Extracting a subsequence from an array - c++

Question

I'm trying to solve an algorithm for extracting a subsequence from an array. It should display the longest subsequence of prime numbers. I have written the whole algorithm but I still get an infinite cycle and I can't figure out where and why. I'm incrementing both indices and modifying the first index at the end, but it is still not working. Thanks a lot !!!
P.S: citire reads the array, prim detects if a number is prime or composed, afisare displays the subsequence and detSecv determines the longest subsequence.
#include <iostream>
#include <math.h>
using namespace std;
void citireSecv(int &n,int x[50])
{
cout<<"Da n: ";
cin>>n;
for(int i=1;i<=n;i++)
{
cout<<"Da un nr: ";
cin>>x[i];
}
}
int prim(int n)
{
int d=2;
while(d<=sqrt(n) && n%d!=0)
{
if(d==2)
d=3;
else
d=d+2;
}
if(d>sqrt(n)) return 1;
else return 0;
}
void afisare(int n,int x[50],int st,int f)
{
for(int i=st;i<=f;i++)
cout<<x[i]<<" ";
}
void detSecv(int n,int x[100],int &st,int &f)
{
st=1; f=0;
int i=1,j;
while(i<=n-1)
{
while(i<=n-1)
{
if(prim(x[i])==0 && prim(x[i+1])==0) i++;
}
j=i+1;
while(j<=n-1)
if(prim(x[j])==0 && prim(x[j+1])==0) j++;
if((j-i) > (f-st))
{
st=i;
f=j;
}
i=j+1;
}
}
int main()
{
int n,x[100],st,f;
citireSecv(n,x);
detSecv(n,x,st,f);
afisare(n,x,st,f);
return 0;
}
Input data:
n=2
First number is: 5
Second number is: 7

Answer 1

Probably just one of many issues with that code:
while(i<=n-1)
{
if(prim(x[i])==0 && prim(x[i+1])==0) i++;
}
j=i+1;
while(j<=n-1)
if(prim(x[j])==0 && prim(x[j+1])==0) j++;
There are two potential infinite loops here. If the conditions in the while don't return true on the first iteration, i (or j) will never get incremented, and you will have your infinite loop. You should almost always increment such variables outside of any conditions.

Answer 2

With a slight change in your code, you make it work, and one thing, you don't need to start array with index 1. you can always start with index zero.
for(int i=1;i<=n;i++)
{
cout<<"Da un nr: ";
cin>>x[i];
}
try to check for a case when no prime subsequence is found, while printing.
void detSecv(int n, int *x, int &start, int &end)
{
start = -1;
end = -1;
int i=0,j;
while(i < n) {
if(prim(x[i])) {
j = i + 1;
while(j < n)
if(prim(x[j])) j++;
else break;
} else {
i++;
continue;
}
if((j-i) > (end - start)) {
start = i;
end = j-1;
}
i=j+1;
}
}

Answer 3

This is a better way to verify if a number is prime or not
bool IsPrime(int number) {
int primeStep = 2;
double stepLimit = sqrt(number);
while(primeStep <= stepLimit)
{
if(number % primeStep == 0)
return false;
primeStep += 1;
}
return true;
}
And nou you can apply that function for each number in your array, and if it's prime , you add it in a new array like this:
void detSecv(int numberOfItems,int *arrayOfNumbers)
{
int arrayOfPrimeNumbers[50] = {};
int index = 0;
for(int i = 0; i < numberOfItems; i++)
{
if(IsPrime(arrayOfNumbers[i])){
arrayOfPrimeNumbers[index] = arrayOfNumbers[i];
index += 1;
}
}
int secondIndex = 0;
while(arrayOfPrimeNumbers[secondIndex] != 0)
{
cout << arrayOfPrimeNumbers[secondIndex] << " ";
secondIndex += 1;
}
}