TypeError when instantiating a deque and then popping-CodePudding

I'm trying to understand why I am getting this TypeError when instantiating my deque. I am solving the "Number of Islands" problem

def bfs(r,c):
    q = collections.deque((r,c))
    while q:
        r_curr, c_curr = q.popleft()
        for dr, dc in dirs:
            r_next, c_next = r_curr   dr, c_curr   dc
            if is_valid(r_next, c_next):
                visited.add((r_next, c_next))
                q.append((r_next, c_next))

Gives me the below error:

TypeError: cannot unpack non-iterable int object
    r_curr, c_curr = q.popleft()

But the below works without error.

def bfs(r,c):
    q = collections.deque()
    q.append((r,c))
    while q:
        r_curr, c_curr = q.popleft()
        for dr, dc in dirs:
            r_next, c_next = r_curr   dr, c_curr   dc
            if is_valid(r_next, c_next):
                visited.add((r_next, c_next))
                q.append((r_next, c_next))

Why does the first method fail but the second method works?

CodePudding user response：

You are getting the error when poplefting from deque not when instantiating it. Because your deque has two elements at the start (r and c) not one tuple (r, c). To have a tuple in deque initialize it like so:

q = collections.deque([(r,c)])

CodePudding user response：

deque takes an iterable, using the elements to form itself. So deque((r, c)) gives you the deque equivalent of (r, c). You want this to be a deque with one item: (r, c), not two items: r and c, so you must use deque(((r, c),)).