On Tue, Apr 22, 2014 at 4:18 PM, Patti Scott <pscott...@yahoo.com.dmarc.invalid> wrote: > I'm practicing with lists. I was looking for documentation on sorting with > cmp() because it isn't immediately clear to me how comparing items two at a > time can sort the entire list.
As you note, comparison itself doesn't sort a list. Comparing elements by itself is a query, not an action. But what it does is give Python enough tools to do the sort for you. Python uses a "comparison" based sorting routine which works by taking in a user-defined notion of when two elements are in order or not. http://en.wikipedia.org/wiki/Comparison_sort As a handwavy explanation of the idea: imagine a list that hasn't been sorted. Python can use the comparison function you give it to repeatedly compare elements in the list. Whenever if it sees disorder, it can swap elements and thereby reduce the disorder in the list. Assuming the comparison is a "good" one, then we can eventually sort the whole list by repeating this over and over. Note that Python will be calling the comparison function on your behalf: you won't be calling it directly yourself. (By "good", we mean a "total ordering" in the sense described in: http://en.wikipedia.org/wiki/Total_order) This is a handwavy explanation because Python does these comparisons and swapping in a fairly sophisticated way to avoid a lot of work. See: http://en.wikipedia.org/wiki/Timsort It maybe that you have not seen instances of functions that take functions as arguments. If so, consider two functions f and g: ######### def f(x): return x * x def g(x): return 2 * x ######### Toy functions, of course. Now they themselves don't do much but compute the square and the double of a number. But they can be passed as arguments to other functions to do something. For example, if we have some list of numbers: ######### numbers = [3, 1, 4, 1, 5, 9, 2, 6] ######### then we may apply f and g pointwise across those functions, using the map() function: ######### print(map(f, numbers)) print(map(g, numbers)) ######### and you'll see that we can compute bulk operations on lists. Again, we're leaving the map() function to call 'f' and 'g' for us. This is an example of a function that can take in other functions. The sorting routine you're looking at is conceptually doing a similar thing by taking in a comparison function, which it will use during its own work. Passing functions as values allows for a notion of "variable" that's really powerful: what is varying isn't just some static piece of plain data, but rather a behavior. _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
