6 replies to this topic

[NicholasIT]

hmm,im doing a question that give a number and showing the prime numb
eg:give 23
it will show 2,3,5,7,11,13,17,19

#include<iostream>

using namespace std;


int main()

{

	int prime=2;


	cout<<"Insert a number";

	cin>>prime;


	for(;prime>=2;prime--)

	{

		if( prime !=prime && prime % prime ==0)

			cout<<prime<<endl;

	}

	

	system("pause");

}

hmm,anyone can give some hints to do it ?

Edited by NicholasIT, 25 June 2010 - 09:46 PM.
addtion info

[opwuaioc]

Well, I think you're showing quick code, right? As in, this isn't what you actually have? If so, then you don't want to define the number that you have your users entering (so remove = 2; in ln. 6). So what you're doing in your for statement as you have it (minus the first part) is using the % operand on the same number (int prime = 2; defines the variable prime, and then you're having them enter the same named variable). You need to differentiate.

For statements look more like this for(int i = 2; i < prime; i++). The if-statement can then divide prime by i and see if the remainder is zero.

As for a quick hint, for prime numbers, "i < prime" is double the effort. There's a way to cut that in half, but I'll let you see if you figure it out. I hope this puts you on the right track, but if you're still having trouble, I'll be back on to see if you get it. :)

[NicholasIT]

your users entering (so remove = 2; in ln. 6) <<<<
hmm,sorry not really understand about that statement ?@@

[opwuaioc]

Sure:

int prime=2;

cout<<"Insert a number";
cin>>prime;

The line "int prime = 2;" is redundant because the user enters a number into a variable with the exact same name as the one you already defined.

Edit: Sorry if I may have misworded that, as I reread what I wrote, I meant that in that line, the "= 2" is redundant.

[NicholasIT]

hmm,i try to modified but i think im not understand it.

#include<iostream>
using namespace std;

int main()
{
	
	int prime;

	cout<<"Insert a number";
	cin>>prime;

	for(int i=2;i<prime;prime--)
	{
		if(prime % i ==1)
			cout<<prime<<endl;
	}
	
	system("pause");
}

hmm,if using the true statement and false statement ?or u showing ur way?@@

[opwuaioc]

I don't know why you switiched from i++ to prime-- - you're testing prime, not i. You won't get the results you want. Also, do "prime % i == 0" for your if-statement, not "== 1".

[NicholasIT]

	int prime;

	cout<<"Insert a number";
	cin>>prime;

	for(int i=2;i<prime;i++)
	{
		if( prime % i== 1)
			cout<<prime<<endl;
		

	}
	
	

i managed to do something like this oni @@