c program

[sekitoleko]

how can i write down a simple c expression that evaluates to the largest multiple of a number n,not greater than a given number m.

[Soujiro]

how can i write down a simple c expression that evaluates to the largest multiple of a number n,not greater than a given number m.

Now that's it.. You need to study first.. Banfa's Tutorial

[DeMan]

Simplest (though probably not most efficient) way would be to start with m and test whether n mod m = 0 if it does we have a winner!! If it doesn't then decrement m and start again.

This is no easy feat, and in fact the strength of public key Cryptography, relies on (roughly) this problem being difficult for large m. If you had a nice long list of primes (where the largest prime is the largest prime below m) [p1, p2, p3, p4, p5, p6, ....], however, you could try a slightly different approach:
(Not in any particular languag but the flow of logic is the important thing)

currentDivisor = 1;
currentPrime = primeList.start();
while(currentDivisor < m)
{
  if(n % currentPrime ==0)
  {
    currentDivisor = currentDivisor * currentPrime;
  }
  else
  {
    currentPrime = primeList.next();
  }
}

NOTICE: the current prime is not changed if the current prime divides and there is good reason for this. If the number is divisible by, say, 9 then we need to test divisibility by 3 more than once.

[Banfa]

Simplest (though probably not most efficient) way would be to start with m and test whether n mod m = 0 if it does we have a winner!! If it doesn't then decrement m and start again.

We are looking for multiples of n so I think you mean

if m mod n == 0 your on to a winner

However if m mod n != 0 then

(m - m mod n) mod n == 0 so (m - m mod n) is your answer.

Actually to avoid an if statement (m - m mod n) is always your answer since if m mod n == 0 then (m - m mod n) == (m - 0) == m.

[DeMan]

Sorry you're right - I shouldn't read without thinking,

You are absolutely right!!

[cworld]

how can i write down a simple c expression that evaluates to the largest multiple of a number n,not greater than a given number m.

yes i think first You need to study some basic Tutorial