How do you round a floating point number in Perl

Question

How can I round a decimal number  floating point  to the nearest integer    e g   1 2   1 1 7   2

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If you decide to use printf or sprintf  note that they use the Round half to even method   foreach my  i   0 5  1 5  2 5  3 5         printf   i - gt    0f n    i      END   0 5 - gt  0 1 5 - gt  2 2 5 - gt  2 3 5 - gt  4

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Output of perldoc -q round   Does Perl have a round   function   What about ceil   and floor    Trig functions   Remember that int   merely truncates toward 0   For rounding to a certain number of digits  sprintf   or printf   is usually the easiest route       printf    3f   3 1415926535           prints 3 142   The POSIX module  part of the standard Perl distribution  implements ceil    floor    and a number of other mathematical and trigonometric functions       use POSIX       ceil     ceil 3 5                            4      floor    floor 3 5                           3   In 5 000 to 5 003 perls  trigonometry was done in the Math  Complex module   With 5 004  the Math  Trig module  part of the standard Perl distribution  implements the trigonometric functions  Internally it uses the Math  Complex module and some functions can break out from the real axis into the complex plane  for example the inverse sine of 2   Rounding in financial applications can have serious implications  and the rounding method used should be specified precisely   In these cases  it probably pays not to trust whichever system rounding is being used by Perl  but to instead implement the rounding function you need yourself   To see why  notice how you ll still have an issue on half-way-point alternation       for   i   0   i  lt  1 01   i    0 05    printf    1f    i       0 0 0 1 0 1 0 2 0 2 0 2 0 3 0 3 0 4 0 4 0 5 0 5 0 6 0 7 0 7     0 8 0 8 0 9 0 9 1 0 1 0   Don t blame Perl   It s the same as in C   IEEE says we have to do this   Perl numbers whose absolute values are integers under 2  31  on 32 bit machines  will work pretty much like mathematical integers  Other numbers are not guaranteed

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If you decide to use printf or sprintf  note that they use the Round half to even method   foreach my  i   0 5  1 5  2 5  3 5         printf   i - gt    0f n    i      END   0 5 - gt  0 1 5 - gt  2 2 5 - gt  2 3 5 - gt  4

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You can either use a module like Math  Round   use Math  Round  my  rounded   round   float      Or you can do it the crude way   my  rounded   sprintf    0f    float

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The following will round positive or negative numbers to a given decimal position   sub round          my   x   pow10            my  a   10     pow10       return  int  x    a      x  lt  0    -0 5   0 5      a

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Whilst not disagreeing with the complex answers about half-way marks and so on  for the more common  and possibly trivial  use-case   my  rounded   int  float   0 5    UPDATE  If it s possible for your  float to be negative  the following variation will produce the correct result   my  rounded   int  float    float abs  float 2    1     With this calculation -1 4 is rounded to -1  and -1 6 to -2  and zero won t explode

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Negative numbers can add some quirks that people need to be aware of   printf-style approaches give us correct numbers  but they can result in some odd displays  We have discovered that this method  in my opinion  stupidly  puts in a - sign whether or not it should or shouldn t  For example  -0 01 rounded to one decimal place returns a -0 0  rather than just 0  If you are going to do the printf style approach  and you know you want no decimal  use  d and not  f  when you need decimals  it s when the display gets wonky    While it s correct and for math no big deal  for display it just looks weird showing something like  -0 0    For the int method  negative numbers can change what you want as a result  though there are some arguments that can be made they are correct    The int   0 5 causes real issues with -negative numbers  unless you want it to work that way  but I imagine most people don t  -0 9 should probably round to -1  not 0  If you know that you want negative to be a ceiling rather than a floor then you can do it in one-liner  otherwise  you might want to use the int method with a minor modification  this obviously only works to get back whole numbers   my  var   -9 1  my  tmpRounded   int  abs  var    0 5    my  finalRounded    var  gt   0   0    tmpRounded   0 -  tmpRounded

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The following will round positive or negative numbers to a given decimal position   sub round          my   x   pow10            my  a   10     pow10       return  int  x    a      x  lt  0    -0 5   0 5      a

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You can either use a module like Math  Round   use Math  Round  my  rounded   round   float      Or you can do it the crude way   my  rounded   sprintf    0f    float

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Output of perldoc -q round   Does Perl have a round   function   What about ceil   and floor    Trig functions   Remember that int   merely truncates toward 0   For rounding to a certain number of digits  sprintf   or printf   is usually the easiest route       printf    3f   3 1415926535           prints 3 142   The POSIX module  part of the standard Perl distribution  implements ceil    floor    and a number of other mathematical and trigonometric functions       use POSIX       ceil     ceil 3 5                            4      floor    floor 3 5                           3   In 5 000 to 5 003 perls  trigonometry was done in the Math  Complex module   With 5 004  the Math  Trig module  part of the standard Perl distribution  implements the trigonometric functions  Internally it uses the Math  Complex module and some functions can break out from the real axis into the complex plane  for example the inverse sine of 2   Rounding in financial applications can have serious implications  and the rounding method used should be specified precisely   In these cases  it probably pays not to trust whichever system rounding is being used by Perl  but to instead implement the rounding function you need yourself   To see why  notice how you ll still have an issue on half-way-point alternation       for   i   0   i  lt  1 01   i    0 05    printf    1f    i       0 0 0 1 0 1 0 2 0 2 0 2 0 3 0 3 0 4 0 4 0 5 0 5 0 6 0 7 0 7     0 8 0 8 0 9 0 9 1 0 1 0   Don t blame Perl   It s the same as in C   IEEE says we have to do this   Perl numbers whose absolute values are integers under 2  31  on 32 bit machines  will work pretty much like mathematical integers  Other numbers are not guaranteed

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If you are only concerned with getting an integer value out of a whole floating point number  i e  12347 9999 or 54321 0001   this approach  borrowed and modified from above  will do the trick   my  rounded   floor  float   0 1

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You can either use a module like Math  Round   use Math  Round  my  rounded   round   float      Or you can do it the crude way   my  rounded   sprintf    0f    float

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You don t need any external module    x 0    1 2   x 1    1 7   foreach   x     print                 -int      lt 0 5    int       int     1      print   n       I may be missing your point  but I thought this was much cleaner way to do the same job   What this does is to walk through every positive number in the element  print the number and rounded integer in the format you mentioned  The code concatenates respective rounded positive integer only based on the decimals  int     basically round-down the number so   -int     captures the decimals  If the decimals are  by definition  strictly less than 0 5  round-down the number  If not  round-up by adding 1

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cat table     perl -ne    d  s   d   s   S     amp  amp  print      int   log  1  log 2     t 2 n

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You can either use a module like Math  Round   use Math  Round  my  rounded   round   float      Or you can do it the crude way   my  rounded   sprintf    0f    float

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See perldoc perlfaq      Remember that int   merely truncates toward 0   For rounding to a   certain number of digits  sprintf   or printf   is usually the   easiest route    printf    3f  3 1415926535      prints 3 142       The POSIX module  part of the standard Perl distribution    implements ceil    floor    and a number of other mathematical   and trigonometric functions   use POSIX   ceil    ceil 3 5     4  floor   floor 3 5     3       In 5 000 to 5 003 perls  trigonometry was done in the Math  Complex module       With 5 004  the Math  Trig module  part of the standard Perl distribution    implements the trigonometric functions       Internally it uses the Math  Complex module and some functions can break   out from the real axis into the complex plane  for example the inverse sine of 2       Rounding in financial applications can have serious implications  and the rounding   method used should be specified precisely   In these cases  it probably pays not to   trust whichever system rounding is being used by Perl  but to instead implement the   rounding function you need yourself       To see why  notice how you ll still have an issue on half-way-point alternation   for   i   0   i  lt  1 01   i    0 05       printf    1f    i    0 0 0 1 0 1 0 2 0 2 0 2 0 3 0 3 0 4 0 4 0 5 0 5 0 6 0 7 0 7 0 8 0 8 0 9 0 9 1 0 1 0       Don t blame Perl   It s the same as in C   IEEE says we have to do   this  Perl numbers whose absolute values are integers under 2  31  on   32 bit machines  will work pretty much like mathematical integers    Other numbers are not guaranteed

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You don t need any external module    x 0    1 2   x 1    1 7   foreach   x     print                 -int      lt 0 5    int       int     1      print   n       I may be missing your point  but I thought this was much cleaner way to do the same job   What this does is to walk through every positive number in the element  print the number and rounded integer in the format you mentioned  The code concatenates respective rounded positive integer only based on the decimals  int     basically round-down the number so   -int     captures the decimals  If the decimals are  by definition  strictly less than 0 5  round-down the number  If not  round-up by adding 1

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My solution for sprintf  if   value    m  d    5          format         d f        if  defined  1          my  coef    0      0  x  1    05                    value    value    coef                value   sprintf    format    value

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If you decide to use printf or sprintf  note that they use the Round half to even method   foreach my  i   0 5  1 5  2 5  3 5         printf   i - gt    0f n    i      END   0 5 - gt  0 1 5 - gt  2 2 5 - gt  2 3 5 - gt  4

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cat table     perl -ne    d  s   d   s   S     amp  amp  print      int   log  1  log 2     t 2 n

User · Answer

loads of reading documentation on how to round numbers  many experts suggest writing your own rounding routines  as the  canned  version provided with your language may not be precise enough  or contain errors   i imagine  however  they re talking many decimal places not just one  two  or three   with that in mind  here is my solution  although not EXACTLY as requested as my needs are to display dollars - the process is not much different  though      sub asDollars        my   cost          my  rv   0     my  negative   0    if   cost      -          negative   1       cost    s  -           my  cost   split        cost        let s get the first 3 digits of  cost 1    my   digit1   digit2   digit3    split      cost 1        now  is  digit3  gt   5      if yes  plus one to  digit2      is  digit2  gt  9 now      if yes   digit2   0   digit1       is  digit1  gt  9 now       if yes   digit1   0   cost 0      if   digit3  gt   5         digit3   0       digit2        if   digit2  gt  9           digit2   0         digit1          if   digit1  gt  9             digit1   0           cost 0                          cost 1     digit1    digit2    if   digit1 ne  0  and  cost 1   lt  10     cost 1      0          and pretty up the left of decimal   if   cost 0   gt  999     cost 0    commafied  cost 0          rv   join       cost      if   negative     rv    -     rv       return  rv     sub commafied               to insert commas before every 3rd number  from the right      positive or negative numbers        my   num          the number to insert commas into     my  negative   0    if   num      -          negative   1       num    s  -           num    s   0        strip LEADING zeros from given number     num    s 0 - g    convert zeros to dashes because     computers     if   num        my  digits   reverse split      num        num            for  my  i   0   i  lt   digits   i    3           num     digits  i         if   digits  i 1      num     digits  i 1           if   digits  i 2      num     digits  i 2           if   i  lt    digits - 3      num                 if   i  gt    digits    last                 num    s            num   join     reverse split      num         num    s - 0 g         if   negative     num    -     num       return  num    a number with commas added    usage  my  prettyNum   commafied 1234567890

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Following is a sample of five different ways to summate values  The first is a naive way to perform the summation  and fails   The second attempts to use sprintf    but it too fails  The third uses sprintf   successfully while the final two  4th  amp  5th  use floor  value   0 5     use strict   use warnings   use POSIX    my  values    26 67 62 51 62 51 62 51 68 82 79 39 79 39    my  total1   0 00   my  total2   0   my  total3   0   my  total4   0 00   my  total5   0   my  value1   my  value2   my  value3   my  value4   my  value5    foreach  value1   values            value2    value1         value3    value1         value4    value1         value5    value1          total1     value1          total2    sprintf   d    value2   100           value3   sprintf   1 2f    value3          value3    s              total3     value3          total4     value4          total5    floor   value5   100 0    0 5         total1    100    total4   floor   total4   100 0    0 5     print   total1    sprintf   011d    total1    n    print   total2    sprintf   011d    total2    n    print   total3    sprintf   011d    total3    n    print   total4    sprintf   011d    total4    n    print   total5    sprintf   011d    total5    n     exit 0       total1  00000044179    total2  00000044179    total3  00000044180    total4  00000044180    total5  00000044180   Note that floor  value   0 5  can be replaced with int  value   0 5  to remove the dependency on POSIX

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Output of perldoc -q round   Does Perl have a round   function   What about ceil   and floor    Trig functions   Remember that int   merely truncates toward 0   For rounding to a certain number of digits  sprintf   or printf   is usually the easiest route       printf    3f   3 1415926535           prints 3 142   The POSIX module  part of the standard Perl distribution  implements ceil    floor    and a number of other mathematical and trigonometric functions       use POSIX       ceil     ceil 3 5                            4      floor    floor 3 5                           3   In 5 000 to 5 003 perls  trigonometry was done in the Math  Complex module   With 5 004  the Math  Trig module  part of the standard Perl distribution  implements the trigonometric functions  Internally it uses the Math  Complex module and some functions can break out from the real axis into the complex plane  for example the inverse sine of 2   Rounding in financial applications can have serious implications  and the rounding method used should be specified precisely   In these cases  it probably pays not to trust whichever system rounding is being used by Perl  but to instead implement the rounding function you need yourself   To see why  notice how you ll still have an issue on half-way-point alternation       for   i   0   i  lt  1 01   i    0 05    printf    1f    i       0 0 0 1 0 1 0 2 0 2 0 2 0 3 0 3 0 4 0 4 0 5 0 5 0 6 0 7 0 7     0 8 0 8 0 9 0 9 1 0 1 0   Don t blame Perl   It s the same as in C   IEEE says we have to do this   Perl numbers whose absolute values are integers under 2  31  on 32 bit machines  will work pretty much like mathematical integers  Other numbers are not guaranteed

User · Answer

If you are only concerned with getting an integer value out of a whole floating point number  i e  12347 9999 or 54321 0001   this approach  borrowed and modified from above  will do the trick   my  rounded   floor  float   0 1

User · Answer

See perldoc perlfaq      Remember that int   merely truncates toward 0   For rounding to a   certain number of digits  sprintf   or printf   is usually the   easiest route    printf    3f  3 1415926535      prints 3 142       The POSIX module  part of the standard Perl distribution    implements ceil    floor    and a number of other mathematical   and trigonometric functions   use POSIX   ceil    ceil 3 5     4  floor   floor 3 5     3       In 5 000 to 5 003 perls  trigonometry was done in the Math  Complex module       With 5 004  the Math  Trig module  part of the standard Perl distribution    implements the trigonometric functions       Internally it uses the Math  Complex module and some functions can break   out from the real axis into the complex plane  for example the inverse sine of 2       Rounding in financial applications can have serious implications  and the rounding   method used should be specified precisely   In these cases  it probably pays not to   trust whichever system rounding is being used by Perl  but to instead implement the   rounding function you need yourself       To see why  notice how you ll still have an issue on half-way-point alternation   for   i   0   i  lt  1 01   i    0 05       printf    1f    i    0 0 0 1 0 1 0 2 0 2 0 2 0 3 0 3 0 4 0 4 0 5 0 5 0 6 0 7 0 7 0 8 0 8 0 9 0 9 1 0 1 0       Don t blame Perl   It s the same as in C   IEEE says we have to do   this  Perl numbers whose absolute values are integers under 2  31  on   32 bit machines  will work pretty much like mathematical integers    Other numbers are not guaranteed

User · Answer

My solution for sprintf  if   value    m  d    5          format         d f        if  defined  1          my  coef    0      0  x  1    05                    value    value    coef                value   sprintf    format    value

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Following is a sample of five different ways to summate values  The first is a naive way to perform the summation  and fails   The second attempts to use sprintf    but it too fails  The third uses sprintf   successfully while the final two  4th  amp  5th  use floor  value   0 5     use strict   use warnings   use POSIX    my  values    26 67 62 51 62 51 62 51 68 82 79 39 79 39    my  total1   0 00   my  total2   0   my  total3   0   my  total4   0 00   my  total5   0   my  value1   my  value2   my  value3   my  value4   my  value5    foreach  value1   values            value2    value1         value3    value1         value4    value1         value5    value1          total1     value1          total2    sprintf   d    value2   100           value3   sprintf   1 2f    value3          value3    s              total3     value3          total4     value4          total5    floor   value5   100 0    0 5         total1    100    total4   floor   total4   100 0    0 5     print   total1    sprintf   011d    total1    n    print   total2    sprintf   011d    total2    n    print   total3    sprintf   011d    total3    n    print   total4    sprintf   011d    total4    n    print   total5    sprintf   011d    total5    n     exit 0       total1  00000044179    total2  00000044179    total3  00000044180    total4  00000044180    total5  00000044180   Note that floor  value   0 5  can be replaced with int  value   0 5  to remove the dependency on POSIX

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Negative numbers can add some quirks that people need to be aware of   printf-style approaches give us correct numbers  but they can result in some odd displays  We have discovered that this method  in my opinion  stupidly  puts in a - sign whether or not it should or shouldn t  For example  -0 01 rounded to one decimal place returns a -0 0  rather than just 0  If you are going to do the printf style approach  and you know you want no decimal  use  d and not  f  when you need decimals  it s when the display gets wonky    While it s correct and for math no big deal  for display it just looks weird showing something like  -0 0    For the int method  negative numbers can change what you want as a result  though there are some arguments that can be made they are correct    The int   0 5 causes real issues with -negative numbers  unless you want it to work that way  but I imagine most people don t  -0 9 should probably round to -1  not 0  If you know that you want negative to be a ceiling rather than a floor then you can do it in one-liner  otherwise  you might want to use the int method with a minor modification  this obviously only works to get back whole numbers   my  var   -9 1  my  tmpRounded   int  abs  var    0 5    my  finalRounded    var  gt   0   0    tmpRounded   0 -  tmpRounded

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Output of perldoc -q round   Does Perl have a round   function   What about ceil   and floor    Trig functions   Remember that int   merely truncates toward 0   For rounding to a certain number of digits  sprintf   or printf   is usually the easiest route       printf    3f   3 1415926535           prints 3 142   The POSIX module  part of the standard Perl distribution  implements ceil    floor    and a number of other mathematical and trigonometric functions       use POSIX       ceil     ceil 3 5                            4      floor    floor 3 5                           3   In 5 000 to 5 003 perls  trigonometry was done in the Math  Complex module   With 5 004  the Math  Trig module  part of the standard Perl distribution  implements the trigonometric functions  Internally it uses the Math  Complex module and some functions can break out from the real axis into the complex plane  for example the inverse sine of 2   Rounding in financial applications can have serious implications  and the rounding method used should be specified precisely   In these cases  it probably pays not to trust whichever system rounding is being used by Perl  but to instead implement the rounding function you need yourself   To see why  notice how you ll still have an issue on half-way-point alternation       for   i   0   i  lt  1 01   i    0 05    printf    1f    i       0 0 0 1 0 1 0 2 0 2 0 2 0 3 0 3 0 4 0 4 0 5 0 5 0 6 0 7 0 7     0 8 0 8 0 9 0 9 1 0 1 0   Don t blame Perl   It s the same as in C   IEEE says we have to do this   Perl numbers whose absolute values are integers under 2  31  on 32 bit machines  will work pretty much like mathematical integers  Other numbers are not guaranteed

User · Answer

Whilst not disagreeing with the complex answers about half-way marks and so on  for the more common  and possibly trivial  use-case   my  rounded   int  float   0 5    UPDATE  If it s possible for your  float to be negative  the following variation will produce the correct result   my  rounded   int  float    float abs  float 2    1     With this calculation -1 4 is rounded to -1  and -1 6 to -2  and zero won t explode

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See perldoc perlfaq      Remember that int   merely truncates toward 0   For rounding to a   certain number of digits  sprintf   or printf   is usually the   easiest route    printf    3f  3 1415926535      prints 3 142       The POSIX module  part of the standard Perl distribution    implements ceil    floor    and a number of other mathematical   and trigonometric functions   use POSIX   ceil    ceil 3 5     4  floor   floor 3 5     3       In 5 000 to 5 003 perls  trigonometry was done in the Math  Complex module       With 5 004  the Math  Trig module  part of the standard Perl distribution    implements the trigonometric functions       Internally it uses the Math  Complex module and some functions can break   out from the real axis into the complex plane  for example the inverse sine of 2       Rounding in financial applications can have serious implications  and the rounding   method used should be specified precisely   In these cases  it probably pays not to   trust whichever system rounding is being used by Perl  but to instead implement the   rounding function you need yourself       To see why  notice how you ll still have an issue on half-way-point alternation   for   i   0   i  lt  1 01   i    0 05       printf    1f    i    0 0 0 1 0 1 0 2 0 2 0 2 0 3 0 3 0 4 0 4 0 5 0 5 0 6 0 7 0 7 0 8 0 8 0 9 0 9 1 0 1 0       Don t blame Perl   It s the same as in C   IEEE says we have to do   this  Perl numbers whose absolute values are integers under 2  31  on   32 bit machines  will work pretty much like mathematical integers    Other numbers are not guaranteed

User · Answer

See perldoc perlfaq      Remember that int   merely truncates toward 0   For rounding to a   certain number of digits  sprintf   or printf   is usually the   easiest route    printf    3f  3 1415926535      prints 3 142       The POSIX module  part of the standard Perl distribution    implements ceil    floor    and a number of other mathematical   and trigonometric functions   use POSIX   ceil    ceil 3 5     4  floor   floor 3 5     3       In 5 000 to 5 003 perls  trigonometry was done in the Math  Complex module       With 5 004  the Math  Trig module  part of the standard Perl distribution    implements the trigonometric functions       Internally it uses the Math  Complex module and some functions can break   out from the real axis into the complex plane  for example the inverse sine of 2       Rounding in financial applications can have serious implications  and the rounding   method used should be specified precisely   In these cases  it probably pays not to   trust whichever system rounding is being used by Perl  but to instead implement the   rounding function you need yourself       To see why  notice how you ll still have an issue on half-way-point alternation   for   i   0   i  lt  1 01   i    0 05       printf    1f    i    0 0 0 1 0 1 0 2 0 2 0 2 0 3 0 3 0 4 0 4 0 5 0 5 0 6 0 7 0 7 0 8 0 8 0 9 0 9 1 0 1 0       Don t blame Perl   It s the same as in C   IEEE says we have to do   this  Perl numbers whose absolute values are integers under 2  31  on   32 bit machines  will work pretty much like mathematical integers    Other numbers are not guaranteed

User · Answer

If you decide to use printf or sprintf  note that they use the Round half to even method   foreach my  i   0 5  1 5  2 5  3 5         printf   i - gt    0f n    i      END   0 5 - gt  0 1 5 - gt  2 2 5 - gt  2 3 5 - gt  4

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loads of reading documentation on how to round numbers  many experts suggest writing your own rounding routines  as the  canned  version provided with your language may not be precise enough  or contain errors   i imagine  however  they re talking many decimal places not just one  two  or three   with that in mind  here is my solution  although not EXACTLY as requested as my needs are to display dollars - the process is not much different  though      sub asDollars        my   cost          my  rv   0     my  negative   0    if   cost      -          negative   1       cost    s  -           my  cost   split        cost        let s get the first 3 digits of  cost 1    my   digit1   digit2   digit3    split      cost 1        now  is  digit3  gt   5      if yes  plus one to  digit2      is  digit2  gt  9 now      if yes   digit2   0   digit1       is  digit1  gt  9 now       if yes   digit1   0   cost 0      if   digit3  gt   5         digit3   0       digit2        if   digit2  gt  9           digit2   0         digit1          if   digit1  gt  9             digit1   0           cost 0                          cost 1     digit1    digit2    if   digit1 ne  0  and  cost 1   lt  10     cost 1      0          and pretty up the left of decimal   if   cost 0   gt  999     cost 0    commafied  cost 0          rv   join       cost      if   negative     rv    -     rv       return  rv     sub commafied               to insert commas before every 3rd number  from the right      positive or negative numbers        my   num          the number to insert commas into     my  negative   0    if   num      -          negative   1       num    s  -           num    s   0        strip LEADING zeros from given number     num    s 0 - g    convert zeros to dashes because     computers     if   num        my  digits   reverse split      num        num            for  my  i   0   i  lt   digits   i    3           num     digits  i         if   digits  i 1      num     digits  i 1           if   digits  i 2      num     digits  i 2           if   i  lt    digits - 3      num                 if   i  gt    digits    last                 num    s            num   join     reverse split      num         num    s - 0 g         if   negative     num    -     num       return  num    a number with commas added    usage  my  prettyNum   commafied 1234567890

[perl] How do you round a floating point number in Perl?

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