Program Fails to output greatest integer - c++

Question

I am writing a program that takes multiple user inputs, and counts how many prime numbers there are, and which number is the greatest and smallest of the inputs. However, I am stuck. when I run the code, it thinks every new input is the greatest number. for example, if I enter 1, 2, 3, 2... It says the greatest number is 2. It takes the last integer inputted and thinks that is the greatest number. I have no started on the lease common number yet, but if I am able to understand how to get the greatest number, I bet I can get the least. Thanks guys!
#include <iostream>
using namespace std;
int main() {
int startstop = 1;
cout << "start program?" << endl;
int begin = 0;
int primecounter = 0;
cin >> begin;
if (begin >= 0) {
while (startstop != 0) {
cout << "enter any number, or press Q to process." << endl;
int oldnum = 0;
int userinput=0;
int newnum = userinput;
int greatestnum = 0;
int greatestcounter = 0;
cin >> userinput;
int x;
bool is_prime = true;
if (userinput == 0 || userinput == 1) {
is_prime = false;
}
// loop to check if n is prime
for (x = 2; x <= userinput / 2; ++x) {
if (userinput % x == 0) {
is_prime = false;
break;
}
}
if (is_prime) {
primecounter++;
}
//check if input is greater than previous input
if (greatestnum > userinput) {
greatestnum = userinput;
}
cout << "prime count: " << primecounter << endl;
cout << "greatest num: " << greatestnum << endl;
userinput = 0;
}
return 0;
}
}

Answer 1

greatestnum is only assined if it's greater than the user input. Since it's initialized to 0, the if statement,
if (greatestnum > userinput) {
greatestnum = userinput;
}
will be false unless the user input is less than zero. If you want to make it the greatest from all user inputs, flip it to < and move the int greatestnum = 0; to right above the if (begin >= 0) {.
if (greatestnum < userinput) {
greatestnum = userinput;
}

Answer 2

#include <iostream>
using namespace std;
int main() {
int startstop = 1;
cout << "start program?" << endl;
int begin = 0;
int primecounter = 0,greatestnum = 0,oldnum = 0,userinput = 0;
cin >> begin;
if (begin >= 0) {
while (startstop != 0) {
cout << "enter any number, or press Q to process." << endl;
cin >> userinput;
int x;
bool is_prime = true;
if (userinput == 0 || userinput == 1) {
is_prime = false;
}
// loop to check if n is prime
for (x = 2; x <= userinput / 2; ++x) {
if (userinput % x == 0) {
is_prime = false;
break;
}
}
if (is_prime) {
primecounter++;
}
//check if input is greater than previous input
if (userinput > oldnum) {
greatestnum = userinput;
oldnum = userinput;
}
cout << "prime count: " << primecounter << endl;
cout << "greatest num: " << greatestnum << endl;
userinput = 0;
}
return 0;
}
}
Initialize 'oldnum=0' , 'userinput=0' and 'greatestnum=0' outside the loop. And there were some error in logic. If userinput is greater the old num the update greatest num=userinput and oldnum=userinput.