[Tutor] is "fold" same as "reduce"?

[Danny Yoo]

On Tue, 22 Jan 2002, Karthik Gurumurthy wrote:

> I got introduced to functional programming through python and i like
> it now..it was'nt the case a couple of months back though. a "lambda"
> used to put me off.

> I was doing some reading and i was just wondering if the "fold"
> construct is same as "reduce" in python?


Yes: Python's 'reduce' is a similar concept to 'fold' in a functional
language.  For example, here's a function that uses reduce to reverse a
list:

###
def reverse(l):
    return reduce(lambda x, y: [y] + x, [[]] + l)
###

Let's see if it works:

###
>>> reverse([1, 2, 3, 4])
[4, 3, 2, 1]
>>> reverse(['larry', 'curly' 'moe'])
['curlymoe', 'larry']
###


reduce() is a surprisingly versatile function; it can be applied to things
that can raise eyebrows.  Here's another application to do filtering:

###
def filter(test_f, l):
    def func(x, y):   
        if test_f(y):
            return x + [y]
        return x
    return reduce(func, [[]] + l)
###

Does this work?

###
>>> filter(isEven, [3, 1, 4, 1, 5, 9, 2, 6])
[4, 2, 6]
###



There's a series of articles of functional programming using Python on
IBM's Developerworks web site:

    http://www-106.ibm.com/developerworks/library/l-prog.html
    http://www-106.ibm.com/developerworks/library/l-prog2.html
    http://www-106.ibm.com/developerworks/library/l-prog3.html

Hey, there's some stuff here that I haven't seen before.  Cool!  *grin* I
think I'll take a closer look at this too.



> and i took a look @ the functional programming article in IBM's
> website. that was a good introduction.

Do you have a link to it?  I'd like to take a look.




> Can someone tell me about other interesting sites that have a good
> discussion on functional programming...with examples.

If you're interested in functional programming, you might want to look at
the Haskell web site:

    http://www.haskell.org/

Haskell is focused on functional programming, and I've heard that it's a
very good language for exploring functional ideas.


You may also find this link handy: I just found it on Google, and it looks
like good stuff:

    http://www.cs.chalmers.se/~rjmh/tutorials.html