Reverse Iteration Through Integers
Benjamin Middaugh
such that 169 becomes 961. I guess I don't know enough yet to figure out
how to do this without a ton of awkward-looking code. I've tried for
loops without much success. I guess I need a good way of figuring out
the length of the input integer so my loop can iterate that many times
to reverse the number, but I keep getting errors that say "TypeError:
'Int' object is not iterable".
Any help would be much appreciated,
Benjamin "Self-Proclaimed Python Newbie"
Lie Ryan
you need to turn the int to a str:
>>> n = 169
>>> rn = int(''.join(reversed(str(n))))
>>> rn
961
>>>
Christian Heimes
> I'm trying to make an integer that is the reverse of an existing integer
> such that 169 becomes 961. I guess I don't know enough yet to figure out
> how to do this without a ton of awkward-looking code. I've tried for
> loops without much success. I guess I need a good way of figuring out
> the length of the input integer so my loop can iterate that many times
> to reverse the number, but I keep getting errors that say "TypeError:
> 'Int' object is not iterable".
>
> Any help would be much appreciated,
Here is a simplistic version that doesn't use fancy math:
>>> str(24)
'24'
>>> str(24)[::-1]
'42'
>>> int(str(24)[::-1])
42
Rhodri James
<benjamin.th...@gmail.com> wrote:
> I'm trying to make an integer that is the reverse of an existing integer
> such that 169 becomes 961. I guess I don't know enough yet to figure out
> how to do this without a ton of awkward-looking code. I've tried for
> loops without much success. I guess I need a good way of figuring out
> the length of the input integer so my loop can iterate that many times
> to reverse the number, but I keep getting errors that say "TypeError:
> 'Int' object is not iterable".
Alternatively, turn your integer into a string, reverse the string, and
turn it back:
rhodri@gnudebst:~$ python
Python 2.6.2 (release26-maint, Apr 19 2009, 01:58:18)
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> n = 123
>>> int(str(n)[::-1])
321
--
Rhodri James *-* Wildebeest Herder to the Masses
Paul Rubin
> I'm trying to make an integer that is the reverse of an existing
> integer such that 169 becomes 961. I guess I don't know enough yet to
> figure out how to do this without a ton of awkward-looking code. I've
> tried for loops without much success. I guess I need a good way of
> figuring out the length of the input integer so my loop can iterate
> that many times to reverse the number, but I keep getting errors that
> say "TypeError: 'Int' object is not iterable".
Sounds like you're working on Euler problems ;-).
The simplest way is to turn the integer to a string:
n_int = 961
n_string = str(n_int)
then reverse the string:
n_string_reversed = reversed(n_string)
and turn it back into an int:
n_int_reversed = int(n_string_reversed)
If you want to peel off digits from an int one by one without string
conversions, it's easiest to do that in reverse order:
n = 961
digits = []
while n > 0:
n,d = divmod(n, 10)
digits.append(d)
Look up the docs for "divmod" for an explanation of that handy function.
Now the above gives you a reversed list of digits--what to do with it
is an exercise for you ;-). Note that if n=0 then you get the empty list.
Yet another way is to use recursion. I'll leave that as an exercise too.
MRAB
> I'm trying to make an integer that is the reverse of an existing integer
> such that 169 becomes 961. I guess I don't know enough yet to figure out
> how to do this without a ton of awkward-looking code. I've tried for
> loops without much success. I guess I need a good way of figuring out
> the length of the input integer so my loop can iterate that many times
> to reverse the number, but I keep getting errors that say "TypeError:
> 'Int' object is not iterable".
>
>
integer (does that actually make sense?), but the digits when it's
represented as a string (and in decimal).
Mick Krippendorf
> Yet another way is to use recursion. I'll leave that as an exercise too.
This time with big numbers:
def trampoline(bouncing, *args, **kwargs):
while bouncing:
result, bouncing, args, kwargs = bouncing(*args, **kwargs)
if result:
return result()
def bouncy(function):
return lambda *args, **kwargs:(None, function, args, kwargs)
def land(result=None):
return lambda:result, None, None, None
def reverse(n):
@bouncy
def rev(i=n, j=0, k=0):
if i:
return rev(*divmod(i, 10), k=(j+k)*10)
return land(j + k)
return trampoline(rev)
print reverse(169883903200298309284038223098439430943092816286 ** 123)
Try it without the @bouncy decoration.
Granted, the code looks like a serious case of Haskell envy, but after
recursion and tail call optimization being cryptic was just the logical
consequence ;-)
Mick.
Jabba Laci
Would someone explain how str[::-1] work? I'm new to Python and I only
saw so far the str[begin:end] notation. What is the second colon?
Thanks,
Laszlo
Chris Rebert
> Hi,
>
> Would someone explain how str[::-1] work? I'm new to Python and I only
> saw so far the str[begin:end] notation. What is the second colon?
Specifies the step value, as in: foo[start:stop:step]
When not specified it, defaults to 1.
So foo[::-1] gives the entire sequence backwards.
Analogously, foo[::2] gives every other item in the sequence, forwards.
Cheers,
Chris
--
http://blog.rebertia.com
alex23
> Would someone explain how str[::-1] work? I'm new to Python and I only
> saw so far the str[begin:end] notation. What is the second colon?
Slice notation is of the form [start:stop:step]. start defaults to the
start of the sequence, stop to the end, and step to 1.
So a slice of [::-1] returns the full sequence in reverse, stepping
back one character at a time from the end of the sequence to the
beginning.
The only mention I could find was http://docs.python.org/dev/3.0/library/functions.html#slice
alex23
> The only mention I could find was http://docs.python.org/dev/3.0/library/functions.html#slice
No idea why that link was the one that came up, this is more
appropriate:
Bearophile
> If you want to peel off digits from an int one by one without string
> conversions, it's easiest to do that in reverse order:
>
> n = 961
> digits = []
> while n > 0:
> n,d = divmod(n, 10)
> digits.append(d)
>
> Look up the docs for "divmod" for an explanation of that handy function.
> Now the above gives you a reversed list of digits--what to do with it
> is an exercise for you ;-). Note that if n=0 then you get the empty list.
I think that with Psyco it's better to avoid divmod().
It's very positive to teach novices to always use tests every time
they write a function, because it makes their programs less buggy and
often allows them to solve the whole programming problem sooner:
def reverse(n):
"""
Reverse the digits of integer n, ignoring trailing zeros.
>>> reverse(12590)
9521
>>> reverse("abc")
Traceback (most recent call last):
...
TypeError: unsupported operand type(s) for divmod(): 'str' and
'int'
>>> [reverse(x) for x in (0, 1, -1, 2, -2L, 100L, -100)]
[0, 1, -1, 2, -2L, 1L, -1]
>>> [reverse(x) for x in (125, 1250, 123456789)]
[521, 521, 987654321]
>>> [reverse(x) for x in (0.0, 1.0, -5.3, 125.0, 1.23456e20)]
[0, 1.0, -5.2999999999999998, 521.0, 654321.0]
>>> str(reverse(169883903200298309284038223098439430943092816286
** 123))[:35]
'65852401624276201339740994895755844'
"""
# code removed ...
if __name__ == "__main__":
import doctest
doctest.testmod()
print "Doctests done"
Using tests (and a bit later to use a versioning system) is among the
things that have to be taught as soon as possible :-)
Bye,
bearophile
Martien Verbruggen
There's also
http://docs.python.org/reference/datamodel.html#index-791
Martien
--
|
Martien Verbruggen | There are only 10 types of people in
first...@heliotrope.com.au | the world; those who understand binary
| and those who don't.