C++ condesing this code - c++

Question

Ok so I've already simplified/condense it while keeping the functionality,but im only doing C++ for a month and a half.Was at 100 lines of code.Is it possible to declare the variable in the functions arguement and then call them without passing values into the arguemnets?
#include <iostream>
#include <windows.h>
using namespace std;
int primeCheck10 (int j)
{
int count=0;
cout<<"Enter a number between 1 and 10___";
cin>>j;
if(j<1 ||j>10)
{
cout<<"Invalid Value\n";
return 0;
}
for(int i=2; i<j; i++)
{
if(j%i==0)
{
count++;
break;
}
}
if(count==0)
cout<<"Prime number\n";
else
cout<<"Not a Prime number\n";
}
int primeCheck100(int j)
{
int count=0;
cout<<"Enter a number between 1 and 100___";
cin>>j;
if(j<1 || j>100){
cout<<"Invalid Value\n";
return 0;
}
for(int i=2; i<j; i++)
{
if(j%i==0)
{
count++;
break;
}
}
if(count==0)
cout<<"Prime number\n";
else
cout<<"Not a Prime number\n";
}
int checkPrime1000(int j)
{
int count=0;
cout<<"Enter a number between 1 and 1000___";
cin>>j;
if(j<1 || j>1000){
cout<<"Invalid Value\n";
return 0;
}
for(int i=2; i<j; i++)
{
if(j%i==0)
{
count++;
break;
}
}
if(count==0)
cout<<"Prime number\n";
else
cout<<"Not a Prime number\n";
}
int main ()
{
system("pause");
return 0;
}

Answer 1

Yes, you can condense all the prime checking into a single function pretty easily. I'd change the structure of the code to just check whether a given number is prime, and return a bool to indicate whether it is or not:'
bool isprime(int n) {
int limit = sqrt(n)+1; // only need to check up to sqrt(n)
if (n == 2)
return true;
if (n == 1 || n % 2 == 0) // check if it's 1 or even
return false;
for (int i = 3; i <= limit; i += 2) // not even -- only check odd numbers
if (n % i == 0)
return false;
return true;
}
Then the code to get input and display results would be separate:
void primecheck(int limit) {
std::cout << "Please enter a number between 1 and " << limit;
int j;
std::cin >> j;
if (j<1 || j > limit)
std::cerr << "Invalid value";
static char const *labels [] = { "Not a prime number\n", "Prime number\n" };
std::cout << labels[isprime(j)];
}
It wouldn't be terribly difficult to make it even shorter than this, but we're reaching the point where it would probably end up less readable if you did so.

Answer 2

If I have unserstood correctly you then you are asking about default arguments. For example
#include <iostream>
void f( int i = 10 )
{
std::cout << "i = " << i << std::endl;
}
int main()
{
f();
}

Answer 3

You could just make one method... and add a bounds
int primecheck(int value, int bounds){
int count=0;
cout <<"Enter a number between 1 and " << bounds << "___";
cin>>value;
if(j<1 || value>bounds){
cout<<"Invalid Value\n";
return 0;
}
for(int i=2; i<value/2; i++)
{
if(value%i==0)
{
count++;
}
}
if(count==0)
cout<<"Prime number\n";
else
cout<<"Not a Prime number\n";
return count;
}
This alg is prob not the best way to check for primes though. for instance for(int i=2; i<j/2; i++) in your loop with optimize it. For example 32, 6x6 = 32, 2x16 = 32, 4x8 = 32. The highest number that can be divisible to j or value is half of it because 1 isn't considered under divisibility so the lowest number would be 2. Which is half.