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Clarification Nested Lists

Hello, I was reading Python Essential Reference by Beazley. It had the following example on nested lists
Quote:
a = [1,”Dave”,3.14, [“Mark”, 7, 9, [100,101]], 10]
a[1] # Retruns Dave
a[3][2] # Returns 9
a[3][3][1] # Returns 101

I just need to confirm something I am having trouble clarifying with the other docs I have found - I think maybe its too simple a question.

Could I reference 7 above with a[4] as well as a[3][1]? So again referring to the above example of a[3][3][1] returns 101 why couldn't it be referenced as a[6][1] or do I need a new [] to represent each nest in the list?

Finally could I negative reference a[-1] to equal 101 or does this create too much confusion ?

Quote:

Originally Posted by flebber

Could I reference 7 above with a[4] as well as a[3][1]? So again referring to the above example of a[3][3][1] returns 101 why couldn't it be referenced as a[6][1] or do I need a new [] to represent each nest in the list?

You seem to be in a bit of a muddle concerning how nested lists work. Let me try and illustrate the above example using a different approach that should make this more clear:

c = [100, 101]
b = ["Mark", 7, 9, c]
a = [1, "Dave", 3.14, b, 10]
 
a[1]       # Returns Dave
a[3]       # Returns the list referenced by b
a[3][2]    # Returns b[2], which is 9
a[3][3][1] # Returns b[3][1], which is the same as c[1], which is 101

Or, to put use an analogy, think of it as a set of boxes with compartments. Box c has two compartments, with 100 and 101 inside it. Box c sites inside the fourth compartment of box b, and box b sits in the fourth compartment of box a.

To get to box b, one must first open the relevant compartment of box a (in programming terms, that's what a[3] does). To get to box c, one must first open the relevant compartment of box b (which is b[3], or a[3][3], since a[3] is just another way of getting b).

Does that make it clearer?

Quote:

Originally Posted by flebber

Finally could I negative reference a[-1] to equal 101 or does this create too much confusion ?

a[-1] references the last element of a, which is 10.

Thanks Arevos, thanks for the reply and clearing that up for me. I thought thats what it was but you know sometimes the simple things trip you up and can be the hardest things to find answers too.

Thanks Again

fleb

Just a small tip, python's interpreter has an interactive mode which you can use to get some hands-on experience with the basics of the language. Just run "python" on the command line.

I can recommend IPython as an "interactive python on steroids". :-)