I have been asked an interview question to reverse an array. I used the following method and it worked as expected:
def reverse(array, i, j):
if i > j: # ensure i <= j
i, j = j, i
while i < j:
array[i], array[j] = array[j], array[i]
i += 1
j -= 1
Now, the interviewer asked me to replace the above while loop by for and I was really confused. Please can someone help me here. Thanks in advance.
This is not a for, but you can also do this:
If you follow the python logic where [i:j] means i <= x < j then you can do this:
array[i:j] = array[j-1:i-1:-1]
Other way, if you want to get the elements where i <= x <= j then you can do as @darksky suggested:
array[i:j+1] = array[j: i - 1: -1]
a[i:j+1] = a[j: i - 1: -1].
You could forget the for loop entirely and do something like this:
def reverse(array, i, j):
# Make sure i is less than j
if i > j:
i, j = j, i
# Reverse array[i:j+1]
section = array[i:j+1]
section.reverse()
array[i:j+1] = section
(The j+1 is to keep consistent with your function's behaviour; both i and j are treated as inclusive, but Python wants [inclusive, exclusive).)
Or if you want to keep the for loop in and avoid standard functions, then you could do something like this:
def reverse(array, i, j):
# Make sure i is less than j
if i > j:
i, j = j, i
# Reverse array[i:j]
count = j - i + 1
for offset in range(0, count // 2):
first = i + offset
second = j - offset
array[first], array[second] = array[second], array[first]
Other answers here perform this in a really smart way, but a for loop should look something like:
for x in range(j - i):
array[i+x], array[j-x] = array[j-x], array[i+x]
i < j Plus, it's the only answer that provides you with a for loop which is what you asked for. Using the implementation exactly as above, but returning array gives: rev([1,2,3,4,5], 1, 3) [1,4,3,2,5] which is as expected.
i == j, shouldn't there be no reversal at all to be done?j < i (two valid indicies, but largest first), it will do no work - not that that is a problem with your loop, I would expect a function written using it to check and swap the values as part of the initial parameter checks.I think more readable example would be:
arr[i,j] = reversed(arr[i, j])
array[i:j] = array[j:i:-1]array[i:j] = array[i:j][::-1]array[i:j] = array[j-1:i-1:-1]. Fora = list(range(10)),a[4:8] = a[8:4:-1]gives you[0, 1, 2, 3, 8, 7, 6, 5, 8, 9]!aafter the operation? What you're quoting is just the slicea[8:4:-1], not the result of the replace operation.