title: why is it compulsory to use using namespace std in c++ language?
body: <p>This is a simple program and my question is to know why is it compulsory to use using namespace std ? why program is not complied without using this ?</p>
  
  <pre class="lang-cpp prettyprint-override"><code>#include &lt;iostream&gt;
  
  using namespace std;
  
  int main(){
  
      int a , b , c , d;
  
      cout &lt;&lt; "Enter First Value" &lt;&lt;endl;
      cin &gt;&gt; a;
  
      cout &lt;&lt; "Enter Second Value" &lt;&lt;endl;
      cin &gt;&gt; b;
  
      cout &lt;&lt; "Enter 1 to add values" &lt;&lt; endl  &lt;&lt; "Enter 2 to subtract values" &lt;&lt;endl  &lt;&lt;"Enter 3 to multiply values" &lt;&lt;endl ;
      cin &gt;&gt; c;
  
      if (c == 1){    
          d = a + b;
          cout &lt;&lt; "After Adding your answer is " &lt;&lt; d &lt;&lt; endl;
      }
  
      else if (c == 2){
          d = a - b;
          cout &lt;&lt; "After Subtracting your answer is " &lt;&lt; d &lt;&lt; endl;
      }
  
      else if (c == 3){
          d = a * b;
          cout &lt;&lt; "After Multiplication your answer is " &lt;&lt; d &lt;&lt; endl;
      }
      else{
          cout &lt;&lt; "You Enter Invalid operation to perform";   
      }
  
      system("pause");  //buitin function to stop the comand screen.
  
      return 0;
  } 
  </code></pre>