Jumping Into C++ Sample Code Help

[lamb]

Not really understanding the code from Chapter 7, the program lists all prime numbers from 1-100.

#include <iostream>


bool isDivisible (int number, int divisor);
bool isPrime (int number);


using namespace std;


int main ()
{
    for ( int i = 0; i < 100; i++ )
    {
        if ( isPrime( i ) )
        {
            cout << i << endl;
        }
    } 
}


bool isPrime (int number)
{
    for ( int i = 2; i < number; i++)
    {
        if ( isDivisible( number, i  ) )
        {
            return false;
        }
    }
    return true;
}


bool isDivisible (int number, int divisor)
{
    return number % divisor == 0;
}

I don't understand how it works, if someone could briefly describe what each function does, that would help considerably.

Thanks in advance,
-lamb

[stahta01]

Read this to understand the isDivisible function.
Modulus Operator in C and C++ - Programming Tutorials - Cprogramming.com

Tim S.

[Elysia]

I don't understand how it works, if someone could briefly describe what each function does, that would help considerably.

I dunno what to say. These functions are as easy as they come. What's the problem?

[Salem]

Add some print statements all over the place, eg

int main ()
{
    for ( int i = 0; i < 100; i++ )
    {
        cout << "for loop i = " << i << endl;
        if ( isPrime( i ) )
        {
            cout << i << endl;
        }
    }
}

Run the code, watch your print statements.

Or run the code in a debugger (consider this an excellent opportunity to learn about this essential tool). Use the debugger to single step the code so you can follow the program flow line by line. Examine the values of variables as you go.

[DRK]

The algorithm is wrong as it prints out numbers 0 and 1 which are not prime.

[lamb]

I dunno what to say. These functions are as easy as they come. What's the problem?

#include <iostream>


bool isDivisible (int number, int divisor);
bool isPrime (int number);


using namespace std;


int main ()
{
	for ( int i = 0; i < 100; i++ ) //Cycles through all values from 1-100.
	{
		if ( isPrime( i ) ) //If it's prime...
		{
			cout << i << endl;//Print it!
		}
	}
}


bool isPrime (int number)
{
	for ( int i = 2; i < number; i++)
	{
		if ( isDivisible( number, i  ) ) //If the two numbers are divisible...
		{
			return false; //I don't care about them!
		}
	}
	return true; //If they're not, tell me!
}


bool isDivisible (int number, int divisor)
{
	return number % divisor == 0; //Only give values that have no remainder(are divisible).
}

So I added comments to show you all my interpretation of the code, what I don't understand is, by the looks of it, the variable 'i'(in main) only gets checked for whether or not it's prime while it is greater than the variable 'i'(in isPrime) and as long 'i'(in main) becomes greater than 'i'(in isPrime), the 'i'(in isPrime) increases by one.

So my question is, how is the program checking the 'i'(in main) by every possible number from 2-100 when the 'i'(in isPrime) is constantly increasing directly with the 'i'(in main).

For instance when 'i'(in main) = 5, won't 'i'(in isPrime) be 5, since 'i'(in isPrime) will increment up to the 'i'(in main)'s value in order to fulfill the loop condition it? Making 5 % 5 have no remainder making the function isPrime return a false value? Also the 'i'(in main) value would never get checked by the values of 'i'(in isPrime) that are below it's current value. For instance wouldn't 5 never get checked by 4 since the loop is constantly increasing?

[laserlight]

Keep in mind that in main and isPrime, i is a local variable. Therefore, the i from main and the i from isPrime are not related. They are different variables that happen to have the same name.

[lamb]

bool isPrime (int number)
{
    for ( int i = 2; i < number; i++)
    {
        if ( isDivisible( number, i  ) ) //If the two numbers are divisible...
        {
            return false; //I don't care about them!
        }
    }
    return true; //If they're not, tell me!
}

Wait so does 'i' start at 2 every time the function is given a new 'number' value?

So like after 'i'(in main aka 'number' in isPrime) is tested @ 5 and moves to 6, will the 'i'(in isPrime) start back at 2 in order to test all values below 5, like this?

TESTING 5 (RESTART)
5/2 5/3 5/4 = PRIME
TESTING 6 (RESTART)
6/2 = COMPOSITE
TESTING 7 (RESTART)
7/2 7/3 7/4 7/5 7/6 = PRIME

[Elysia]

"i" starts at 2 every time the function is called. Keep in mind that it's IsPrime's "i", and not any other "i"s (say, main's). Locals variables may have the same names as variables in other functions, but they are in no way related.

[lamb]

"i" starts at 2 every time the function is called. Keep in mind that it's IsPrime's "i", and not any other "i"s (say, main's). Locals variables may have the same names as variables in other functions, but they are in no way related.

Ah thank-you, I'm aware of the difference between variables of the same name in different functions, just wasn't sure on the starting at 2 each time a new input is entered.