Runtime error in code (C++) - c++

Question

i am a beginner to c++ but i wouldn't have asked this question if i didnt spend hours on it.
The code is about finding primes between two numbers in the most efficient way where maximum limit is 10^9.
The following code gives me runtime error but i have no idea why.. help
#include <iostream>
#include <stdio.h>
#include <math.h>
using namespace std;
long int prime[32000];
bool isprime(long int a){
for(long int i = 3; i <= 32000; i+=2){
if(a%i == 0){
return false;
}
}
return true;
}
void generateprimes(){
long int a = 0;
for(long int i = 3; i < 31623 ; i+=2){
if(isprime(i)){
prime[a] = i;
a++;
}
}
}
bool newisprime(long int a){
long int x =0;
for(long int i = prime[x]; i <= sqrt(a); i = prime[++x]){
if(a%i == 0){
return false;
}
}
return true;
}
void generateprimes_inbetween(long int n,long int m){
if(n%2 == 0){
++n;
}
if(n == 1){
printf("2\n");
n = 3;
}
for(long int i = n; i <= m ; i+=2){
if(newisprime(i)){
printf("%d\n",i);
}
}
}
int main() {
long int a,b,c;
scanf("%ld",&a);
generateprimes();
for(long int i = 0; i < a ; i++){
scanf("%ld %ld",&b,&c);
generateprimes_inbetween(b,c);
printf("\n");
}
return 0;
}

Answer 1

In isprime() you loop through ALL numbers in your array prime[]. But at startup, as it's global data, most of them will be 0, so that a%i will result in a fatal divide by 0.
You have somewhere to keep track of the numer of primes that you've stored in your array and only test against the primes that you've stored there.
Supposing that it's homework and you're not allowed to use vectors, you could do it as follows:
const size_t max_primes = 32000; // avoid hard coded values
unsigned long prime[max_primes] {2, 3}; // prefilled values
size_t nprimes = 2; // number of primes in the array
bool isprime(unsigned long a){
for(size_t i = 0; i < nprimes; i++){
if(a%prime[i] == 0)
return false;
}
return true;
}
void generateprimes(){
nprimes = 2;
for(unsigned long i = 3; nprimes<max_primes && i < ULONG_MAX; i += 2){
if(isprime(i)){
prime[nprimes] = i;
nprimes++;
}
}
}
bool newisprime(unsigned long a){
size_t x = 0;
for(unsigned long i = prime[x]; i <= sqrt(a) && x<nprimes; i = prime[++x]){
if(a%i == 0)
return false;
}
if(x == nprimes) {
cout << "Attention: Reaching end of prime table !!" << endl;
}
return true;
}
Some remarks:
for the index, it's safer to use the unsigned type size_t.
make sure that whenever you use an index, it remains within the bounds
as you work with positive numbers, it could make sense to use unsigned long