How do I return the product of a while loop

Question

I don't get the concept of loops yet. I got the following code:

x=0
while  x < n:
   x = x+1
   print x

which prints 1,2,3,4,5.

That's fine, but how do I access the computation, that was done in the loop? e.g., how do I return the product of the loop( 5*4*3*2*1)?

Thanks.

Edit:

That was my final code:

def factorial(n):
   result = 1
   while n >= 1:
      result = result *n
      n=n-1
   return result

Change the calculation and at the end of the loop `x` will contain the calculated value... (note: use `math.factorial` if that's your real world use) — Jon Clements, Jul 16 '13 at 13:01
possible duplicate of [Is there a built-in product() in Python?](http://stackoverflow.com/questions/7948291/is-there-a-built-in-product-in-python) — njzk2, Jul 16 '13 at 13:29

score 3 · Answer 1 · answered Jul 16 '13 at 13:02

3

By storing that product and returning that result:

def calculate_product(n):
    product = 1
    for x in range(n):
        product *= x + 1

    return product

Now we have a function that produces your calculation, and it returns the result:

print calculate_product(5)

answered Jul 16 '13 at 13:02

Martijn Pieters

1,048,767
296
4,058
3,343

Isn't it faster to do the `+ 1` calculation only once for range instead of n times in the loop? – dansalmo Jul 16 '13 at 14:54
@dansalmo Probably true, but this is arguably cleaner, and the performance difference shouldn't really amount to anything. – Slater Victoroff Jul 16 '13 at 14:55
@Slater Tyranus, I agree it looks cleaner at first glance, but it does not represent the factorial algorithm as cleanly as it is generally represented. – dansalmo Jul 16 '13 at 15:03
1

@dansalmo: If you are concerned with speed, then use the `math` module; C functions beat Pure Python any day. – Martijn Pieters Jul 16 '13 at 15:04
I am mentioning it as a more general point, not as applying only to factorial. – dansalmo Jul 16 '13 at 15:08

Vladimir · Accepted Answer · 2013-07-16T13:33:36.433

You want to introduce one more variable (total) which contains accumulated value of a bunch of actions:

total = 1
x = 1
while x <= 5:
   total *= x
   x += 1
   print x, total
print 'total:', total

Actually, more pythonic way:

total = 1
n = 5
for x in xrange(1, n + 1):
    total *= x
print total

Note, that the initial value of total must be 1 and not 0 since in the latter case you will always receive 0 as a result (0*1*.. is always equals to 0).

score 3 · Answer 3 · answered Jul 16 '13 at 13:22

3

A "one-liner"

>>> import operator
>>> reduce(operator.mul, xrange(1, n + 1))
120
>>>

answered Jul 16 '13 at 13:22

John

13,197
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101

rumpelsepp · Answer 4 · 2014-04-09T09:07:09.253

2

use a for loop:

sum_ = 1
for i in range(1, 6):
    sum_ *= i
print sum_

edited Apr 09 '14 at 09:07

answered Jul 16 '13 at 13:06

rumpelsepp

103
10

Must be `range(1, 6)` to match OPs question – dansalmo Jul 16 '13 at 14:38

Slater Victoroff · Answer 5 · 2013-07-16T15:12:49.847

Alternately you could use the yield keyword which will return the value from within the while loop. For instance:

def yeild_example():
    current_answer = 1
    for i in range(1,n+1):
        current_answer *= i
        yield current_answer

Which will lazily evaluate the answers for you. If you just want everything once this is probably the way to go, but if you know you want to store things then you should probably use return as in other answers, but this is nice for a lot of other applications.

This is called a generator function with the idea behind it being that it is a function that will "generate" answers when asked. In contrast to a standard function that will generate everything at once, this allows you to only perform calculations when you need to and will generally be more memory efficient, though performance is best evaluated on a case-by-case basis. As always.

**Edit: So this is not quite the question OP is asking, but I think it would be a good introduction into some of the really neat and flexible things about python.

Your example needs `for` instead of `while` and `n+1` instead of `n` to be correct. — dansalmo, Jul 16 '13 at 14:46
Mentioning the phrase "generator function" in your description would also be helpful. — dansalmo, Jul 16 '13 at 15:04

score 1 · Answer 6 · answered Jul 16 '13 at 15:46

If you prefer to keep your while loop structure, you could do it like (there are 1000 +1 ways to do it ...):

x=1
result = 1
while  x <= n:
   x += 1
   result *= x

Where result will store the factorial. You can then just return or print out result, or whatever you want to do with it.

score 0 · Answer 7 · answered Jul 16 '13 at 14:05

to access the computation done in the loop, you must use counter(with useful and understandable name), where you will store the result of computation. After computation you just return or use the counter as the product of the loop.

sum_counter=0
x=0
while  x < 10:
   sum_counter +=x
   x+=1
print sum_counter

How do I return the product of a while loop

7 Answers7