Not declared in this scope

**vart** · 03-09-2009

Try not to use 'return' in loops or functions, but affect a local variable which you return always in the end (although here it doesn't really matter).

Of course it matters - now you return false even if the number is prime
you do not check 2 to be a divider
you check all numbers even if the 3 is a divider

so better stop fixing functions in this way - you enter too many bugs

**laserlight** · 03-09-2009

Originally Posted by Taka

I'm sorry I'm trying to understand what you are saying but I am only learning C++ and I'm not really getting what you are trying to tell me.

Okay, which part would you like me to explain further?

Originally Posted by Taka

I've been told I need to leave the True/False statements in there thats why I didn't take them out.

If those are the requirements of your instructors, then so be it, but be aware that your instructors are teaching you to do something that is unnecessary.

By the way, this statement is not quite correct, since 2 is the even prime number: "Every even number is not prime". Not only that, but it looks like you wrote isPrime() as if it were isComposite().

Originally Posted by MarkZWEERS

Try not to use 'return' in loops or functions, but affect a local variable which you return always in the end (although here it doesn't really matter).

I do not subscribe to the notion of strict "single entry, single exit", and as vart pointed out, you did not practice what you preached since you returned false instead of isprime at the end. Furthermore, without a break, you potentially loop far more than necessary than with a version that uses an immediate return.

**MarkZWEERS** · 03-09-2009

so better stop fixing functions in this way - you enter too many bugs

sorry, I was too rapid in that..

**Taka** · 03-09-2009

I'm not understanding the

Code:

 int& number

part.

Wouldn't I need to put it as

Code:

 int &number

and then wouldn't I need a

Code:

 *number

somewhere in the code?

I played around with the bool in the code. I'll take it out when I submit but it does make it cleaner and clearer.

Code:

#include <iostream>
#include <math.h>

using namespace std;

bool isPrime(int);
void getNumber(int& number);

int main()
{
   int number;

   getNumber(number);

   if (isPrime(number)) 
      cout << "\n" << number << "is a prime number\n";
   else 
      cout << "\n" << number << "is not a prime number\n";

   return 0;
}

void getNumber(int& number)
{
   cout << "Please enter a positive number ";
   cin >> number;
   if (!cin.good())
   {
      printf("Invalid number entered\n");
      exit(1);
   }
}

bool isPrime(int number)
{
   int count, s;

   /* Every even number is not prime */
   if (number % 2 == 0) return true;

   /* check every odd number up to the square root of the number */
   s = sqrt(number);
   for (count=3; count<=s; count+=2);
   {
      if (number % count == 0) return true;
   }
   return false;
}

**laserlight** · 03-09-2009

Originally Posted by Taka

I'm not understanding the

Code:

 int& number

part.

Wouldn't I need to put it as

Code:

 int &number

and then wouldn't I need a

Code:

 *number

somewhere in the code?

Have you learnt about pass by reference yet? The spacing has no meaning in this case, because either way it says that the parameter named number is a reference to an int, so whatever int you pass as an argument to the function can be modified from within the function. This use of & is different from its use as the address of operator.

**Taka** · 03-09-2009

We haven't gone into it alot yet which is probably why I'm struggling to understand it.

It compiles without the & but when I try to compile it with it, it comes back with:

prime.cpp: In function 'bool isPrime(int)':
prime.cpp:42: warning: converting to 'int' from 'double'

Undefined first referenced
symbol in file
getNumber(int&) /var/tmp//ccy5ogNG.o
ld: fatal: Symbol referencing errors. No output written to a.out
collect2: ld returned 1 exit status

**laserlight** · 03-09-2009

The compiler is complaining because sqrt() returns a double, but you put its return value into an int without explicitly casting.

**Taka** · 03-09-2009

Yeah I'm working on fixing that, but then the main error is that it doesn't create the output because of the

Code:

 getNumber(int&)

so have I written this incorrectly?

**laserlight** · 03-09-2009

What is your current code?

**Taka** · 03-09-2009

Code:

#include <iostream>
#include <math.h>

using namespace std;

bool isPrime(int);
void getNumber(int& number);

int main()
{
   int number;

   getNumber(number);

   if (isPrime(number)) 
      cout << "\n" << number << "is a prime number\n";
   else 
      cout << "\n" << number << "is not a prime number\n";

   return 0;
}

void getNumber(int& number)
{
   cout << "Please enter a positive number ";
   cin >> number;
   if (!cin.good())
   {
      printf("Invalid number entered\n");
      exit(1);
   }
}

bool isPrime(int number)
{
   int count, s;

   /* Every even number is not prime */
   if (number % 2 == 0) return true;

   /* check every odd number up to the square root of the number */
   s = sqrt(number);
   for (count=3; count<=s; count+=2);
   {
      if (number % count == 0) return true;
   }
   return false;
}

**matsp** · 03-09-2009

Compiling your latest code with gcc-mingw-3.4.5, it gives no errors, just the warning about converting double to int without cast.

The program runs, but it does not recognise valid primes, such as 31 or 17 as primes, so you probably have a bug in your "is this a prime" function.

--
Mats

**Taka** · 03-09-2009

OK I will check my compiler. I am using "g++ -lm"

I'm not very good at math so any hints on where my logic is wrong in isPrime?

**Litz** · 03-09-2009

Your "isPrime" function is wrong. 2 is a prime number (and the only even one if I recall correctly), yet you have a line of code where you do something like: If number%2 == 0, return true. So you're basically saying every even number will return true.

Didn't check the rest since that already seems wrong.

**Taka** · 03-09-2009

Originally Posted by Litz

Your "isPrime" function is wrong. 2 is a prime number (and the only even one if I recall correctly), yet you have a line of code where you do something like: If number%2 == 0, return true. So you're basically saying every even number will return true.

Didn't check the rest since that already seems wrong.

It works for 2:
./a.out
Please enter a positive number 2

2is a prime number