Re: Modular exponentiations in bc

On Thu, 17 Apr 2008 22:28:26 +0000, Dances With Crows wrote:

K. Jennings staggered into the Black Sun and said:
I would be interested to compute modular exponentiations in bc. One
can't use a^b % c for arbitrarily large a, b and c, for bc seems to
carry out this computation in the straightforward way: First compute
a^b and then reduce the result mod c. There is no way to compute a^b in
a reasonable time (or store the result) when a and b are large enough.

Looking into the bc 1.06 sources, a function called bc_raisemod() is
defined that seems to do what I need - however, I have no clue as to
how to invoke it from the bc command line. Ideas, anyone?

I looked at the source and I *think* the operator you want is | .
According to eval.c, this operator calls dc_modexp(), which calls
bc_raisemod() . I don't know for sure whether it will work according to
what you want. (All the math I need can be handled by normal operators,
which is nice.)

Thanks for your feedback. Alas, the | operator is not mentioned
in bc's doc, and all my attempts to get bc to recognize it have so far
failed.

However, it can be used all right from dc.

.

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