Wrong results with "mod"

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Matthew Durkin
Matthew Durkin 2019 年 2 月 10 日
回答済み: John D'Errico 2019 年 2 月 10 日
Why does Matlab give me the wrong answer to mod(14^27,55) = 32, it should equal 9. What am I doing wrong?
  2 件のコメント
Matthew Durkin
Matthew Durkin 2019 年 2 月 10 日
Never mind, I found out I need to use "powermod" to get the answer.
the cyclist
the cyclist 2019 年 2 月 10 日
Maybe you should copy your comment to an answer, and accept it.

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回答 (1 件)

John D'Errico
John D'Errico 2019 年 2 月 10 日
This fails because MATLAB cannot represent an integer larger than 2^53-1 as an exact integer, when working in double precision.
14^27 is far larger than that of course. So it fails. This leaves you with two basic options, to use symbolic arithmetic, (or perhaps my VPI toolbox) or to use powermod.
mod(sym(14)^27,55)
ans =
9
powermod(14,27,55)
ans =
9
The powermod solution is better of course, in the sense that the huge number 14^27 never had to be fully computed.

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