Help with reduce().

[EricIDLE]

im sorry i still dont understand reduce can you explain it in simpiler
terms?

Steve Holden <sholden at holdenweb.com> wrote in message
news:Sds27.29662$F%5.1909477 at e420r-atl2.usenetserver.com...
> "EricIDLE" <grayson-wilson at home.com> wrote in message
> news:3ur27.663627$166.13698874 at news1.rdc1.bc.home.com...
> > Ok well I have ANOTHER problem with one of the examples in my book (By
The
> > Way, "Teach yourself python in 24 hours" is an extremely poorly written
> book
> > and I do not reccomend it to any newbies who are just starting off)
> > any ways the example is as follows.
> >
> > n = range(1,11)
> > def mult(x,y):
> >     return x * y
> > f = reduce(mult,n)
> > print f
> >
> > I get everything up to reduce. I relize mult() takes the first two
numbers
> > in range N which would be 1, 2 and multiplys them together to get 2...
ok
> > fine im good ... now comes the tricky part!! what does reduce do? Does
it
> > take the whole range and make it into a number (12345678910) then
subtract
> 2
> > (the result of the function mult) or what does it do.
> >
> The reduce() function takes a function as its first argument and a
sequence
> as its second. There's also an optional third argument, which we can
ignore
> for now. The function is applied to the first two items of the sequence,
> then to that result and the third item of the sequence, then to that
result
> and the fourth item, and so on until the sequence is exhausted. Here's a
> loop that might make it a bit clearer:
>
> >>> def mult(x, y):
> ...  return x * y
> ...
> >>> for i in range(2, 11):
> ...  n = range(1, i)
> ...  print i, ":", reduce(mult, n)
> ...
> 2 : 1
> 3 : 2
> 4 : 6
> 5 : 24
> 6 : 120
> 7 : 720
> 8 : 5040
> 9 : 40320
> 10 : 362880
>
> Each result extends the value by one multiplication (by one less than I
> because of the way range works).
>
> > And i have another question (not related to first question)
> >
> > x = ['Thu', 'Aug', '5', '06:06:27', 'MDT', '1999']
> > def strp(x,y):
> >  return x + ' ' + y
> > r = reduce(strp,x)
> > print r
> >
> > my problem is this line return "x + ' ' + y" what does it return the
value
> > of. I would understand it if it was x + y but the qutation marks are
what
> > throw me off what do they mean?
> >
> This is another case where the interactive interpreter is your friend. Try
> it and see...
>
> >>> x = 'one'
> >>> y = 'two'
> >>> x + y
> 'onetwo'
> >>> x + ' ' + y
> 'one two'
>
> You can see that the second statement constructs a string from *three*
> values: the string bound to x, the string consisting of a single space,
and
> the string bound to y.
>
> So all your second example does is append al lthe strings together, with
> spaces in between them. It takes 'Thu' and 'Aug" and puts them together
with
> a space in between them, that takes *that* result and '5' and puts them
> together with a space between them, and ... you probably see the point
now.
>
> When you give reduce() a thrid argument, it starts with that and the first
> element of the sequence rather than the first two elements of the
sequence,
> allowing you to put an initial value into the string of function
> evaluations:
>
> >>> x = ['Thu', 'Aug', '5', '06:06:27', 'MDT', '1999']
> >>> def strp(x, y):
> ...  return x + ' ' + y
> ...
> >>> print reduce(strp, x, "Date is:")
> Date is: Thu Aug 5 06:06:27 MDT 1999
>
> I hope the book also (though possibly later) points out that a much better
> way to do the same thing would be
>
> x = ['Thu', 'Aug', '5', '06:06:27', 'MDT', '1999']
> r = string.join(x, " ")
>
> which uses a function from the string module. And, using string methods,
you
> could even abreviate it to:
>
> " ".join(x)
>
> which tells the string containing a single space to insert itself between
> the elements of the list, returning the resulting long string. But that
> probably won't appear until you're about ten hours into the book! Good
luck.
>
> regards
>  Steve
> --
> http://www.holdenweb.com/
>
>
>
>
>