[Tutor] Project Euler #8

[Alan Gauld]

On 31/05/13 21:49, Nick Shemonsky wrote:

> I did stumble upon using reduce ...but I didn't really
> understand what it was doing

>> def product(intlist):
>>      return reduce(operator.mul, intlist)

I explain reduce thusly in my Functional Programming topic:

---------------------
The reduce function is a little less obvious in its intent. This 
function reduces a list to a single value by combining elements via a 
supplied function. For example we could sum the values of a list and 
return the total like this:

def add(i,j): return i+j
print reduce(add, numbers)

As before we could do this more conventionally like this:

res = 0
for i in range(len(numbers)): # use indexing
    res = res + numbers[i]
print res

While that produces the same result in this case, it is not always so 
straightforward. What reduce actually does is call the supplied function 
passing the first two members of the sequence and replaces them with the 
result. In other words a more accurate representation of reduce is like 
this:

def reduce(numbers):
   L = numbers[:] # make a copy of original
   while len(L) >= 2:
      i,j = L[0],L[1] # use tuple assignment
      L = [i+j] + L[2:]
   return L[0]
----------------------

HTH

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/