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cpp/seive-of-Eratosthenes.cpp

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+// C++ program to print all primes smaller than or equal to 
+// n using Sieve of Eratosthenes 
+#include <bits/stdc++.h> 
+using namespace std; 
+
+void SieveOfEratosthenes(int n) 
+{ 
+    // Create a boolean array "prime[0..n]" and initialize 
+    // all entries it as true. A value in prime[i] will 
+    // finally be false if i is Not a prime, else true. 
+    bool prime[n+1]; 
+    memset(prime, true, sizeof(prime)); 
+
+    for (int p=2; p*p<=n; p++) 
+    { 
+        // If prime[p] is not changed, then it is a prime 
+        if (prime[p] == true) 
+        { 
+            // Update all multiples of p greater than or  
+            // equal to the square of it 
+            // numbers which are multiple of p and are 
+            // less than p^2 are already been marked.  
+            for (int i=p*p; i<=n; i += p) 
+                prime[i] = false; 
+        } 
+    } 
+
+    // Print all prime numbers 
+    for (int p=2; p<=n; p++) 
+       if (prime[p]) 
+          cout << p << " "; 
+} 
+
+// Driver Program to test above function 
+int main() 
+{ 
+    int n = 30; 
+    cout << "Following are the prime numbers smaller "
+         << " than or equal to " << n << endl; 
+    SieveOfEratosthenes(n); 
+    return 0; 
+}