Split an integer into digits to compute an ISBN checksum

Question

I m writing a program which calculates the check digit of an ISBN number  I have to read the user s input  nine digits of an ISBN  into an integer variable  and then multiply the last digit by 2  the second last digit by 3 and so on  How can I  split  the integer into its constituent digits to do this  As this is a basic homework exercise I am not supposed to use a list

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Recursion version   def int digits n       return  n  if n lt 10 else int digits n 10   n 10

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After own diligent searches I found several solutions  where each has advantages and disadvantages  Use the most suitable for your task   All examples tested with the CPython 3 5 on the operation system GNU Linux Debian 8     Using a recursion  Code  def get digits from left to right number  lst None          Return digits of an integer excluding the sign          if lst is None          lst   list        number   abs number       if number  lt  10          lst append number          return tuple lst       get digits from left to right number    10  lst      lst append number   10       return tuple lst    Demo  In  121   get digits from left to right -64517643246567536423  Out 121    6  4  5  1  7  6  4  3  2  4  6  5  6  7  5  3  6  4  2  3   In  122   get digits from left to right 0  Out 122    0    In  123   get digits from left to right 123012312312321312312312  Out 123    1  2  3  0  1  2  3  1  2  3  1  2  3  2  1  3  1  2  3  1  2  3  1  2      Using the function divmod  Code  def get digits from right to left number          Return digits of an integer excluding the sign          number   abs number       if number  lt  10          return  number         lst   list        while number          number  digit   divmod number  10          lst insert 0  digit       return tuple lst    Demo  In  125   get digits from right to left -3245214012321021213  Out 125    3  2  4  5  2  1  4  0  1  2  3  2  1  0  2  1  2  1  3   In  126   get digits from right to left 0  Out 126    0    In  127   get digits from right to left 9999999999999999  Out 127    9  9  9  9  9  9  9  9  9  9  9  9  9  9  9  9      Using a construction tuple map int  str abs number     In  109   tuple map int  str abs -123123123     Out 109    1  2  3  1  2  3  1  2  3   In  110   tuple map int  str abs 1412421321312     Out 110    1  4  1  2  4  2  1  3  2  1  3  1  2   In  111   tuple map int  str abs 0     Out 111    0       Using the function re findall  In  112   tuple map int  re findall r  d   str 1321321312     Out 112    1  3  2  1  3  2  1  3  1  2   In  113   tuple map int  re findall r  d   str -1321321312     Out 113    1  3  2  1  3  2  1  3  1  2   In  114   tuple map int  re findall r  d   str 0     Out 114    0       Using the module decimal  In  117   decimal Decimal 0  as tuple   digits Out 117    0    In  118   decimal Decimal 3441120391321  as tuple   digits Out 118    3  4  4  1  1  2  0  3  9  1  3  2  1   In  119   decimal Decimal -3441120391321  as tuple   digits Out 119    3  4  4  1  1  2  0  3  9  1  3  2  1

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Use the body of this loop to do whatever you want to with the digits  for digit in map int  str my number

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How about a one-liner list of digits     ldigits   lambda n  l     not n and l or l insert 0 n 10  or ldigits n 10 l

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Similar to this answer but more a more  pythonic  way to iterate over the digis would be   while number         pop  the rightmost digit     number  digit   divmod number  10

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Just create a string out of it   myinteger   212345 number string   str myinteger    That s enough  Now you can iterate over it   for ch in number string      print ch   will print each digit in order   Or you can slice it   print number string  2    first two digits print number string -3     last three digits print number string 3    forth digit     Or better  don t convert the user s input to an integer  the user types a string   isbn   raw input   for pos  ch in enumerate reversed isbn        print   d    d is  d    pos   2  int ch   int ch     pos   2    For more information read a tutorial

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I have made this program and here is the bit of code that actually calculates the check digit in my program       Get the 10 digit number     number input  Please enter ISBN number           Explained below     no11      int number 0   11     int number 1   10     int number 2   9     int number 3   8                 int number 4   7     int number 5   6     int number 6   5     int number 7   4                int number 8   3     int number 9   2   11        Round to 1 dp     no11   round no11  1        explained below     no11   str no11  split            get the remainder and check digit     remainder   no11 1      no11    11 - int remainder         Calculate 11 digit ISBN     print  Correct ISBN number is     number   str no11     Its a long line of code  but it splits the number up  multiplies the digits by the appropriate amount  adds them together and divides them by 11  in one line of code  The  split   function just creates a list  being split at the decimal  so you can take the 2nd item in the list and take that from 11 to find the check digit  This could also be made even more efficient by changing these two lines       remainder   no11 1      no11    11 - int remainder     To this       no11    11 - int no11 1      Hope this helps

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number 10  x  10   You can use this in a loop  where number is the full number  x is each iteration of the loop  0 1 2 3     n  with n being the stop point  x   0 gives the ones place  x   1 gives the tens  x   2 gives the hundreds  and so on  Keep in mind that this will give the value of the digits from right to left  so this might not be the for an ISBN but it will still isolate each digit

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On Older versions of Python     map int str 123     On New Version 3k  list map int str 123

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Convert it to string and map over it with the int   function   map int  str 1231231231

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Converting to str is definitely slower then dividing by 10   map is sligthly slower than list comprehension   convert to string with map 2 13599181175 convert to string with list comprehension 1 92812991142 modulo  division  recursive 0 948769807816 modulo  division 0 699964046478   These times were returned by the following code on my laptop   foo        def foo limit       return sorted set map sum  map lambda x  map int  list str x     map lambda x  x   9  range limit        foo  i       bar        def bar limit       return sorted set  sum  int i  for i in str n    for n in  k  9 for k in range limit       bar  i       rac        def digits n       return  n  if n lt 10 else digits n   10   n    10   def rabbit limit       return sorted set  sum digits n   for n in  k  9 for k in range limit       rabbit  i       rab        def sum digits number     result   0   while number      digit   number    10     result    digit     number    10   return result  def rabbit limit       return sorted set  sum digits n  for n in  k  9 for k in range limit       rabbit  i        import timeit  print  convert to string with map   timeit timeit foo   100  number 10000  print  convert to string with list comprehension   timeit timeit bar   100  number 10000  print  modulo  division  recursive   timeit timeit rac   100  number 10000  print  modulo  division   timeit timeit rab   100  number 10000

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while number      digit   number   10        do whatever with digit        remove last digit from number  as integer      number     10   On each iteration of the loop  it removes the last digit from number  assigning it to digit   It s in reverse  starts from the last digit  finishes with the first

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Answer  165  Method  brute-force  Here is a tiny bit of Python  version 2 7  code to count em all   from math import sqrt  floor is ps   lambda x  floor sqrt x      2    x count   0 for n in range 1002  10000  3       if n   11 and is ps sum map int  str n              count    1         print    i   s     count  n

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list of ints    int i  for i in str ISBN     Will give you a ordered list of ints  Of course  given duck typing  you might as well work with str ISBN    Edit  As mentioned in the comments  this list isn t sorted in the sense of being ascending or descending  but it does have a defined order  sets  dictionaries  etc in python in theory don t  although in practice the order tends to be fairly reliable   If you want to sort it   list of ints sort    is your friend  Note that sort   sorts in place  as in  actually changes the order of the existing list  and doesn t return a new list

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Just assuming you want to get the i-th least significant digit from an integer number x  you can try    abs x   10  i    10   i-1     I hope it helps

[python] Split an integer into digits to compute an ISBN checksum

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