python – How to peek front of deque without popping?

python – How to peek front of deque without popping?

TL;DR: assuming your deque is called d, just inspect d[-1], since the rightmost element in a deque is the front (you might want to test before the length of the deque to make sure its not empty). Taking @asongtoruins suggestion, use if d: to test whether the deque is empty (its equivalent to if len(d) == 0:, but more pythonic)

Why not converting to list?

Because deques are indexable and youre testing the front. While a deque has an interface similar to a list, the implementation is optimized for front- and back- operations. Quoting the documentation:

Deques support thread-safe, memory efficient appends and pops from
either side of the deque with approximately the same O(1) performance
in either direction.

Though list objects support similar operations, they are optimized for
fast fixed-length operations and incur O(n) memory movement costs for
pop(0) and insert(0, v) operations which change both the size and
position of the underlying data representation.

Converting to list might be desirable if you have lots of operations accessing the middle of the queue. Again quoting the documentation:

Indexed access is O(1) at both ends but slows to O(n) in the middle.
For fast random access, use lists instead.

Conversion to list is O(n), but every subsequent access is O(1).

Here is a simple implementation that allowed me to check the front of the queue before popping (using while and q[0]):

Apply your own condition against q[0], before q.popleft(), below:

testLst = [100,200,-100,400,340]

while q:
    print({}{}.format(length of queue: , len(q)))
    print({}{}.format(head: , q[0]))



deque([100, 200, -100, 400, 340])
length of queue: 5
head: 100

deque([200, -100, 400, 340])
length of queue: 4
head: 200

deque([-100, 400, 340])
length of queue: 3
head: -100

deque([400, 340])
length of queue: 2
head: 400

length of queue: 1
head: 340

python – How to peek front of deque without popping?

You can simply find the last element using my_deque[-1] or my_deque[len(my_deque)-1] .

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