Newbie questions on Python

[idkfaidkfaidkfa]

Hi all,
i'm programming in python for the first time (usually i use C as programming language). I don't understand these results:

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

[3, 2, 1]

what does a[2::-1] means?
Thanks

 

[Chris Angelico]

Hi all,
i'm programming in python for the first time (usually i use C as programming language). I don't understand these results:

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

[3, 2, 1]

what does a[2::-1] means?

That's taking a slice. This page has something to say on the subject:

http://docs.python.org/3.3/tutorial/introduction.html

By the way, regarding your email address: there are no cheat codes in
Python... either that, or Python *is* a cheat code.

ChrisA
[[ VERY HAPPY CODING ADDED ]]

 

[Matt Jones]

When slicing: l[start:end:step]

In your example of "a[2::-1]" you are reversing the list by using a step of
-1, then you are slicing at index 2 (third element).

*Matt Jones*

Hi all,
i'm programming in python for the first time (usually i use C as

programming language). I don't understand these results:

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

[3, 2, 1]

what does a[2::-1] means?

That's taking a slice. This page has something to say on the subject:

http://docs.python.org/3.3/tutorial/introduction.html

By the way, regarding your email address: there are no cheat codes in
Python... either that, or Python *is* a cheat code.

ChrisA
[[ VERY HAPPY CODING ADDED ]]

 

[Neil Cerutti]

Hi all,
i'm programming in python for the first time (usually i use C as programming language). I don't understand these results:

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

[3, 2, 1]

The third item is the "step". The default value is 1. If you
provide a negative step, your slice will be in reverse. So you
are getting item 2 through 0 in reverse order in your result
slice.

Imagine something like the following for loop taking place
somewhere:

for (int i = 2; i <= 0; --i) {
fprintf(a);
}

 

[Lele Gaifax]

Imagine something like the following for loop taking place
somewhere:

for (int i = 2; i <= 0; --i) {
fprintf(a);
}

Neil most probably meant

for (int i = 2; i >= 0; --i) {
fprintf(a);
}

where "fprintf" is actually a fictitious "do_something" function.

ciao, lele.

 

[Neil Cerutti]

Imagine something like the following for loop taking place
somewhere:

for (int i = 2; i <= 0; --i) {
fprintf(a);
}

Neil most probably meant

for (int i = 2; i >= 0; --i) {
fprintf(a);
}

where "fprintf" is actually a fictitious "do_something" function.

ciao, lele.

Thanks for the correction.

 

[Chris Angelico]

ROFLMAO. Incidentally, my son used to use IDDQD rather than IDKFA.

I of course spurned all such, since I preferred to do it the hard way.
Thus I was Doomed.

I'd sometimes use IDDQD/IDFA (no K) and then see how quickly I could
blitz the levels, with proper navigation. There's something inherently
fun about blasting through everything with the rocket launcher.

Mind you, I was really cheap, so I never bought Doom... and was just
playing through the demo over and over.

ChrisA