CODEWITHSUNDEEP
python
THEWHY
THEWHY

Reputation: 1

Why is this custom reverse function acting strange?

I needed to write a custom reverse function, here is the code:

def reverse(x):
    length = len(x)
    out = x
    for i in range(length):
        out[length-i-1] = x[i]
    return out

print(reverse([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]))

but for some reason this outputs [1, 2, 3, 4, 5, 6, 6, 5, 4, 3, 2, 1], does anyone know why this is happening?

Upvotes: 0

Views: 65

Answers (4)

Charles Dassonville
Charles Dassonville

Reputation: 207

Because you set out = x. And in python this is reference typed.

So in your loop, when you do out[0] = x[10], it sets out[0] to x[10], but it also sets x[0] to x[10] as x and out have the same reference.

If you want to create a reverse function from scratch, here is one of the best practices:

def reverse(my_list):
    i = 0             # first item
    j = len(my_list)-1   # last item
    while i<j:
        my_list[i], my_list[j] = my_list[j], my_list[i]
        i += 1
        j -= 1
    return my_list

Upvotes: 1

megaleunam
megaleunam

Reputation: 1

def reverse(x):
    length = len(x)
    out = []
    for i in range(1,length+1):
        out.append(x[-i])
    
    return out

Upvotes: -1

chepner
chepner

Reputation: 531868

out = x doesn't make a copy: it just makes the name out refer to the same list as the name x.

While you can make a copy with something like out = x.copy() or out = list(x), there's little reason to do so because you'll simply be overwriting every element of the resulting copy. Instead, just build a new list:

def reverse(x):
    length = len(x)
    out = []
    for i in range(length):
        out.append(x[length-i-1])
    return out

There are other, simpler and/or faster ways to produce a reversed list (list comprehensions, slicing, using the reversed type, etc) but I'll let you experiment with them on your own.

Upvotes: 2

rchome
rchome

Reputation: 2723

out = x makes the variable out point to the same list as x. You probably want to do out = x.copy().

Upvotes: 0

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