(a^b) mod c
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hi all, i need your help. i want to use (a^b) mod c
but in matlab if i use mod((a^b),c);
it's return inf
because a,b,c are too big.
are you know how to do that with another way, in matlab?? for example use a loop ??
1 个评论
Oleg Komarov
2011-6-29
- What do you mean by too big?
- Are they scalars?
回答（2 个）
Jan
2011-6-29
If b is not prime, but x * y:
(a^b) mod c = (a^x mod c)^y mod c
But if b is prime, this does not help to avoid the overflow. Then I'd try FEX:John's VPI or FEX:Ben Barrowes' Multiple Precision Toolbox.
EDITED: A loop method:
function r = ApowerBmodC(a, b, c)
r = 1;
for i = 1:b
r = mod(a * r, c);
end
Then "ApowerBmodC(1234, 5678, 1256)" replies 1112. For these numbers the "(a^x mod c)^y mod c" does not help, because 5678 = 2*17*167 and "mod(1234^2, 1256)^17" is 4.38e45 such that trailing figures are all zero due to rounding.
3 个评论
zikrullah muhammad
2011-6-29
Jan
2011-6-29
Have you looked at the two posted links?
Sean de Wolski
2011-6-29
VPI is back !!
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2011-6-29
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