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+/*
+Challenge 198
+
+Task 2: Prime Count
+Submitted by: Mohammad S Anwar
+You are given an integer $n > 0.
+
+Write a script to print the count of primes less than $n.
+
+Example 1
+Input: $n = 10
+Output: 4 as in there are 4 primes less than 10 are 2, 3, 5 ,7.
+Example 2
+Input: $n = 15
+Output: 6
+Example 3
+Input: $n = 1
+Output: 0
+Example 4
+Input: $n = 25
+Output: 9
+*/
+
+#include <iostream>
+
+// https://en.wikipedia.org/wiki/Primality_test
+bool is_prime(int n) {
+    if (n==2 || n==3) return true;
+    if (n<=1 || n%2==0 || n%3==0) return false;
+    for (int i=5; i*i<=n; i+=6)
+        if (n%i==0 || n%(i+2)==0) return false;
+    return true;
+}
+
+int next_prime(int p) {
+    p++;
+    while (!is_prime(p)) p++;
+    return p;
+}
+
+int count_primes_below_n(int n) {
+    int count=0;
+    int p=2;
+    while (p<n) {
+        count++;
+        p=next_prime(p);
+    }
+    return count;
+}
+
+int main(int argc, char* argv[]) {
+    argv++; argc--;
+    if (argc!=1) {
+        std::cerr << "usage: ch-2 n" << std::endl;
+        return EXIT_FAILURE;
+    }
+
+    int n=atoi(argv[0]);
+    int count=count_primes_below_n(n);
+    std::cout << count << std::endl;
+}