[Tutor] Sequence Comparison

[Danny Yoo]

On Mon, 12 Jan 2004, Hameed Khan wrote:

> i was reading the Sequence comparison section of python documentation.
> but i really did not understand from it. if anyne of you can explain it
> to me then it will be very kind of you. the below is the examples with
> the questions i have in mind.
>
> >>> (1, 2, 3)              < (1, 2, 4)
> True
> # Q.1 why it returns True when 1 is not lower then 1
> and 2 is not lower then 2. only 3 is lower then 4
                             ^^^^^^^^^^^^^^^^^^^^^^

Hi Hameed,

That's exactly right.  *grin* Python is doing a lexicographical
"dictionary" based sort.


You should have already seen this kind of sort before, in a regular
English dictionary.  Say that we have two words, like "abc" and "abd".
If those were real words, which one would come first in a dictionary?
And why?

A more concrete example: think about the words "account" and "accountant".
Which one would come first in a dictionary, and why?

The tuples above are being sorted for the same reason.






> >>> (1, 2, 3, 4)           < (1, 2, 4)
> True
> # the above 2 question (Q.1 and Q.2) applies to this
> comparison because this tuples have different sizes


Imagine that the system has two arrows pointed to the first components of
each item:

(1, 2, 3, 4)    (1, 2, 4)
 ^               ^
 A               B


Let's call them Arrow A and Arrow B.  If Arrows A and B are pointed to the
same thing, they'll move to the right one place:


(1, 2, 3, 4)    (1, 2, 4)
    ^               ^
    A               B


Again, they point to the same thing.  Let's move Arrows A and B again:



(1, 2, 3, 4)    (1, 2, 4)
       ^               ^
       A               B


3 is less than 4, so (1, 2, 3, 4) is less than (1, 2, 4).


Does this make sense so far?









> >>> (1, 2)                 < (1, 2, -1)
> True


Let's use the arrow notation again:

   (1, 2)          (1, 2, -1)
    ^               ^
    A               B

Equal.  Move the arrows to the right one:


   (1, 2)          (1, 2, -1)
       ^               ^
       A               B

Equal again.  Let's move the arrows to the right again:

   (1, 2)          (1, 2, -1)
          ^                ^
          A                B



Arrow A has fallen off!  In this case, (1, 2) is less than (1, 2, -1)
because its arrow has fallen off first.





> # i understand it a little bit that the elements of second tuple
> converted to the type of elements of first tuple and then compared but
> what if we compare
>
> (1,"string",3) == (1.0,2.0,3.0)


Good question.  The equality (==) comparison between a string and a number
is always False, according to the Python language spec.  According to:

    http://www.python.org/doc/current/ref/comparisons.html

"""The operators <, >, ==, >=, <=, and != compare the values of two
objects. The objects need not have the same type. If both are numbers,
they are converted to a common type. Otherwise, objects of different types
always compare unequal, and are ordered consistently but arbitrarily."""


So we will get a False value off:

   "string" == 2.0

And that's what forces:

    (1, "string", 3) == (1.0, 2.0, 3.0)

to evaluate to False.




Please feel free to ask more questions.  Hope this helps!