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Carl_Mesaros
Joined: 28 Nov 2016 Posts: 8

Posted: Thu May 04, 2017 7:06 pm Post subject: concerning the MOD intrinsic function 


Dear Silverfrost community:
I do not know how to use the MOD intrinsic function. Could you
suggest ways that I can learn how to use this function? Thank you.
Best regards,
Carl Mesaros 

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JohnCampbell
Joined: 16 Feb 2006 Posts: 1846 Location: Sydney

Posted: Mon May 08, 2017 7:00 am Post subject: 


Try something like the following. This may show some of how it works.
Code:  integer i,j
do i = 10,10
j = mod(i,4)
write (*,*) i,j, mod(i,4)
end do
end 
Note:
mod(I+p,p) = mod(I,p)
mod (I,p) =  mod(I,p)
mod(I,p) = mod(I,p) { which can be annoying when 0 < I < p, as mod(I+p,p) = mod(I,p) but mod(Ip,p) /= mod(I,p) } 

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mecej4
Joined: 31 Oct 2006 Posts: 749

Posted: Mon May 08, 2017 12:35 pm Post subject: Re: 


JohnCampbell wrote: 
mod(I+p,p) = mod(I,p)

Not always!
Counterexample: mod(2, 3) = 2, but mod(2+3, 3) = 1
It is quite easy to make such slips if one or both the arguments to the mod() function, or its sister function modulo(), are negative. Modular arithmetic can be counterintuitive if one is not used to it. For example, (a + b) mod c need not equal (a mod c) + (b mod c); consider a=1, b=1, c=2.
The rule is: Given two numbers (real or integer) A and P, of the same type and kind, find the integer K = INT(A/P) in the case of mod() and K = FLOOR(A/P) in the case of modulo(). The value of the function is A  K*P, and '/' has its mathematical meaning, not "integer division".
The functions can be used in a number of situations where one has to find the nearest from a set of discrete values. I use the mod() function frequently in an iterative algorithm to monitor the progress: IF (mod(ITER, PRNTINTVL) == 0) WRITE(*,*)... 

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JohnCampbell
Joined: 16 Feb 2006 Posts: 1846 Location: Sydney

Posted: Tue May 09, 2017 12:33 am Post subject: 


For reporting progress I try to use an alternative form to "IF (mod(ITER, PRNTINTVL) == 0) WRITE(*,*)..."
IF ( ITER >= Next_PRNT_ITER ) THEN
WRITE(*,*)...
Next_PRNT_ITER = Next_PRNT_ITER + PRNTINTVL
END IF
This can create greater flexibility by varying PRNTINTVL or
starting with a different Next_PRNT_ITER. The following all give a different initial report, which can be useful for confirming the initial cycling:
Next_PRNT_ITER = PRNTINTVL
Next_PRNT_ITER = 0
Next_PRNT_ITER = PRNTINTVL 

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