Hi, as a complement to Torbjörn answer:
1) the decimal string 0.693...875 cannot be represented exactly within 3000 bits, thus the mpf_set_str() necessarily performs some rounding 2) the mpf_get_str() also has to perform some rounding, since you convert a 3000-bit number into a 99-digit decimal string 3) mpf functions do not specify any rounding, see the GMP reference manual: Note that the 'mpf' functions are _not_ intended as a smooth extension to IEEE P754 arithmetic. In particular results obtained on one computer often differ from the results on a computer with a different word size. New projects should consider using the GMP extension library MPFR (<https://www.mpfr.org/>) instead. MPFR provides well-defined precision and accurate rounding, and thereby naturally extends IEEE P754. 4) with MPFR, if you tell mpfr_set_str to round up and mpfr_get_str to round to nearest, you get: $ cat f.c #include <stdio.h> #include <mpfr.h> int main() { mpfr_t x; mpfr_init2(x,3000); mpfr_set_str(x,"0.6931471805599453094172321214581765680755001343602552541206800094933936219696947156058633269964186875",10,MPFR_RNDU); char str[300]; mpfr_exp_t myexp; mpfr_get_str(str, &myexp, 10, 99, x, MPFR_RNDN); mpfr_clear(x); printf("%s\n",str); } $ gcc f.c -lmpfr $ ./a.out 693147180559945309417232121458176568075500134360255254120680009493393621969694715605863326996418688 Best regards, Paul Zimmermann PS: followups about MPFR should go on the MPFR list > From: 赵世忠 <szz...@sei.ecnu.edu.cn> > Date: Sun, 30 Jul 2023 08:41:43 +0800 (GMT+08:00) > > mpf_t x; > mpf_init2(x,3000); > mpf_set_str(x,"0.6931471805599453094172321214581765680755001343602552541206800094933936219696947156058633269964186875",10); > char str[300]; > mp_exp_t myexp; > mpf_get_str(str, &myexp, 10, 99, x); > mpf_clear(x); > > > printf("%s\n",str); > > > The last digit should be '8', but mpf_get_str outputs '7'. > > > > > > > _______________________________________________ > gmp-bugs mailing list > gmp-bugs@gmplib.org > https://gmplib.org/mailman/listinfo/gmp-bugs > _______________________________________________ gmp-bugs mailing list gmp-bugs@gmplib.org https://gmplib.org/mailman/listinfo/gmp-bugs
