Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #103997 > unrolled thread

list reversal error

Back to article view | Back to comp.lang.python

Contents

  list reversal error vlyamtsev@gmail.com - 2016-03-03 14:51 -0800
    Re: list reversal error John Gordon <gordon@panix.com> - 2016-03-03 23:08 +0000
      Re: list reversal error Joel Goldstick <joel.goldstick@gmail.com> - 2016-03-03 18:13 -0500
      Re: list reversal error MRAB <python@mrabarnett.plus.com> - 2016-03-03 23:20 +0000
    Re: list reversal error Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-03 23:20 +0000
    Re: list reversal error Gary Herron <gherron@digipen.edu> - 2016-03-03 15:15 -0800
    Re: list reversal error Steven D'Aprano <steve@pearwood.info> - 2016-03-04 11:06 +1100

#103997 — list reversal error

i have list of strings "data" and i am trying to build reverse list data1
data1 = []
for i in range(len(data)):
   j = len(data) - i
   data1.append(data[j])

but i have the following error:
data1.append(data[j])
IndexError: list index out of range
 
am i doing it wrong?
Thanks

[toc] | [next] | [standalone]

#103999

In <8b3d06eb-0027-4396-bdf8-fee0cc9ff771@googlegroups.com> vlyamtsev@gmail.com writes:

> i have list of strings "data" and i am trying to build reverse list data1
> data1 = []
> for i in range(len(data)):
>    j = len(data) - i
>    data1.append(data[j])

> but i have the following error:
> data1.append(data[j])
> IndexError: list index out of range
>  
> am i doing it wrong?
> Thanks

Python lists are zero-indexed, meaning a list of five items will have
indexes 0 to 4.

The first time through your loop, i is 0, so

    j = len(data) - i 

evaluates to

    j = len(data)

which would yield 5 for a five-element list, but the last actual element
is in data[4].

-- 
John Gordon                   A is for Amy, who fell down the stairs
gordon@panix.com              B is for Basil, assaulted by bears
                                -- Edward Gorey, "The Gashlycrumb Tinies"

[toc] | [prev] | [next] | [standalone]

#104000

On Thu, Mar 3, 2016 at 6:08 PM, John Gordon <gordon@panix.com> wrote:

> In <8b3d06eb-0027-4396-bdf8-fee0cc9ff771@googlegroups.com>
> vlyamtsev@gmail.com writes:
>
> > i have list of strings "data" and i am trying to build reverse list data1
> > data1 = []
> > for i in range(len(data)):
> >    j = len(data) - i
> >    data1.append(data[j])
>
> > but i have the following error:
> > data1.append(data[j])
> > IndexError: list index out of range
> >
> > am i doing it wrong?
> > Thanks
>
> Python lists are zero-indexed, meaning a list of five items will have
> indexes 0 to 4.
>
> The first time through your loop, i is 0, so
>
>     j = len(data) - i
>
> evaluates to
>
>     j = len(data)
>
> which would yield 5 for a five-element list, but the last actual element
> is in data[4].
>

>>> s = "123"
>>> s2 = s[::-1]
>>> s2
'321'
>>>

Use reverse slice

>
> --
> John Gordon                   A is for Amy, who fell down the stairs
> gordon@panix.com              B is for Basil, assaulted by bears
>                                 -- Edward Gorey, "The Gashlycrumb Tinies"
>
> --
> https://mail.python.org/mailman/listinfo/python-list
>



-- 
Joel Goldstick
http://joelgoldstick.com/ <http://joelgoldstick.com/stats/birthdays>
http://cc-baseballstats.info/

[toc] | [prev] | [next] | [standalone]

#104001

On 2016-03-03 23:08, John Gordon wrote:
> In <8b3d06eb-0027-4396-bdf8-fee0cc9ff771@googlegroups.com> vlyamtsev@gmail.com writes:
>
>> i have list of strings "data" and i am trying to build reverse list data1
>> data1 = []
>> for i in range(len(data)):
>>    j = len(data) - i
>>    data1.append(data[j])
>
>> but i have the following error:
>> data1.append(data[j])
>> IndexError: list index out of range
>>
>> am i doing it wrong?
>> Thanks
>
> Python lists are zero-indexed, meaning a list of five items will have
> indexes 0 to 4.
>
> The first time through your loop, i is 0, so
>
>      j = len(data) - i
>
> evaluates to
>
>      j = len(data)
>
> which would yield 5 for a five-element list, but the last actual element
> is in data[4].
>
A simpler alternative is to use 'reversed':

data1 = list(reversed(data))

[toc] | [prev] | [next] | [standalone]

#104002

On 03/03/2016 22:51, vlyamtsev@gmail.com wrote:
> i have list of strings "data" and i am trying to build reverse list data1
> data1 = []
> for i in range(len(data)):
>     j = len(data) - i
>     data1.append(data[j])
>
> but i have the following error:
> data1.append(data[j])
> IndexError: list index out of range

At the first pass through the for loop, j is effectively set to 
len(data).  As Python is indexed from zero this will always take you 
beyond the end of data, hence the IndexError.

>
> am i doing it wrong?
> Thanks
>

You are doing things the difficult way.  First up, using the construct:-

for i in range(len(data)):

is usually not needed in Python.

There is a reversed function 
https://docs.python.org/3/library/functions.html#reversed which will do 
the job for you.

data1 = list(reversed(data))

Or use the slice notation

data1 = data[::-1]

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence

[toc] | [prev] | [next] | [standalone]

#104003

On 03/03/2016 02:51 PM, vlyamtsev@gmail.com wrote:
> i have list of strings "data" and i am trying to build reverse list data1
> data1 = []
> for i in range(len(data)):
>     j = len(data) - i
>     data1.append(data[j])
>
> but i have the following error:
> data1.append(data[j])
> IndexError: list index out of range
>   
> am i doing it wrong?
> Thanks

Look at the values (say with a print) you get from your line
     j = len(data) - i
You'll find that that produces (with a list of 4 elements for example) 
4,3,2,1 when in fact you want 3,2,1,0. Soo you really want
     j = len(data) - i -1


Better yet, use more of Python with
     data1 = list(reversed(data))

Or don't even make a new list, just reverse the original list in place
 >>> L=[1,2,3]
 >>> L.reverse()
 >>> L
[3, 2, 1]

Or even better, if you simply want to iterate through the original list, 
but in reverse order:
     for datum in reversed(data):
         ... whatever with datum ...
which wastes no time actually reversing the list, but simply loops 
through them back to front.


Gary Herron

-- 
Dr. Gary Herron
Department of Computer Science
DigiPen Institute of Technology
(425) 895-4418

[toc] | [prev] | [next] | [standalone]

#104006

On Fri, 4 Mar 2016 09:51 am, vlyamtsev@gmail.com wrote:

> i have list of strings "data" and i am trying to build reverse list data1

Use a slice with a negative step-size and defaults for the start and end
positions.


data1 = data[::-1]




-- 
Steven

[toc] | [prev] | [standalone]

Back to top | Article view | comp.lang.python

csiph-web