A pretty straight forward task, calculate the number of primes between 2 integers.
Given 2 integers A ≤ B < 10
5 what’s the number of primes in range from A to B inclusive.
Note: A prime number is a positive integer greater than 1 and is divisible by 1 and itself only. For N to be prime it is enough to test the divisibility of numbers less than or equal to square root of N.
Input
As many as 1000 lines, each line contains 2 integers A and B separated by a space. Input is terminated when A = B = -1 (Do not process this line).
Output
For every line in input – except for the last line where A = B = -1 - print the number of prime numbers between A and B inclusive.
Sample Input
0 9999
1 5
-1 -1
Sample Output
1229
3
#include<iostream>
#include<algorithm>
using namespace std;
const int N=100010;
int st[N];
int main()
{
for(int i=2;i<N;i++)
{
if(!st[i])
{
for(int j=i+i;j<N;j+=i)
st[j]=1;
}
}
st[0]=0;
st[1]=0;
for(int i=2;i<N;i++)
{
if(st[i]==0) st[i]=1;
else st[i]=0;
st[i]+=st[i-1];
}
int a,b;
while(cin>>a>>b)
{
if(a==-1&&b==-1)break;
cout<<st[b]-st[a-1]<<endl;
}
}