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Prime numbers... again.

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Prime numbers... again., Yea. I suck.

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Adot2the	25 Mar, 2008 - 11:45 AM Post #1
New D.I.C Head Joined: 7 Feb, 2008 Posts: 37	I have tried searching for previous posts about finding prime numbers, but I can't really seem to find any that can help me figure out how to impliment them into my code. So I thought I would post what I need help with, and what I have so far. Here it goes. This is only one step to my project, I hope to figure this out so I can do the other half on my own. Write a program so that the user will enter two integers say x and y. As long as x>y, the program will print all primes between x and y inclusive. Also, write a function named findPrime which accepts two integer parameters say a and b. This function will print all prime numbers from a to b by calling the function isPrime. Have your program call this function busing the user input x and y. My code: CODE // Project 4 IsPrime.cpp : Defines the entry point for the console application. // #include "stdafx.h" #include <iostream> #include <stdlib.h> using namespace std; int main() { int x; int y; printf ( "Please enter two positive integers-->"); scanf ( "%d" "%d" , &x, &y ); printf ( "\nPRIME: "); } int findPrime ( int a, int b) If 3 and 30 are entered for x and y, the program should produce the following output: Prime: 3 5 7 11 13 17 19 23 29 (Prime count =9) I'm not really sure where to start, not sure how to call isPrime to check to see if its prime or not, pretty sure I know how to write a code to figure out the prime number for 1 number, but not to list in between, just really confused, I know I need a for loop somewhere, and of course a while loop, I could really use help on how the program should look, (an algorithm would be nice). Well, any tips, pointers, or anything else would be greatly appreciated. Adot2the~
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steelersfan	25 Mar, 2008 - 12:13 PM Post #2
New D.I.C Head Joined: 5 Mar, 2008 Posts: 24	try out my previous posts and responces to them. i have a very similar program that calculated up to the first 100,000 primes and then printed out a section of primes such as your x-y..let me know if it helps..if not i can try to give you some clips of my program to help you <http://www.dreamincode.net/forums/index.php?showtopic=45246&hl=prime>
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Adot2the	25 Mar, 2008 - 12:23 PM Post #3
New D.I.C Head Joined: 7 Feb, 2008 Posts: 37	still lost, not sure how to write my functions, or how my program should look thanks anyway.
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steelersfan	25 Mar, 2008 - 01:25 PM Post #4
New D.I.C Head Joined: 5 Mar, 2008 Posts: 24	haha well then what functions do you need to have and such? algorithm for your program would be kind of like: ask user for x and y inputs calculate prime numbers between x and y using a for loop print prime numbers calculated i know it seems vague but that could maybe help..is there a specific number the user is suppose to not go over or anything??
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Adot2the	25 Mar, 2008 - 04:16 PM Post #5
New D.I.C Head Joined: 7 Feb, 2008 Posts: 37	QUOTE(steelersfan @ 25 Mar, 2008 - 02:25 PM) haha well then what functions do you need to have and such? algorithm for your program would be kind of like: ask user for x and y inputs calculate prime numbers between x and y using a for loop print prime numbers calculated i know it seems vague but that could maybe help..is there a specific number the user is suppose to not go over or anything?? well if the user enters 3 and 30, it should calculate all of the prime numbers between the two, and not go over 30 if thats what you mean. But no, it should be able to calculate the primes between any range of numbers, the only problem that I have is writing functions to do so, I don't know what an IsPrime function should look like, and a FindPrime function should look like. I don't know how to call the functions to print the numbers out.
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Adot2the	26 Mar, 2008 - 02:15 AM Post #6
New D.I.C Head Joined: 7 Feb, 2008 Posts: 37	can anyone show me how it would look?
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Sepanto

26 Mar, 2008 - 02:58 AM

Post #7

D.I.C Head

Joined: 20 Mar, 2008
Posts: 97

Linux

Dream Kudos: 50
My Contributions

Let's say we have a function int prime(int a, int

cpp


int prime(int x,int y)
{
int i,j;
bool x=true;
if (y>x){
for (i=a;i<b;i++)
{   for (j=2;j<i;j++)
         if (i%j=0)
            x=false;
     if (x==true)
         return x;
}
return -1;
}
else
return -1;
}
//this means if no prime is found return -1.
return -1;
}

now my idea for the main was something like this:

cpp


int main()
{
int x,y,i,prime;
scanf("%d%d",&x,&y);
prime=x;
do {
prime=prime(prime,y);
printf("%d",prime);
}while(prime!=-1);
return 0;
}

I hope i didn't complicate it to something unworking, but the algorithem seems right.

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Adot2the	27 Mar, 2008 - 12:57 AM Post #8
New D.I.C Head Joined: 7 Feb, 2008 Posts: 37	hmm not sure why im getting errors. CODE // Project 4 IsPrime.cpp : Defines the entry point for the console application. // #include "stdafx.h" #include <iostream> #include <stdlib.h> #include <conio.h> using namespace std; int isprime( int a, int b) { int i; int p; for (i=2;a<b;i++) if (a%i!=0) { p++; return 0; else return i } } int main() { int x; int y; printf ( "Please enter two positive integers-->"); scanf ( "%d" "%d" , &x, &y ); printf ( "\nPRIME: "); isprime(i); return 0; } Just need the program to call isprime to keep printing the value of i. The variable i should be the prime numbers between x and y. confused lol.
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Adot2the	28 Mar, 2008 - 12:09 AM Post #9
New D.I.C Head Joined: 7 Feb, 2008 Posts: 37	bump, could use help.. lol.
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steelersfan	28 Mar, 2008 - 03:36 AM Post #10
New D.I.C Head Joined: 5 Mar, 2008 Posts: 24	what is the error message you are getting back??
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steelersfan	28 Mar, 2008 - 03:41 AM Post #11
New D.I.C Head Joined: 5 Mar, 2008 Posts: 24	should you maybe also call the isprime function before you call your print function? maybe im wrong though and also your calculations aren't going to be correct you need another nested for loop...like Sepanto showed you above in his first bit of code he posted
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Amadeus	28 Mar, 2008 - 06:24 AM Post #12
g++ -o drink whiskey.cpp Joined: 12 Jul, 2002 Posts: 12,170 Thanked 33 times Dream Kudos: 25 My Contributions	You are supplying a variable named i to the function isprime(). you have not declared a variable named i, nor have you instantiated it.
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