# Factorizing in C++

Can anyone help me out in improving the following program to find factors of number $$n$$ as it doesn't work for higher numbers.

 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 #include #include void main() { clrscr(); cin>>n; for (int a=1;a

Note by Pranjal Jain
3 years, 9 months ago

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You can firstly improve by running the loop till $$i \leq \frac{n}{2}$$. Or you can try this:

 1 2 3 4 5 6 7 8 9 int n; cin>>n; for(int l=1, int h=n ; l<=h ; l++,h=n/l) { if(n%l==0) { cout<

- 3 years, 9 months ago

Yes, I noticed $$\frac{n}{2}$$ thing. Can you explain algorithm you used? Thanks.

- 3 years, 9 months ago

So basically we can pair up the divisors for any given $$n$$. So, l will have the smaller one and h will store the larger one ( such that $$l \times h =n$$ ) and we are incrementing l by one each time and correspondingly decreasing h. So h will decrease much faster and we would be skipping lot of numbers which anyway we need not check.

PS: I forgot to include the line to check whether $$l= h$$ that is if n is a perfect square.

- 3 years, 9 months ago

It showed many errors. Which compiler are you using? And I believe the condition should be if (n%h==0)

- 3 years, 9 months ago

We can bring down $$\frac{n}{2}$$ to $$\lfloor\sqrt{n}\rfloor$$. I still can't understand how will it help while dealing with higher numbers. Let me compile.

- 3 years, 9 months ago

How many times should i try to help before someone notices Pranjal Jain . I have included a few ideas in the following from Sudeep Salgia as well:

 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 #include #include #include int main() { double m,i,k=10; cin>>m; for(i=1;i

Here are the factors of $$10000000099$$

And here are the factors of $$7296872389761$$

- 3 years, 9 months ago

Thanks. Sorry for not noticing. I am quite inactive now a days on B'ant. (Turned off all email notifications as well)

- 3 years, 9 months ago

- 3 years, 9 months ago

Try long double it has the max and min values $$10^{4932}\quad and \quad 10^{-4932}$$ respectively and has a memory of 10 bytes. Its the biggest as far as I know.

- 3 years, 9 months ago

Modulus doesn't work with floating data types.

- 3 years, 9 months ago

Maybe you could try using long variable or long long . But that won't be really necessary if you are writing this for boards

- 3 years, 9 months ago

I program for fun. -_- I hate boards

- 3 years, 9 months ago

Oh ! So have you studied any algorithm books like CLRS ? Or are you active on websites like topcoder , spoj ?

- 3 years, 9 months ago

Currently I am inactive due to JEE. But I will be active after 2-3 months on Project Euler.

- 3 years, 9 months ago

Try Java Codes,They are always better than C++

- 3 years, 9 months ago

For that, I'll have to learn java properly.

- 3 years, 9 months ago

Try this out

 1 2 3 4 5 6 7 8 double n,i; cin>>n; for(i=1;i<=(n/2);i++) { double k=floor(n/i); if (i*k==n) cout<

- 3 years, 9 months ago

Why are you taking

 1 2 Double k=floor(n/i); If(i*k==n)

to check if $$i$$ is a factor of $$n$$ instead of

 1 If(n%i == 0)

- 3 years, 9 months ago

Big numbers cannot be stored in long. Therefore we need something which is even bigger, like float or double. The problem with using "%" function with float/double is that it will not work. "%" is meant to work only with integers(int or long or short), It will not work with float and double as they are decimal values. Does 100%2.3 make sense?

Hence we use a workaround to find the true factors of n by using floor.

- 3 years, 9 months ago

Ouch. I didn't notice that you used double. But then why not use long long? long long has a higher range than a double for the same memory, since a double can only hold 15 significant digits whereas long long can hold upto 18.

Note I don't use C++ so I have no idea if long long is used commonly.

- 3 years, 9 months ago

Nope, long long doesn't work in c++. That's the biggest problem here.

- 3 years, 9 months ago