File: gawk.info,  Node: Arbitrary Precision Integers,  Next: Checking for MPFR,  Prev: FP Math Caution,  Up: Arbitrary Precision Arithmetic

16.5 Arbitrary-Precision Integer Arithmetic with 'gawk'
=======================================================

When given the '-M' option, 'gawk' performs all integer arithmetic using
GMP arbitrary-precision integers.  Any number that looks like an integer
in a source or data file is stored as an arbitrary-precision integer.
The size of the integer is limited only by the available memory.  For
example, the following computes 5^4^3^2, the result of which is beyond
the limits of ordinary hardware double-precision floating-point values:

     $ gawk -M 'BEGIN {
     >   x = 5^4^3^2
     >   print "number of digits =", length(x)
     >   print substr(x, 1, 20), "...", substr(x, length(x) - 19, 20)
     > }'
     -| number of digits = 183231
     -| 62060698786608744707 ... 92256259918212890625

   If instead you were to compute the same value using
arbitrary-precision floating-point values, the precision needed for
correct output (using the formula 'prec = 3.322 * dps') would be 3.322 x
183231, or 608693.

   The result from an arithmetic operation with an integer and a
floating-point value is a floating-point value with a precision equal to
the working precision.  The following program calculates the eighth term
in Sylvester's sequence(1) using a recurrence:

     $ gawk -M 'BEGIN {
     >   s = 2.0
     >   for (i = 1; i <= 7; i++)
     >       s = s * (s - 1) + 1
     >   print s
     > }'
     -| 113423713055421845118910464

   The output differs from the actual number,
113,423,713,055,421,844,361,000,443, because the default precision of 53
bits is not enough to represent the floating-point results exactly.  You
can either increase the precision (100 bits is enough in this case), or
replace the floating-point constant '2.0' with an integer, to perform
all computations using integer arithmetic to get the correct output.

   Sometimes 'gawk' must implicitly convert an arbitrary-precision
integer into an arbitrary-precision floating-point value.  This is
primarily because the MPFR library does not always provide the relevant
interface to process arbitrary-precision integers or mixed-mode numbers
as needed by an operation or function.  In such a case, the precision is
set to the minimum value necessary for exact conversion, and the working
precision is not used for this purpose.  If this is not what you need or
want, you can employ a subterfuge and convert the integer to floating
point first, like this:

     gawk -M 'BEGIN { n = 13; print (n + 0.0) % 2.0 }'

   You can avoid this issue altogether by specifying the number as a
floating-point value to begin with:

     gawk -M 'BEGIN { n = 13.0; print n % 2.0 }'

   Note that for this particular example, it is likely best to just use
the following:

     gawk -M 'BEGIN { n = 13; print n % 2 }'

   When dividing two arbitrary precision integers with either '/' or
'%', the result is typically an arbitrary precision floating point value
(unless the denominator evenly divides into the numerator).

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

   (1) Weisstein, Eric W. 'Sylvester's Sequence'.  From MathWorld--A
Wolfram Web Resource
().