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-   -   Finding LCM & GCD in C (http://www.go4expert.com/articles/finding-lcm-gcd-c-t457/)

 oleber 23Jul2007 18:33

Re: Finding LCM & GCD in C

Lets see the performance.

:confused: Which is the fastest algorithm?

:) GCM seems to be at most n
:( LCM seems to be at most n*m

:p so, if you do lcm(a,b) := (a*b)/gcm(a,b), you get a n complexity

:eek: This is mathematics.

 oleber 24Jul2007 13:50

Re: Finding LCM & GCD in C

Swamp is the subject of another post:

 shabbir 24Jul2007 14:17

Re: Finding LCM & GCD in C

Quote:
 Originally Posted by oleber Swamp is the subject of another post: http://www.go4expert.com/showthread.php?t=1732
Could not get what you meant here.

 oleber 24Jul2007 14:26

Re: Finding LCM & GCD in C

clocking and shabbir are speaking of swaping variables.

 clocking 24Jul2007 19:11

Re: Finding LCM & GCD in C

:) hi !
thanks for your idea. But I only want someone tell me about that algorithm.
Swapping is not difficult, but I'm interested in the easier way solving it.
Code:

```tmp = ts[i]; ts[i] = ts[j]; ts[j] = tmp;```
can't we use tmp ?

 Balaselvi 27Feb2008 10:13

Re: Finding LCM & GCD in C

Hi,

This is Bala.
I want to know how to calculate GCD of many numbers, ex 2048 values.

Re: Finding LCM & GCD in C

Code:

```int main() {     int n1,n2,n3,n;     cout<<" Numbers are : ";     cin>>n1>>n2>>n3;     n=FindGcd(FindGcd(n1,n2),n3);     cout<<"GCD of n1,n2,and n3 is "<<n<<endl;     return 0; } int FindGcd(int num1, int num2) {     int temp;        while(num2!=0)     {         temp = num2;         num2 = num1%num2;         num1 = temp;     }        return num1; }```

 aisha.ansari84 5Mar2008 18:22

Re: Finding LCM & GCD in C

can you please tell me when the loop will actually stop

 zeeme 22Feb2009 20:00

Re: Finding LCM & GCD in C

correct code is this 100% checked!

for(n=1;n%a != 0 || n%b != 0;n++);
return n

 vanishing_stapler 15May2009 01:55

Re: Finding LCM & GCD in C

When I searched for lcm c++, this thread had the top two spots, so I figured that I'd join and put this here.

For finding the LCM (Least Common Multiple) of two integers, a and b, in C++, use:

for(n=a;n%b != 0;n+=a);
return n;

For maximum speed, try to set it up so that the larger number is "a" and the smaller one is "b".

The reasoning:
The least common multiple (LCM) of two numbers, a and b, is the smallest number that is a multiple of "a" AND a multiple of "b". If we already know that the LCM is a multiple of "a", then why not count by "a"? When we count by "a", starting at "a", we are testing every number that is a multiple of "a" already, and only have to see if it is a multiple of "b" too. This code does just that. When "a", the number we're counting by, is larger than 1, the process will be much faster than with those other suggestions, because it's taking bigger steps (at the same speed per step) to get to the same solution.

If "a" is a billion, then this will be at least a billion times faster than this:

for(n=1;n%a != 0 || n%b != 0;n++);
return n;

(even though both work)

I hope this helps.

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