Lists Like tuples, but mutable. Formed with brackets: Assignment: >>> a = [1,2,3] Or with a constructor function: a = list(some_other_container) Subscription (again, zero based indexing): >>> a[0] 1 >>> a[len(a) - 1] 3

Assignment a = (1,2,3) a[0] = 10 # Breaks the program. a = [1,2,3] a[0] = 10 # Fine a = [10,20,30] # Assigns a new list, # rather than altering the old one. Essentially the same kind of operation as : b = 100 b = 200

For mutables, if you change a variable's content, it changes for all attached labels. You can still disconnect it and connect it to something new. >>> a = [10] # List containing 10. >>> b = a >>> b[0] 10 >>> b[0] = 20 >>> a[0] # Changing b changes a, as they refer to the same 20 # object. >>> b = [30] # Disconnect b from [10] and attach it to a new list. Essentially a new "b". >>> a[0] 30 The what of variables

Slices: extended indexing Extended indexing is a way of referencing values in a sequence. These are sometimes called a 'slice' (essentially slice objects are generated, containing the indices generated by a range). a[i:j] returns all the elements between i and j, including a[i] but not a[j] a[i:j:k] returns the same, but stepping k numbers each time. Slices must have at least [:] (slice everything), but the rest is optional. If j > len(a), the last position is used. If i is None or omitted, zero is used. If i > j, the slice is empty. a[:2] # First two values. a[-2:] # Last two values. “For non-negative indices, the length of a slice is the difference of the indices, if both are within bounds. For example, the length of word[1:3] is 2.”

Assigning slices While you can use slices as indices with immutable sequences, they can be used with mutable sequences like lists for assignment as well. >>> a = [0,1,2,3,4] >>> b = [10,20,30] >>> a[1:3] = b >>> a [0,10,20,30,3,4] # Note we replace 2 values with 3. >>> b = [10,20] >>> a[1:5] = b [0,10,20] # We replace 4 values with 2.

List related functions All the same functions as immutable sequences, plus these "in place" functions (i.e. functions that change the original, rather than returning a copy): del(a[:]) Delete a slice. a.clear() Empty the list. a.extend(b) Extends a with the contents of b (i.e. a += b) a.insert(i, x) Inserts x into a at the index position i. a.pop(i) Returns the item at i and removes it from a. a.remove(x) Remove the first item from a equal to x a.reverse() Reverses the items of a. https://docs.python.org/3/tutorial/datastructures.html#more-on-lists

2D lists There is nothing to stop sequences containing other sequences. You can therefore built 2D (or more) sequences. a = [[1,2,3],[10,20,30],[100,200,300]] They can be regular (i.e. have all second dimension sequences the same length) or irregular. Reference like this: >>> a[0] [1,2,3] >>> a[0][0] 1

NB: 2D sequences Note that tuples can contain lists. This seems odd, as lists are mutable, but it is only the object references/links in the tuple that are immutable, not the objects themselves. Tuples of numbers and strings are immutable because numbers and strings are immutable. >>> a = ([1,2],[10,20]) >>> a[0][0] = 100 >>> a ([100,2],[10,20]) >>> a[0] = [1000,2000] TypeError: 'tuple' object does not support item assignment

NB: 2D sequences Finally, you can combine two lists or tuples into a list of tuples like this: >>> a = [1,2,3,4,5] >>> b = [10,20,30,40,50] >>> c = zip(a,b) >>> d = list(c) >>> d [(1,10),(2,20),(3,30),(4,40),(5,50)] As we'll see in the module on control flow, this allows us to process two sets of number simultaneously. List(seq) https://docs.python.org/3/faq/programming.html#how-do-i-create-a-multidimensional-list

Command line arguments When you run a Python program at the command line, you can pass information into it. This is not unusual. When you click on a Word file in Windows, what is happening in the background is Word is being activated and passed the filename to open. Indeed, when you run a Python program, you are doing this: python helloworld.py you're saying "run the python program, and the thing after its name is the file for it to execute." The elements passed into a program like this are called "command line arguments". They are used to quickly say how a program should work without re-writing the code. One place lists are used is in getting data into a program.

Command line arguments We can actually get our code to record command line arguments. python ourfile.py arg1 arg2 arg3 Inside the program, these are made available in a list called sys.argv argv[0] is actually the script name (and, depending on OS, path) If run using the -c option of the interpreter, argv[0] == "-c". After argv[0] come the other arguments as strings.

Example #args.py import sys print ("Hello " + sys.argv[1]) > python args.py Dave > Hello Dave