How to resolve a Java Rounding Double issue

Question

Seems like the subtraction is triggering some kind of issue and the resulting value is wrong   double tempCommission   targetPremium doubleValue   rate doubleValue   100d    78 75   787 5   10 0 100d  double netToCompany   targetPremium doubleValue   - tempCommission    708 75   787 5 - 78 75  double dCommission   request getPremium   doubleValue   - netToCompany    877 8499999999999   1586 6 - 708 75  The resulting expected value would be 877 85   What should be done to ensure the correct calculation

User · Answer

This is a fun issue   The idea behind Timons reply is you specify an epsilon which represents the smallest precision a legal double can be  If you know in your application that you will never need precision below 0 00000001 then what he suggests is sufficient to get a more precise result very close to the truth  Useful in applications where they know up front their maximum precision  for in instance finance for currency precisions  etc   However the fundamental problem with trying to round it off is that when you divide by a factor to rescale it you actually introduce another possibility for precision problems  Any manipulation of doubles can introduce imprecision problems with varying frequency  Especially if you re trying to round at a very significant digit  so your operands are  lt  0  for instance if you run the following with Timons code   System out println round  1515476 0    0 00001    0 00001     Will result in 1499999 9999999998 where the goal here is to round at the units of 500000  i e  we want 1500000   In fact the only way to be completely sure you ve eliminated the imprecision is to go through a BigDecimal to scale off  e g   System out println BigDecimal valueOf 1515476 0  setScale -5  RoundingMode HALF UP  doubleValue       Using a mix of the epsilon strategy and the BigDecimal strategy will give you fine control over your precision  The idea being the epsilon gets you very close and then the BigDecimal will eliminate any imprecision caused by rescaling afterwards  Though using BigDecimal will reduce the expected performance of your application   It has been pointed out to me that the final step of using BigDecimal to rescale it isn t always necessary for some uses cases when you can determine that there s no input value that the final division can reintroduce an error  Currently I don t know how to properly determine this so if anyone knows how then I d be delighted to hear about it

User · Answer

As the previous answers stated  this is a consequence of doing floating point arithmetic   As a previous poster suggested  When you are doing numeric calculations  use java math BigDecimal   However  there is a gotcha to using BigDecimal  When you are converting from the double value to a BigDecimal  you have a choice of using a new BigDecimal double  constructor or the BigDecimal valueOf double  static factory method  Use the static factory method   The double constructor converts the entire precision of the double to a BigDecimal while the static factory effectively converts it to a String  then converts that to a BigDecimal   This becomes relevant when you are running into those subtle rounding errors  A number might display as  585  but internally its value is  0 58499999999999996447286321199499070644378662109375   If you used the BigDecimal constructor  you would get the number that is NOT equal to 0 585  while the static method would give you a value equal to 0 585    double value   0 585  System out println new BigDecimal value    System out println BigDecimal valueOf value      on my system gives   0 58499999999999996447286321199499070644378662109375 0 585

User · Answer

So far the most elegant and most efficient way to do that in Java       double newNum   Math floor num   100   0 5    100

User · Answer

double rounded   Math rint toround   100    100

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double rounded   Math rint toround   100    100

User · Answer

Another example   double d   0  for  int i   1  i  lt   10  i          d    0 1    System out println d         prints 0 9999999999999999 not 1 0   Use BigDecimal instead   EDIT   Also  just to point out this isn t a  Java  rounding issue  Other languages exhibit  similar  though not necessarily consistent  behaviour  Java at least guarantees consistent behaviour in this regard

User · Answer

Another example   double d   0  for  int i   1  i  lt   10  i          d    0 1    System out println d         prints 0 9999999999999999 not 1 0   Use BigDecimal instead   EDIT   Also  just to point out this isn t a  Java  rounding issue  Other languages exhibit  similar  though not necessarily consistent  behaviour  Java at least guarantees consistent behaviour in this regard

User · Answer

To control the precision of floating point arithmetic  you should use java math BigDecimal  Read The need for BigDecimal by John Zukowski for more information   Given your example  the last line would be as following using BigDecimal   import java math BigDecimal   BigDecimal premium   BigDecimal valueOf  1586 6    BigDecimal netToCompany   BigDecimal valueOf  708 75    BigDecimal commission   premium subtract netToCompany   System out println commission           premium     -     netToCompany     This results in the following output   877 85   1586 6 - 708 75

User · Answer

Any time you do calculations with doubles  this can happen  This code would give you 877 85   double answer   Math round dCommission   100000    100000 0

User · Answer

To control the precision of floating point arithmetic  you should use java math BigDecimal  Read The need for BigDecimal by John Zukowski for more information   Given your example  the last line would be as following using BigDecimal   import java math BigDecimal   BigDecimal premium   BigDecimal valueOf  1586 6    BigDecimal netToCompany   BigDecimal valueOf  708 75    BigDecimal commission   premium subtract netToCompany   System out println commission           premium     -     netToCompany     This results in the following output   877 85   1586 6 - 708 75

User · Answer

To control the precision of floating point arithmetic  you should use java math BigDecimal  Read The need for BigDecimal by John Zukowski for more information   Given your example  the last line would be as following using BigDecimal   import java math BigDecimal   BigDecimal premium   BigDecimal valueOf  1586 6    BigDecimal netToCompany   BigDecimal valueOf  708 75    BigDecimal commission   premium subtract netToCompany   System out println commission           premium     -     netToCompany     This results in the following output   877 85   1586 6 - 708 75

User · Answer

Save the number of cents rather than dollars  and just do the format to dollars when you output it   That way you can use an integer which doesn t suffer from the precision issues

User · Answer

Better yet use JScience as BigDecimal is fairly limited  e g   no sqrt function   double dCommission   1586 6 - 708 75  System out println dCommission    gt  877 8499999999999  Real dCommissionR   Real valueOf 1586 6 - 708 75   System out println dCommissionR    gt  877 850000000000

User · Answer

Any time you do calculations with doubles  this can happen  This code would give you 877 85   double answer   Math round dCommission   100000    100000 0

User · Answer

As the previous answers stated  this is a consequence of doing floating point arithmetic   As a previous poster suggested  When you are doing numeric calculations  use java math BigDecimal   However  there is a gotcha to using BigDecimal  When you are converting from the double value to a BigDecimal  you have a choice of using a new BigDecimal double  constructor or the BigDecimal valueOf double  static factory method  Use the static factory method   The double constructor converts the entire precision of the double to a BigDecimal while the static factory effectively converts it to a String  then converts that to a BigDecimal   This becomes relevant when you are running into those subtle rounding errors  A number might display as  585  but internally its value is  0 58499999999999996447286321199499070644378662109375   If you used the BigDecimal constructor  you would get the number that is NOT equal to 0 585  while the static method would give you a value equal to 0 585    double value   0 585  System out println new BigDecimal value    System out println BigDecimal valueOf value      on my system gives   0 58499999999999996447286321199499070644378662109375 0 585

User · Answer

It s quite simple   Use the   2f operator for output  Problem solved   For example   int a   877 8499999999999  System out printf  Formatted Output is    2f   a     The above code results in a print output of  877 85  The   2f operator defines that only TWO decimal places should be used

User · Answer

This is a fun issue   The idea behind Timons reply is you specify an epsilon which represents the smallest precision a legal double can be  If you know in your application that you will never need precision below 0 00000001 then what he suggests is sufficient to get a more precise result very close to the truth  Useful in applications where they know up front their maximum precision  for in instance finance for currency precisions  etc   However the fundamental problem with trying to round it off is that when you divide by a factor to rescale it you actually introduce another possibility for precision problems  Any manipulation of doubles can introduce imprecision problems with varying frequency  Especially if you re trying to round at a very significant digit  so your operands are  lt  0  for instance if you run the following with Timons code   System out println round  1515476 0    0 00001    0 00001     Will result in 1499999 9999999998 where the goal here is to round at the units of 500000  i e  we want 1500000   In fact the only way to be completely sure you ve eliminated the imprecision is to go through a BigDecimal to scale off  e g   System out println BigDecimal valueOf 1515476 0  setScale -5  RoundingMode HALF UP  doubleValue       Using a mix of the epsilon strategy and the BigDecimal strategy will give you fine control over your precision  The idea being the epsilon gets you very close and then the BigDecimal will eliminate any imprecision caused by rescaling afterwards  Though using BigDecimal will reduce the expected performance of your application   It has been pointed out to me that the final step of using BigDecimal to rescale it isn t always necessary for some uses cases when you can determine that there s no input value that the final division can reintroduce an error  Currently I don t know how to properly determine this so if anyone knows how then I d be delighted to hear about it

User · Answer

Any time you do calculations with doubles  this can happen  This code would give you 877 85   double answer   Math round dCommission   100000    100000 0

User · Answer

I would modify the example above as follows   import java math BigDecimal   BigDecimal premium   new BigDecimal  1586 6    BigDecimal netToCompany   new BigDecimal  708 75    BigDecimal commission   premium subtract netToCompany   System out println commission           premium     -     netToCompany     This way you avoid the pitfalls of using string to begin with  Another alternative   import java math BigDecimal   BigDecimal premium   BigDecimal valueOf 158660  2   BigDecimal netToCompany   BigDecimal valueOf 70875  2   BigDecimal commission   premium subtract netToCompany   System out println commission           premium     -     netToCompany     I think these options are better than using doubles  In webapps numbers start out as strings anyways

User · Answer

To control the precision of floating point arithmetic  you should use java math BigDecimal  Read The need for BigDecimal by John Zukowski for more information   Given your example  the last line would be as following using BigDecimal   import java math BigDecimal   BigDecimal premium   BigDecimal valueOf  1586 6    BigDecimal netToCompany   BigDecimal valueOf  708 75    BigDecimal commission   premium subtract netToCompany   System out println commission           premium     -     netToCompany     This results in the following output   877 85   1586 6 - 708 75

User · Answer

See responses to this question   Essentially what you are seeing is a natural consequence of using floating point arithmetic   You could pick some arbitrary precision  significant digits of your inputs   and round your result to it  if you feel comfortable doing that

User · Answer

Another example   double d   0  for  int i   1  i  lt   10  i          d    0 1    System out println d         prints 0 9999999999999999 not 1 0   Use BigDecimal instead   EDIT   Also  just to point out this isn t a  Java  rounding issue  Other languages exhibit  similar  though not necessarily consistent  behaviour  Java at least guarantees consistent behaviour in this regard

User · Answer

It s quite simple   Use the   2f operator for output  Problem solved   For example   int a   877 8499999999999  System out printf  Formatted Output is    2f   a     The above code results in a print output of  877 85  The   2f operator defines that only TWO decimal places should be used

User · Answer

See responses to this question   Essentially what you are seeing is a natural consequence of using floating point arithmetic   You could pick some arbitrary precision  significant digits of your inputs   and round your result to it  if you feel comfortable doing that

User · Answer

Save the number of cents rather than dollars  and just do the format to dollars when you output it   That way you can use an integer which doesn t suffer from the precision issues

User · Answer

Save the number of cents rather than dollars  and just do the format to dollars when you output it   That way you can use an integer which doesn t suffer from the precision issues

User · Answer

I would modify the example above as follows   import java math BigDecimal   BigDecimal premium   new BigDecimal  1586 6    BigDecimal netToCompany   new BigDecimal  708 75    BigDecimal commission   premium subtract netToCompany   System out println commission           premium     -     netToCompany     This way you avoid the pitfalls of using string to begin with  Another alternative   import java math BigDecimal   BigDecimal premium   BigDecimal valueOf 158660  2   BigDecimal netToCompany   BigDecimal valueOf 70875  2   BigDecimal commission   premium subtract netToCompany   System out println commission           premium     -     netToCompany     I think these options are better than using doubles  In webapps numbers start out as strings anyways

User · Answer

So far the most elegant and most efficient way to do that in Java       double newNum   Math floor num   100   0 5    100

User · Answer

Although you should not use doubles for precise calculations the following trick helped me if you are rounding the results anyway    public static int round Double i        return  int  Math round i     i  gt  0 0    0 00000001   -0 00000001        Example       Double foo   0 0      for  int i   1  i  lt   150  i              foo    0 00010            System out println foo       System out println Math round foo   100 0    100 0       System out println round foo 100 0    100 0     Which prints   0 014999999999999965 0 01 0 02   More info  http   en wikipedia org wiki Double precision

User · Answer

See responses to this question   Essentially what you are seeing is a natural consequence of using floating point arithmetic   You could pick some arbitrary precision  significant digits of your inputs   and round your result to it  if you feel comfortable doing that

User · Answer

Any time you do calculations with doubles  this can happen  This code would give you 877 85   double answer   Math round dCommission   100000    100000 0

User · Answer

See responses to this question   Essentially what you are seeing is a natural consequence of using floating point arithmetic   You could pick some arbitrary precision  significant digits of your inputs   and round your result to it  if you feel comfortable doing that

User · Answer

As the previous answers stated  this is a consequence of doing floating point arithmetic   As a previous poster suggested  When you are doing numeric calculations  use java math BigDecimal   However  there is a gotcha to using BigDecimal  When you are converting from the double value to a BigDecimal  you have a choice of using a new BigDecimal double  constructor or the BigDecimal valueOf double  static factory method  Use the static factory method   The double constructor converts the entire precision of the double to a BigDecimal while the static factory effectively converts it to a String  then converts that to a BigDecimal   This becomes relevant when you are running into those subtle rounding errors  A number might display as  585  but internally its value is  0 58499999999999996447286321199499070644378662109375   If you used the BigDecimal constructor  you would get the number that is NOT equal to 0 585  while the static method would give you a value equal to 0 585    double value   0 585  System out println new BigDecimal value    System out println BigDecimal valueOf value      on my system gives   0 58499999999999996447286321199499070644378662109375 0 585

User · Answer

Although you should not use doubles for precise calculations the following trick helped me if you are rounding the results anyway    public static int round Double i        return  int  Math round i     i  gt  0 0    0 00000001   -0 00000001        Example       Double foo   0 0      for  int i   1  i  lt   150  i              foo    0 00010            System out println foo       System out println Math round foo   100 0    100 0       System out println round foo 100 0    100 0     Which prints   0 014999999999999965 0 01 0 02   More info  http   en wikipedia org wiki Double precision

User · Answer

Better yet use JScience as BigDecimal is fairly limited  e g   no sqrt function   double dCommission   1586 6 - 708 75  System out println dCommission    gt  877 8499999999999  Real dCommissionR   Real valueOf 1586 6 - 708 75   System out println dCommissionR    gt  877 850000000000

User · Answer

As the previous answers stated  this is a consequence of doing floating point arithmetic   As a previous poster suggested  When you are doing numeric calculations  use java math BigDecimal   However  there is a gotcha to using BigDecimal  When you are converting from the double value to a BigDecimal  you have a choice of using a new BigDecimal double  constructor or the BigDecimal valueOf double  static factory method  Use the static factory method   The double constructor converts the entire precision of the double to a BigDecimal while the static factory effectively converts it to a String  then converts that to a BigDecimal   This becomes relevant when you are running into those subtle rounding errors  A number might display as  585  but internally its value is  0 58499999999999996447286321199499070644378662109375   If you used the BigDecimal constructor  you would get the number that is NOT equal to 0 585  while the static method would give you a value equal to 0 585    double value   0 585  System out println new BigDecimal value    System out println BigDecimal valueOf value      on my system gives   0 58499999999999996447286321199499070644378662109375 0 585

User · Answer

Another example   double d   0  for  int i   1  i  lt   10  i          d    0 1    System out println d         prints 0 9999999999999999 not 1 0   Use BigDecimal instead   EDIT   Also  just to point out this isn t a  Java  rounding issue  Other languages exhibit  similar  though not necessarily consistent  behaviour  Java at least guarantees consistent behaviour in this regard

User · Answer

Save the number of cents rather than dollars  and just do the format to dollars when you output it   That way you can use an integer which doesn t suffer from the precision issues

[java] How to resolve a Java Rounding Double issue

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