String manipulation: Giving a string input, containing comments, implement a method to output the given string excluding the comments.

Question

in Java · Accepted Answer

public class RemoveCommentFromString {

    public enum State {
        OutsideComment,
        InsideLineComment,
        InsideBlockComment,
        InsideBlockCommentNoNewLineYet
    }

    public static String removeComments(String str) {
        State state = State.OutsideComment;
        StringBuilder endResult = new StringBuilder();
        Scanner s = new Scanner(str);
        s.useDelimiter("");
        
        while (s.hasNext()) {
            String c = s.next();
            switch (state) {
                case OutsideComment:
                    if (c.equals("/") && s.hasNext()) {
                        String c2 = s.next();
                        if (c2.equals("/")) {
                            state = State.InsideLineComment;
                        } else if (c2.equals("*")) {
                            state = State.InsideBlockCommentNoNewLineYet;
                        } else {
                            endResult.append(c).append(c2);
                        }
                    } else {
                        endResult.append(c);
                    }
                    break;
                    
                case InsideLineComment:
                    if (c.equals("
")) {
                        endResult.append("
");
                        state = State.OutsideComment;
                    }
                    break;
                    
                case InsideBlockCommentNoNewLineYet:
                    if (c.equals("
")) {
                        endResult.append("
");
                        state = State.InsideBlockComment;
                    }
                    break;
                    
                case InsideBlockComment:
                    if (c.equals("*") && s.hasNext()) {
                        String c2 = s.next();
                        if (c2.equals("/")) {
                            state = State.OutsideComment;
                        }
                    }
                    break;
            }
        }
        
        return endResult.toString();
    }

    public static void main(String[] args) {
        String str = "start_code();// first comment
" +
                "more_code(); /* block comment
"
                + "second line of block comment */
"
                + "more_code_after_block();";
        
        System.out.println("Scanner: "  + removeComments(str));
    }
}

Python using a DFA · Answer

comments_dfa = (
    ({
        '/': 1,
        'oth': 0,
        'in_comment': False,
    }, {
        '/': 2,
        '*': 3,
        'oth': 0,
        'in_comment': False,
    }, {
        '
': 0,
        'oth': 2,
        'in_comment': True,
    }, {
        '*': 4,
        'oth': 3,
        'in_comment': True,
    }, {
        '/': 0,
        'oth': 3,
        'in_comment': True,
    })
)

def remove_comments(input_string):
    result = ''
    cs = 0
    comment_status = False
    for c in input_string:
        if not comments_dfa[cs]['in_comment'] or c == '
':
            result += c

        if comments_dfa[cs]['in_comment'] and not comment_status:
            result = result[:-2]

        comment_status = comments_dfa[cs]['in_comment']

        if c in comments_dfa[cs]:
            cs = comments_dfa[cs][c]
        else:
            cs = comments_dfa[cs]['oth']

    return result

print remove_comments("""
    This is a /* test */ and / I want to check // what happends here
    123 /* Is this system */ working? or not???
    // Testing new line
""")

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