File: gawk.info,  Node: Setting the rounding mode,  Prev: Setting precision,  Up: FP Math Caution

16.4.5 Setting the Rounding Mode
--------------------------------

The 'ROUNDMODE' variable provides program-level control over the
rounding mode.  The correspondence between 'ROUNDMODE' and the IEEE
rounding modes is shown in *note Table 16.5: table-gawk-rounding-modes.


Rounding mode                    IEEE name              'ROUNDMODE'
---------------------------------------------------------------------------
Round to nearest, ties to even   'roundTiesToEven'      '"N"' or '"n"'
Round toward positive infinity   'roundTowardPositive'  '"U"' or '"u"'
Round toward negative infinity   'roundTowardNegative'  '"D"' or '"d"'
Round toward zero                'roundTowardZero'      '"Z"' or '"z"'
Round away from zero                                    '"A"' or '"a"'

Table 16.5: 'gawk' rounding modes

   'ROUNDMODE' has the default value '"N"', which selects the IEEE 754
rounding mode 'roundTiesToEven'.  In *note Table 16.5:
table-gawk-rounding-modes, the value '"A"' selects rounding away from
zero.  This is only available if your version of the MPFR library
supports it; otherwise, setting 'ROUNDMODE' to '"A"' has no effect.

   The default mode 'roundTiesToEven' is the most preferred, but the
least intuitive.  This method does the obvious thing for most values, by
rounding them up or down to the nearest digit.  For example, rounding
1.132 to two digits yields 1.13, and rounding 1.157 yields 1.16.

   However, when it comes to rounding a value that is exactly halfway
between, things do not work the way you probably learned in school.  In
this case, the number is rounded to the nearest even digit.  So rounding
0.125 to two digits rounds down to 0.12, but rounding 0.6875 to three
digits rounds up to 0.688.  You probably have already encountered this
rounding mode when using 'printf' to format floating-point numbers.  For
example:

     BEGIN {
         x = -4.5
         for (i = 1; i < 10; i++) {
             x += 1.0
             printf("%4.1f => %2.0f\n", x, x)
         }
     }

produces the following output when run on the author's system:(1)

     -3.5 => -4
     -2.5 => -2
     -1.5 => -2
     -0.5 => 0
      0.5 => 0
      1.5 => 2
      2.5 => 2
      3.5 => 4
      4.5 => 4

   The theory behind 'roundTiesToEven' is that it more or less evenly
distributes upward and downward rounds of exact halves, which might
cause any accumulating round-off error to cancel itself out.  This is
the default rounding mode for IEEE 754 computing functions and
operators.

                     Rounding Modes and Conversion

   It's important to understand that, along with 'CONVFMT' and 'OFMT',
the rounding mode affects how numbers are converted to strings.  For
example, consider the following program:

     BEGIN {
         pi = 3.1416
         OFMT = "%.f"        # Print value as integer
         print pi            # ROUNDMODE = "N" by default.
         ROUNDMODE = "U"     # Now change ROUNDMODE
         print pi
     }

Running this program produces this output:

     $ gawk -M -f roundmode.awk
     -| 3
     -| 4

   The other rounding modes are rarely used.  Rounding toward positive
infinity ('roundTowardPositive') and toward negative infinity
('roundTowardNegative') are often used to implement interval arithmetic,
where you adjust the rounding mode to calculate upper and lower bounds
for the range of output.  The 'roundTowardZero' mode can be used for
converting floating-point numbers to integers.  When rounding away from
zero, the nearest number with magnitude greater than or equal to the
value is selected.

   Some numerical analysts will tell you that your choice of rounding
style has tremendous impact on the final outcome, and advise you to wait
until final output for any rounding.  Instead, you can often avoid
round-off error problems by setting the precision initially to some
value sufficiently larger than the final desired precision, so that the
accumulation of round-off error does not influence the outcome.  If you
suspect that results from your computation are sensitive to accumulation
of round-off error, look for a significant difference in output when you
change the rounding mode to be sure.

   ---------- Footnotes ----------

   (1) It is possible for the output to be completely different if the C
library in your system does not use the IEEE 754 even-rounding rule to
round halfway cases for 'printf'.