how to calculate modulus in matlab ?

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ahmed
ahmed 2013-5-15
编辑： James Tursa 2023-5-31
Hi everyone,
the question seems simple as using mod works for small numbers not for large numbers.
i want to calculate mod( (4^15)*(21^13),47) the matlab ans= 21 but the correct ans = 3 using the windows calculator. is there any way in matlab to calculate such modulus ?
Thanks in advance.

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James Tursa
James Tursa 2013-5-17
See this newsgroup thread where Bruno Luong gives advice on how to do this calculation without using the symbolic toolbox:
http://www.mathworks.com/matlabcentral/newsreader/view_thread/329023

更多回答（3 个）

Roger Stafford
Roger Stafford 2013-5-15
The number you describe is far too large for accurate numerical computation using only 'double' floating points numbers. However, the 'mod' function also works with symbolic numbers using the symbolic toolbox. You can compute with these to any accuracy you wish.
Walter Roberson
Walter Roberson 2013-5-15
  1 个评论
Roger Stafford
Roger Stafford 2013-5-16
The number 21^13 as computed with 'double' is too large to be computed exactly. Its least five bits must necessarily be rounded to zeros to fit within 53 bits and consequently it gives:
mod(21^13,47) = 2,
whereas the true modulo 47 value is:
mod(mod(21^7,47)*mod(21^6,47),47) = 7.

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ahmed
ahmed 2013-5-17
Thanks for your help any way , but even the powermod function or any of the above functions in url links can calculate the correct answer which is 3 .
trying:
powermod( (4^15) * (21^13) , 1 , 47 ) gives 21 but the correct ans using windows calculator is 3 and it is equivelant to the answer in my cryptography study book .

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