Not understanding this small piece of code

[Ashwin Rao]

I was reading about slicing a list in python. All was well until I started the next topic that said there was a third parameter to the slicing called as step. I understood that except for why does the code below print the shoppingList in the reverse order. Can anyone help me? Thanks!

shoppingList = ['apple','orange','banana']
print shoppingList[::-1]

[chris webster]

Because you are telling it to take steps of "-1" i.e. step backwards. Here's a couple more examples using a backward step:

>>> nums = [1,2,3,4,5,6,7,8,9]
>>> nums[::-2]  # step backwards and take every 2nd item
[9, 7, 5, 3, 1]

>>> nums[3::-1] # step backwards from item 3
[4, 3, 2, 1]

>>> nums[7:4:-1] # step backwards from item 7 and stop before you reach item 4
[8, 7, 6]

I suggest you simply play around with these slices/steps to get a feel for how they work.

[Frank vanJossum]

-1 will go backwards. It takes negative steps, which start from the end of the set.

The slicing syntax is:
a[start:end:step]
To get a more clear picture, try a longer list:
shoppingList = ['apple','orange','banana','strawberry','lemon','coconut'] # different step sizes print shoppingList[::1] print shoppingList[::2] print shoppingList[::3] # fun with index print shoppingList[0:len(shoppingList):1] print shoppingList[0:2] print shoppingList[2:4]