Starting With the GNU MPFR Library

The following program computes a lower bound on 1+1/1!+1/2!+...+1/100! using a 200-bit precision:

• mpfr_t s, t, u; declares three floating-point variables s, t, u;
• mpfr_init2 (t, 200); initializes the variable t with a 200-bit precision;
• mpfr_set_d (t, 1.0, MPFR_RNDD); sets the value of t to the double-precision number 1.0 rounded toward minus infinity (here no rounding is involved since 1 is represented exactly as a double-precision number and also as a 200-bit MPFR number);
• the statement mpfr_mul_ui (t, t, i, MPFR_RNDU); multiplies t in place by the unsigned integer i, where the result is rounded toward plus infinity;
• mpfr_div (u, u, t, MPFR_RNDD); divides u by t, rounding the result toward minus infinity, and stores it in u;
• the statement mpfr_out_str (stdout, 10, 0, s, MPFR_RNDD); prints the value of s in base 10, rounded toward minus infinity, where the third argument 0 means that the number of printed digits is automatically chosen from the precision of s (note: mpfr_printf could also be used instead of printf, mpfr_out_str and putchar);
• finally the mpfr_clear and mpfr_free_cache calls free the space used by the MPFR variables and caches.

Note: with this program, you need MPFR 3.0 or later.

#include <stdio.h>

#include <gmp.h>
#include <mpfr.h>

int main (void)
{
  unsigned int i;
  mpfr_t s, t, u;

  mpfr_init2 (t, 200);
  mpfr_set_d (t, 1.0, MPFR_RNDD);
  mpfr_init2 (s, 200);
  mpfr_set_d (s, 1.0, MPFR_RNDD);
  mpfr_init2 (u, 200);
  for (i = 1; i <= 100; i++)
    {
      mpfr_mul_ui (t, t, i, MPFR_RNDU);
      mpfr_set_d (u, 1.0, MPFR_RNDD);
      mpfr_div (u, u, t, MPFR_RNDD);
      mpfr_add (s, s, u, MPFR_RNDD);
    }
  printf ("Sum is ");
  mpfr_out_str (stdout, 10, 0, s, MPFR_RNDD);
  putchar ('\n');
  mpfr_clear (s);
  mpfr_clear (t);
  mpfr_clear (u);
  mpfr_free_cache ();
  return 0;
}

The result of this program is:

$ ./sample
Sum is 2.7182818284590452353602874713526624977572470936999595749669131e0

Note that the fix that adds the exponent e0 in the mpfr_out_str output is mentioned on the MPFR 4.0.2 page.

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