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I am trying to learn basic python and I can't seem to be able to return values using a while loop. I am copying the syntax from guides I am reading but it doesn't seem to work when I run it. I may have a different version, I am not sure.

Anyways I was just messing around and got this code

def fact(x):
 count = 1
 sum = 0
 while count <= x:
     sum =  count * sum
     count + 1
 else:
     return sum

I don't even know if this correctly computes factorial, I don't care, I am just trying to get it to return a value. What is wrong? Why can't I use return? I am not sure what is wrong, when I replace return with 

       display,

it still doesn't work.

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3
  • @Keyser: if, while, for, try all have an else clause. Commented Dec 4, 2012 at 23:35
  • @Keyser: It lets you put in code optimizations which would normally take half a dozen lines to implement normally. Commented Dec 4, 2012 at 23:37
  • @IgnacioVazquez-Abrams Not sure I follow. I meant the fact that it seems like it's just an extra line of code. We're going off-topic now though :p Thanks for clarifying. Commented Dec 4, 2012 at 23:39

4 Answers 4

Reset to default
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In your while loop, you aren't actually setting count equal to count + 1 - you are just stating that count + 1, which doesn't reassign the variable:

In [1]: count = 1

In [2]: count + 1
Out[2]: 2

In [3]: count
Out[3]: 1

You will want to use something like count += 1. Also, you are going to have an issue with:

sum =  count * sum

Since it will initially be 0, you are resetting it to 0 each time, regardless of what count is. You can try setting it to 1 if that still works for you, but you may need to think through what you're trying to do. The else statement with while is used, but here you can probably just exit the block after your while condition is met and return the variable that way.

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9 Comments

Odd, I have not seen the += operator in any of the tutorials yet.
@Jordan Definitely comes in handy :)
Assignment is not an operator in Python. Just saying.
@IgnacioVazquez-Abrams Is that directed at me? Are you saying = is not an operator? Then what is it?
@IgnacioVazquez-Abrams What do you mean by side effects? I think I understand why it is wrong to use in that case because it is just reassigning that variable to the same value every time and never actually changing it's value.
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def fact(x):
    count = 1
    sumi = 1
    while count <= x:
        sumi =  count * sumi
        count = count + 1

    return sumi

Some mistakes here :

  • sumi = 0 at the beginning is wrong, it will make your function always returns 0
  • count + 1 does not change the value of count if you don't assign it to anything
  • put your return statement after the while. When the loop is done it will return the value of sumi
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Comments

1

Try:

def fact(x):
    count = 1
    sum = 1
    while count <= x:
        sum *= count
        count += 1
    return sum

Although this is probably nicer:

def fact(x):
    prod = 1
    for i in range(1, x+1):
        prod *= i
    return prod

If you start the counting variable at 0 you'll get 0 out, since 0 * n == 0 for all n.

The else is also completely useless here.

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4 Comments

Except there is. It isn't needed in this case, but it shouldn't affect anything.
fixed in edit. Didn't know while..else -- seems like an odd feature, though.
It's rarely used, but it saves you having to create a sentinel; just break out instead.
@MikeLarsen: your suggested fix still doesn't work. I know it's hard to get your answers in quick enough for relatively easy Python questions, but try to run a quick test of your answer before you post it. IPython is great for quickly writing and testing code snippets.
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So I don't know what you are following to work with structures of python but you cannot call else by itself. You will never return since you have no judgement value to test the else statement. It needs to be attached to an if statement to work.

For example you would want to do something like:

if x == 1:
   return blah
else:
   return foo

I also don't know for sure but that code may not even run at all. I don't know what display is either but I'm pretty sure its not what you want.

Please read this: http://docs.python.org/2/tutorial/controlflow.html

This will help you learn about control structures so that you can make your loop do what you want.

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1 Comment

I dislike that guide because it doesn't even include while in it, which seems more obvious for this example.

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