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Remove comments from a given C/C++ program

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Given a C/C++ program, remove comments from it. 

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The idea is to maintain two flag variables, one to indicate that a single line comment is started, another to indicate that a multiline comment is started. When a flag is set, we look for the end of comment and ignore all characters between start and end.

Following is C++ implementation of above idea.


// C++ program to remove comments from a C/C++ program
#include <iostream>
using namespace std;
string removeComments(string prgm)
    int n = prgm.length();
    string res;
    // Flags to indicate that single line and multiple line comments
    // have started or not.
    bool s_cmt = false;
    bool m_cmt = false;
    // Traverse the given program
    for (int i=0; i<n; i++)
        // If single line comment flag is on, then check for end of it
        if (s_cmt == true && prgm[i] == '\n')
            s_cmt = false;
        // If multiple line comment is on, then check for end of it
        else if  (m_cmt == true && prgm[i] == '*' && prgm[i+1] == '/')
            m_cmt = false,  i++;
        // If this character is in a comment, ignore it
        else if (s_cmt || m_cmt)
        // Check for beginning of comments and set the appropriate flags
        else if (prgm[i] == '/' && prgm[i+1] == '/')
            s_cmt = true, i++;
        else if (prgm[i] == '/' && prgm[i+1] == '*')
            m_cmt = true,  i++;
        // If current character is a non-comment character, append it to res
        else  res += prgm[i];
    return res;
// Driver program to test above functions
int main()
    string prgm = "   /* Test program */ \n"
                  "   int main()  \n"
                  "   {           \n"
                  "      // variable declaration \n"
                  "      int a, b, c;    \n"
                  "      /* This is a test  \n"
                  "          multiline     \n"
                  "          comment for   \n"
                  "          testing */      \n"
                  "      a = b + c;       \n"
                  "   }           \n";
    cout << "Given Program \n";
    cout << prgm << endl;
    cout << " Modified Program ";
    cout << removeComments(prgm);
    return 0;


Given Program 
   /* Test program */ 
   int main()  
      // variable declaration 
      int a, b, c;    
      /* This is a test  
          comment for   
          testing */      
      a = b + c;       

 Modified Program     
   int main()  
            int a, b, c;    
      a = b + c;       

Time  Complexity :O(N), where N is length of string.

Space Complexity: O(1), since no extra space used.

Last Updated : 31 Jan, 2023
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