What does list[x::y] do? [duplicate]

Question

Possible Duplicate:
Good Primer for Python Slice Notation

I've been reading over some sample code lately and I've read quite a few websites but I just can't seem to get the query right to give me an answer I'm looking for. If anyone could help me out I'd appreciate it.

Answer 1

It slices

x[startAt:endBefore:skip] 

if you use skip = 2, every other element the list beginning at startAt and ending at endBefore will be selected. [Remember: indices live BETWEEN list elements]

To see this, enter

x = range(100) 

at the Python prompt. Then try these things

x[::2] x[::3] x[10:40:6] 

and see what happens.

Answer 2

L[x::y] means a slice of L where the x is the index to start from and y is the step size. Here are some examples you can try in the interpreter

>>> L=range(20) >>> L [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] 

If you want every 3rd element

>>> L[::3] [0, 3, 6, 9, 12, 15, 18] 

Now every third element starting from L[1]

>>> L[1::3] [1, 4, 7, 10, 13, 16, 19] 

Now every third element starting from L[2]

>>> L[2::3] [2, 5, 8, 11, 14, 17] 

You can specify a negative step to go backwards

>>> L[::-1] [19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0] 

You can also assign to this slice, but the value must have the same length as the slice you are replacing

>>> L[::3]=[0,0,0,0,0,0,0] >>> L [0, 1, 2, 0, 4, 5, 0, 7, 8, 0, 10, 11, 0, 13, 14, 0, 16, 17, 0, 19] 

Finally, you can delete every 3rd element like this

>>> del L[::3] >>> L [1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 19]