Don't understand boolean function variation

[mdtexeira]

For self-study, I've been working on loops and functions and such, and wrote a fairly obvious little program to take user input and output all of the primes up to the users choice. The thing is, it was returning all odd numbers, and I eventually figured out why, but I don't understand why.

  #include "stdafx.h"
#include "../../std_lib_facilities.h"

vector<int> primes;

bool numberIsPrime(int n)
{
	for (int p = 0; p < primes.size(); ++p)
		if (n%primes[p] == 0)
			return false;
}

int main()
{
	cout << "Please enter the max integer you wish to see all primes up to:\n";
	int max = 0;
	cin >> max;
	primes.push_back(2);

	for (int i = 3; i < max; ++i)
	{
		if (numberIsPrime(i))
			primes.push_back(i);
	}

	cout << "The prime numbers from 1-" << max << " are:\n";
	for (int p = 0; p < primes.size(); ++p)
		cout << primes[p] << '\n';

	return 0;
}

This is the working code. The code returning all odds had only one difference, in that it had:

else return true;

at the tail end of the function. Can someone explain why that would make a difference, because I'm not getting my head around it?

[Chervil]

    if (n%primes[p] == 0)
        return false;
    else 
        return true;

When you do this, there is no longer a loop. It will always exit from the for-loop on the very first pass, that is, when testing that n may or may not be divided by the first prime number - which is 2. Thus, instead of a test for primeness, it becomes a test for odd / even.

edit: AbstractionAnon (below) is correct.

[AbstractionAnon]

Your bool function never returns true, so a number will never be considered to be prime.

An else return true; would be wrong. That would return true (is prime) after checking only the first entry in the vector. You want return true; after line 10 (no else). You want to return true only after you've examined all the numbers in the vector.

bool numberIsPrime(int n)
{   for (int p = 0; p < primes.size(); ++p)
		if (n%primes[p] == 0)
			return false;
    return true;			
}

[mdtexeira]

Why does the code above work then? Without any return true, the bool function is apparently being returned as true.

Also, if I nested the 'if' and 'else' statements in a block, would the 'else' statement still close the loop?

[Chervil]

Why does the code above work then? Without any return true, the bool function is apparently being returned as true.

It doesn't 'work'.
g++ issues the message,

[Warning] control reaches end of non-void function [-Wreturn-type]

Though the compiler still generates some sort of code and you can try to run the program, the message is letting you know that there is a problem. That is, the function was supposed to return a value, but it did not do so. What that means is the compiler substituted some arbitrary value. It is letting you know that the results cannot be trusted.

Also, if I nested the 'if' and 'else' statements in a block, would the 'else' statement still close the loop?

Without seeing the code the question is not clear, but it sounds like the same problem still exists.