How do I round numbers in a list?

**zmbd** · Sep 12 '12, 10:51 AM

So what error message are you getting?
So what output, if any, are you getting?
Hint: round(i,0) returns a double so if you want to round and then return the integer part only...

-z

**Miguel Valenzue** · Sep 12 '12, 11:15 AM

Okay, here's the code again:

Code:

myList = [10.5,25.1,350.6,4,5,6]

for i in myList:
    int(i)

print myList

for i in myList:
    round(i)

print myList

for i in range(len(myList)):
    myList[i] == round(i,0)

print myList

Here's my output

[10.5, 25.1, 350.6, 4, 5, 6]
[10.5, 25.1, 350.6, 4, 5, 6]
[10.5, 25.1, 350.6, 4, 5, 6]

My desirec output is
[11, 25, 351, 4, 5, 6]

No errors. Just doesn't do anything.
When you say it returns a double. clueless on what that implies.

**zmbd** · Sep 12 '12, 11:37 AM

Returns a data type of "double" vs integer, long, etc..
Data types are covered in most good books and you can google on them as related to python/ruby/vba/.net etc...

My hint has the solution.

-z

**Miguel Valenzue** · Sep 12 '12, 01:29 PM

I will call you Gollum from now own, master of riddles. ;)
Okay, I understand double.
So if round is returning a double, then I need to convert that into an integer.

I've tried that (at least I think) with this code.

Code:

for i in myList:
    int((round(i)))

print myList

But on a deeper level, is my logic right? to redefine the numbers (long) in a list by just using the for loop?

**bvdet** · Sep 12 '12, 03:14 PM

In your first example, you are using the comparison operator "==" when you should be using the assignment operator "=". Also, you should combine int() and round() to achieve the desired output.

Code:

>>> int(round(12.5, 0))
13
>>> x == int(round(12.5, 0))
False
>>> x = int(round(12.5, 0))
>>> x
13
>>>

A list comprehension is the easiest way.

Code:

>>> x = [1,2,3,11.5]
>>> newlist = [int(round(n, 0)) for n in x]
>>> newlist
[1, 2, 3, 12]
>>>

**zmbd** · Sep 12 '12, 04:12 PM

bvdet
sigh... you let 'em off the hook. :(
-z

**bvdet** · Sep 12 '12, 04:18 PM

Sorry about that zmbd. If he was going to "get it" he already would have.

**Miguel Valenzue** · Sep 12 '12, 07:23 PM

Well, first of all, thanks both of you. I appreciate your time.
But after looking at the way I was approaching it, I want to learn a bit more here.
I got the code to work so now my vector multiplication that I was going for is complete.

But this was my thought process.

If I put

Code:

int(12.5,0)

in the command line of python, it returns 13

So that was fine with me. I get that.

But this was my thinking.
If I have a list, I can do something real basic like this

Code:

myList =[4,3,5,6,1]

for range in (len(myList)):
     i=*3

That returns a new list of
12,9,15,6,1

So my thought process was, why can't I just use the round or integer function within the same statement such as

Code:

for range in (len(myList)):
     i=int(i)

But when I do this, it returns a number like 2.000000001 or something like that.

Anyhow, that's what I don't get so as part of my learning experience, can you help me understand what is happening? I see that round returns a double like you said, but how does that double go back to being an int?

I mean, why doesn´t the following code work?

Code:

for range in (len(myList)):
     i=int(round(i,0))

thanks again
Oh, and bvdet,
I didn´t know you could use list comprehension like that.
I redid all my code and cut it down by about 25% with that new found knowledge.

**bvdet** · Sep 12 '12, 09:47 PM

In your for loop, range is used as a temporary identifier for the expression len(myList), which returns an integer. You cannot iterate on an integer. Also, you will mask the built-in function range(). I think you meant to do this:

Code:

>>> myList = [10.2, 12.9]
>>> for i in range(len(myList)):
... 	myList[i]=int(round(myList[i],0))
... 	
>>> myList
[10, 13]
>>>

The temporary identifier i can be used as the index for getting each list element and reassigning the element at that index. Identifier i will retain the last value assigned to it. Built-in function range() returns a list.

Code:

>>> range(len(myList))
[0, 1]
>>> i
1
>>>

HTH

How do I round numbers in a list?

How do I round numbers in a list?

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