Python float to int conversion

Question

Basically  I m converting a float to an int  but I don t always have the expected value   Here s the code I m executing   x   2 51  print  --------- 251 0   y   251 0 print y  print int y    print  --------- 2 51   100   y   x   100 print y  print int y    print  --------- 2 51   1000   10   y   x   1000   10 print y  print int y    print  --------- 2 51   100   10   10   y   x   100   10   10 print y  print int y    x   4 02 print  --------- 402 0   y   402 0 print y  print int y    print  --------- 4 02   100   y   x   100 print y  print int y    print  --------- 4 02   1000   10   y   x   1000   10 print y  print int y    print  --------- 4 02   100   10   10   y   x   100   10   10 print y  print int y     And here s the result  first value is the result of the operation  second value is int   of the same operation    --------- 251 0 251 0 251 --------- 2 51   100 251 0 250 --------- 2 51   1000   10 251 0 251 --------- 2 51   100   10   10 251 0 250 --------- 402 0 402 0 402 --------- 4 02   100 402 0 401 --------- 4 02   1000   10 402 0 401 --------- 4 02   100   10   10 402 0 401   2 51 and 4 02 are the only values that lead to that strange behaviour on the 2 50 -  5 00 range  Every other two digits value in that range converts to int without any problem when given the same operations   So  what am I missing that leads to those results  I m using Python 2 7 2 by the way

User · Answer

Languages that use binary floating point representations (Python is one) cannot represent all fractional values exactly. If the result of your calculation is 250.99999999999 (and it might be), then taking the integer part will result in 250.

A canonical article on this topic is What Every Computer Scientist Should Know About Floating-Point Arithmetic.

User · Answer

2 51   100   250 999999999997   The int   function simply truncates the number at the decimal point  giving 250  Use   int round 2 51 100      to get 251 as an integer  In general  floating point numbers cannot be represented exactly  One should therefore be careful of round-off errors  As mentioned  this is not a Python-specific problem  It s a recurring problem in all computer languages

User · Answer

gt  gt  gt  x   2 51  gt  gt  gt  x 100 250 99999999999997   the floating point numbers are inaccurate  in this case  it is 250 99999999999999  which is really close to 251  but int   truncates the decimal part  in this case 250   you should take a look at the Decimal module or maybe if you have to do a lot of calculation at the mpmath library http   code google com p mpmath

User · Answer

What Every Computer Scientist Should Know About Floating-Point Arithmetic  Floating-point numbers cannot represent all the numbers   In particular  2 51 cannot be represented by a floating-point number  and is represented by a number very close to it    gt  gt  gt  print    16f    2 51 2 5099999999999998  gt  gt  gt  2 51 100 250 99999999999997  gt  gt  gt  4 02 100 401 99999999999994   If you use int  which truncates the numbers  you get   250 401   Have a look at the Decimal type

User · Answer

int converts by truncation  as has been mentioned by others  This can result in the answer being one different than expected  One way around this is to check if the result is  close enough  to an integer and adjust accordingly  otherwise the usual conversion  This is assuming you don t get too much roundoff and calculation error  which is a separate issue  For example   def toint f       trunc   int f      diff   f - trunc        trunc is one too low     if abs f - trunc - 1   lt  0 00001          return trunc   1       trunc is one too high     if abs f - trunc   1   lt  0 00001          return trunc - 1       trunc is the right value     return trunc   This function will adjust for off-by-one errors for near integers  The mpmath library does something similar for floating point numbers that are close to integers

[python] Python float to int conversion

Examples related to python

Examples related to floating-point

Examples related to int