Slice a list of lists?

[Jonno]

I know that I can index into a list of lists like this:
a=[[1,2,3],[4,5,6],[7,8,9]]
a[0][2]=3
a[2][0]=7

but when I try to use fancy indexing to select the first item in each
list I get:
a[0][:]=[1,2,3]
a[:][0]=[1,2,3]

Why is this and is there a way to select [1,4,7]?

 

[Andreas Waldenburger]

I know that I can index into a list of lists like this:
a=[[1,2,3],[4,5,6],[7,8,9]]
a[0][2]=3
a[2][0]=7

but when I try to use fancy indexing to select the first item in each
list I get:

Let me write out in words what you're doing, and it should become clear:

a[0][:]=[1,2,3]

Here you're making a list of all elements of the first element of a.

a[:][0]=[1,2,3]

And here you're selecting the first element of all elements of a.

Huh. Not quite as clear as I hoped. But ponder on this for a few
moments. It'll dawn on you eventually.

Why is this and is there a way to select [1,4,7]?

zip(*a)[0]

(or rather list(zip(*a)[0]), if you definitely need a list and not a
tuple)


/W

 

[Andreas Waldenburger]

Let me rephrase what I wrote a bit.

a[0][:]=[1,2,3]

Here you're making a list of all elements of the first element of a.

That is, you're making a copy of the first element of a.

a[:][0]=[1,2,3]

And here you're selecting the first element of all elements of a.

That is, you're taking the first element of a copy of a.

I hope this is a little less confusing.

/W