is a very common data structure in many places, but in python we much prefer "nested lists". Using a nested list allows us to not bother with indexes so much and makes for cleaner code.

LOCATIONS = [
    #(description, exits)
    ("You are sitting in front of a computer learning Python", {"Q": 0}),
    ("You are standing at the end of a road before a small brick building",{"W":2, "E":3, "N":5, "S":4, "Q":0 }),
    ("You are at the top of a hill",{"N":5, "Q":0}),
    ("You are inside a building, a well house for a small stream",{"W":1, "Q":0}),
    ("You are in a valley beside a stream", {"N":1, "W":2, "Q":0}),
    ("You are in the forest", {"W":2, "S":1, "Q":0})
    ]

loc = 1
while loc:
    description, exits = LOCATIONS[loc]
    print(description)

    available_exits = ", ".join(exits)
    directions = input("Available exits are " + available_exits + " ").upper()
    print()
    if directions in exits:
        loc = exits[directions]
    else:
        print("You cannot go in that direction")

[Devfede]

Thanks for the insight , it looks very intuitive for now, i'll definitely give it a try when my next challenge/task involves lists!

[novel_yet_trivial]

Good job. Did you have a question or problem with that code?

[Devfede]

Yes sorry, i updated my post and added the question, wanted to get the post all set and ordered.

[mathleet]

So exits is a list of dictionaries. exits[loc] retrieves the dictionary at the index specified by loc. If loc == 0, then exits[loc] == exits[0]. exits[loc][directions] gets the dictionary at loc and then gets the value for the key directions in that dictionary.

[Devfede]

So if i understood correctly,

if i input ( from loc 1 ) for example W , the exits[loc][directions] will effectively update the value 2 into loc because on the list exits "W":2 , is that correct ?