MATLAB: Mod gives incorrect result

mod function

Hi Mr/Miss,

I am Eda Nur Dagasan. I am a master's student. I am writing from Turkey. I use mod(x,y) function for my Matlab project. But I have a problem with mod(x,y) function. When I use mod(146^23,187) process, a result of this process is wrong. When I do this (mod(146^23,187) ) process with a calculator, the result is 5, but ı use mod(x,y) function the result is 141 and this result is wrong. I attached screenshots about this problem. Could you help me?

Best Answer

See this thread:

https://www.mathworks.com/matlabcentral/answers/338182-how-to-get-mod-of-large-numbers

Related Solutions

MATLAB: How to get mod of large numbers

If you compute (5652^42)*(23^77) to use the naive approach for computing this quantity, the resulting number is much greater than flintmax. If we look at the spacing between that number and the next largest number, we see that the spacing is (much) greater than 5929.

>> x = (5652^42)*(23^77)
x =
     2.78954779454946e+262
>> eps(x)
ans =
     3.49595995098571e+246
>> (x + 5929) == x
ans =
  logical
   1

As Sean stated in this answer from a while ago, you could use Symbolic Math Toolbox. Or you could use one of the algorithms described in this Wikipedia page to perform these computations with numbers no greater than 5652^2.

MATLAB: How to differentiate between powermod(a,b,c) and mod(a^b,c)

104^45 exceeds 2^53, which is the largest number for which double precision is able to distinguish between adjacent integers. 2^53+1 and 2^53+2 are stored the same way in double precision.

So taking mod() of such a large number is going to be inaccurate, because double precision is not storing the actual number, just the closest approximation it can get.

powermod computes in a way that avoids the overflow problem.

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Related Solutions

MATLAB: How to get mod of large numbers

MATLAB: How to differentiate between powermod(a,b,c) and mod(a^b,c)

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