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Author: MathProgrammer

C Programs/C Programs 23/Bear and Prime Numbers.cpp

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+#include <cstdio>
+#include <vector>
+
+using namespace std;
+
+void precompute(vector <int> &is_prime, int N)
+{
+    is_prime[0] = is_prime[1] = false;
+    vector <int> primes;
+
+    for(int i = 2; i <= N; i++)
+    {
+        if(is_prime[i])
+            primes.push_back(i);
+
+        for(int j = 0; j < primes.size() && i*primes[j] <= N; j++)
+        {
+            is_prime[i*primes[j]] = false;
+
+            if(i%primes[j] == 0) break;
+        }
+    }
+}
+
+int main()
+{
+    int no_of_elements;
+    scanf("%d", &no_of_elements);
+
+    const int MAX_N = 1e7;
+    vector <int> frequency(MAX_N + 1, 0);
+    while(no_of_elements--)
+    {
+        int element;
+        scanf("%d", &element);
+        frequency[element]++;
+    }
+
+    vector <int> is_prime(MAX_N + 1, true);
+    precompute(is_prime, MAX_N);
+
+    vector <int> no_of_multiples(MAX_N + 1, 0);
+    for(int i = 1; i <= MAX_N; i++)
+    {
+        if(!is_prime[i]) continue;
+
+        for(int multiple = i; multiple <= MAX_N; multiple += i)
+        {
+            no_of_multiples[i] += frequency[multiple];
+        }
+    }
+
+    vector <int> answer(MAX_N + 1, 0);
+    for(int i = 1; i <= MAX_N; i++)
+        answer[i] = answer[i - 1] + no_of_multiples[i];
+
+    int no_of_queries;
+    scanf("%d", &no_of_queries);
+
+    while(no_of_queries--)
+    {
+        int left, right;
+        scanf("%d %d", &left, &right);
+
+        right = min(right, MAX_N);
+        left = min(left, MAX_N);
+
+        printf("%d\n", answer[right] - answer[left - 1]);
+    }
+
+    return 0;
+}