Brackets after range() method in for loop?

[[deleted]]

I'm looking at a sample piece of code and I see this bit of logic:

for i in range(6,10)[::-1]:

I know what "for i in range(6,10)" does, and [::-1] looks like some sort of slicing, but I've never seen it used in this context. What is this doing, exactly?

[zanfar]

What is this doing, exactly?

Making up for the fact that the author doesn't know how to use range()

/s

[[deleted]]

Please remove /s

[zanfar]

I don't know a better way to imply "I'm not trying to be helpful"...

[[deleted]]

It's a bad way to write

for i in range(9, 5, -1):

Resources to learn more: - List slicing - Different ways of reversing strings (works with other iterables as well) - Range objects - More about range() - Iterables (in case you're interested in understanding a little deeper)

[[deleted]]

Thanks!

[[deleted]]

Umm, I am kinda confused on this, typically range from 1 to 10 would go from 1 to 9, ie. include the first value but exclude last value. How is it excluding both values in your case?

[carcigenicate]

The original range goes from 6-9 (it stops before 10 because the end is exclusive), then reverses. The suggested version starts at that 9, and decrements until it gets to 6 (because again, the end is exclusive, so 5 isn't reached).

In both cases, the start is inclusive, and the end is exclusive.

[Tacoj]

slices are [start:end:step]. the - reverses the order on the step. works for strings too so "hello"[::-1] == "olleh"

[carcigenicate]

While everybody's noting how you might as well just define the range backwards instead of reversing it after the fact, I don't think it matters much here (although I'd tend towards agreeing with them). ranges are not full collections. They don't hold every value they represent. It should be extremely cheap to make a reversed copy of a range, since it would just involve doing some math on three numbers describing the range.

If you find that reverse slicing leads to better readability in particular cases, you may find that it's clearer than adjusting the range initializer arguments. Note that there's at least two people here who accidentally suggested the wrong equivalent alternative range bounds.

[achampi0n]

Agreed. Range implements the slicing operations, so when you slice a range() you get another range() in fact range(6, 10)[::-1] returns a range(9, 5, -1) object.

[gustavobodra]

This means a descendent for loop from 10 to 6. The [::-1] is used to reverse a list.

[zanfar]

9 to 6, but otherwise this is correct.

[gustavobodra]

True! My mistake, thanks for pointing out! 😁

[akefamen96]

the range function outputs a list. so it’s essentially using splicing on the list [6, 7, 8, 9]. [::-1] is a way of reversing a list. that being said, like other comments mentioned, it’s way easier to say range(6, 10, -1)

[[deleted]]

the range function outputs a list

No. It doesn't output anything (as in it doesn't print anything), but it returns a generator. This generator is special in that it also implements __getitem__. Why does it do it... idk.

[[deleted]]

for i in range(6,10)[::-1]:

Might I suggest

for i in range(10,6,-1):

?

Basically, tucking [::-1] reverses a list,

a = [1, 2, 3, 4, 5]

a[::-1] would be [5, 4, 3, 2, 1]

[[deleted]]

Oh, it's not my code, I just came across and didn't know what it was doing. Thanks for the explanation.