# using function to display prime numbers before user input not working.

#### Killerbee65

Joined May 15, 2017
243
C++:
#include <iostream>
using namespace std;

void display_purpose();
void get_input(int&, int&, int&);
bool isPrime(int);

int main()
{
display_purpose();
int num;
bool flag;

cout << "Enter any number(should be positive integer): ";
cin >> num;

if (num == 0 || num == 1) {
cout << num << " is not a prime number, nor does it have any before it\n";
}
else {
flag = isPrime(num);
if (flag == true)
cout << num << " is a prime number\n";
else
cout << num << " is not a prime number\n";
}
int min, max;
get_input(min, max, num);

cout << "\nPrime numbers smaller than " << num << " are: ";

// Smallest prime number is 2
for (int i = min; i <= max; i++)
{
if (isPrime(i))
cout << i << " ";
}
cout << endl;
return 0;
}

void display_purpose() {
cout << "The purpose of this program is to display all the prime numbers smaller than or equal to the given integer.\n";
}

bool isPrime(int num)
{
bool flag=true;

for(int i = 2; i <= num / 2; i++) {
if(num % i == 0) {
flag = false;
break;
}
}
static int times = 0;
if (times == 0)
{
cout << "This function is called for first time\n";
times++;
}
else
{
times++;
cout << "This function has been called " << times << " times so far\n";
}
return flag;
}
void get_input(int &maximum, int &mininum, int &num)
{
maximum = 10000;
mininum = 1;
if(num < mininum || num > maximum)
{
cout << "Error: the input must be between 1 and 10,000\n";
}
}
I'm not exactly sure why the code doesn't work when I input let's say 7 the program is supposed to display all the prime numbers before 7 and 7 (doesn't display user input if the input isn't a prime number). The problem is I'm not really sure why it doesn't work, comparing it to online solutions I can't see much of a difference.

#### WBahn

Joined Mar 31, 2012
26,308
Does this even compile? Unless the rules for C++ are significantly different than C, I don't see how?

What have you done to test it?

Have you tested the three functions you define at the bottom?

#### Killerbee65

Joined May 15, 2017
243
Does this even compile? Unless the rules for C++ are significantly different than C, I don't see how?

What have you done to test it?

Have you tested the three functions you define at the bottom?
It compiles without issue and no warnings, The only issue is it won't display the prime numbers prior to the user's input. As for the rules for C++, I'm almost certain c programming works in c++ with a few exceptions here and there. I tested each function by taking the code and putting it in the main code but seeing how some pieces of the code require a function call to work, I assumed those special cases would work smoothly. But from my point of view, each function works just fine and the problem might lie in the main function. The reason I say this is because the isPrime function only checks to see if the number inputted by the user is a prime number and every prime number before that (displays "this function ran x amount of times" for every "prime check").