gmp precision

[jorgen]

When using the gmp library for high precision calculations, is it right that if I use mpf_set_d(...) to assign a double value to an mp float, the precision of other mp floats used together with this one is truncated to double precision, potentially losing information?

As a test I use this program:

	mpf_t x, y, z;
	mpf_init2(x, 1000000);
	mpf_init2(y, 1000000);
	mpf_init2(z, 1000000);
	mpf_set_str(x, "1e0", 10);
	mpf_set_d(y, 3.3124365842565);
	mpf_set_d(z, 3.3124365842566);
	mpf_mul(y, x, y);
	mpf_mul(z, x, z);
	mpf_sub(x, y, z);
	cout << setprecision(100) << x << endl;

(Notice the difference in the last digits of y and z), and get the output -9.9920...e-14. If I add even more digits to y and z I get output 0. Thus it seems that although I initialized the variables to 1000000 bits the precision is still around 15 digits.

(Also: according to the tutorial at http://gmplib.org/manual/ it should not be necessary to use init2 like this but only init, however to get high enough precision in other small test programs I have to use init2)

[helios]

You should ask this in the GMP mailing list. You're more likely to get good replies.

[jorgen]

Will do, thanks!