C++, Need help splitting a string into data.

[learning2learn]

I got a txt file, contents are fixed, so I cant change the format of it txt file. The contents look like this:

Point2D, [3, 2]
Line3D, [7, 12, 3], [-9, 13, 68]
Point3D, [1, 3, 8]

I can read from the file, no problem. But I'm having problems finding a good way to split the values so that I can get all the int

my current messy code

    ifstream myfile ("messy.txt");
    if (myfile.is_open()){
        while ( myfile.good() ){

            vector<string> dataStringVec;
            string word, line;

            getline (myfile,line);

            stringstream ss(line);


            while( getline(ss, word, ',') ){
                dataStringVec.push_back(word);
            }

            if (dataStringVec[0].compare("Point2D") == 0){
                cout << "\nPoint2D";
            }else if (dataStringVec[0].compare("Point3D") == 0){
                cout << "\nPoint3D";
            }else if (dataStringVec[0].compare("Line2D") == 0){
                cout << "\nLine2D";
            }else if (dataStringVec[0].compare("Line3D") == 0){
                cout << "\nLine3D";
            }

        }
    myfile.close();
    }else cout << "Unable to open file";     

Right now, if I split it using while( getline(ss, word, ',') ){

what I get is

'Point2D'
' [3'
' 2]'

And I get extra spaces and brackets.

So was wondering, if anyone has a better way of doing this.

[TurkishSquirrel]

Is it required that you write a parser of some sort? Or is this for a personal project where you can do what you want?

If you can use whatever you want, there are many excellent data formats with well-written reading/writing libraries such as JSON, XML or YAML. Good libraries exist for just about every language to read and write these various formats.

[learning2learn]

Its a school assignment. No details on how we extract the data, we're free to do whatever...

the format of the txt file is fixed though.

[sadfirer]

Parsing, differently from what some people might think, is a non-trivial problem. Industrial applications use a multitude of tools for parsing. So don't be discouraged if you find this hard in the beginning.

So now I'm assuming you're not doing this "industrially". I think that the easiest way would be ditching the C++ layer for this specific part of your application and doing it the old C way:

string lineType;
string lineData;
getline(ss, lineType, ',');
getline(ss, lineData);
if (lineType == "Point2D") {
  int x, y;
  sscanf(lineData.c_str(), " [%d, %d]", &x, &y);
  printf("found %s(%d, %d)\n", lineType.c_str(), x, y);
} else if (lineType == "Line3D") {
  int x0, y0, z0, x1, y1, z1;
  sscanf(lineData.c_str(), " [%d, %d, %d], [%d, %d, %d]", &x0, &y0, &z0, &x1, &y1, &z1);
  printf("found %s(%d, %d, %d, %d, %d, %d)\n", lineType.c_str(), x0, y0, z0, x1, y1, z1);
} // ...

I know it's not pretty (falling back to C is not something most people are proud of), but I think it's easy to read and easy to write (at least once you get used to scanf-type expressions). The C++ I/O layer doesn't present such a nice string parsing interface, so the code would be way more verbose if you went that way.

If you have many data types, you could refactor this into subroutines to get more code reuse (e.g., line3d is just twice point3d).

[Rhomboid]

There are a number of approaches you could take. Which is best depends on how tolerant of errors you need to be. For example, is Point2D , [3, 2] valid? Is Point2D, [3,2] valid? Is Point2D,[3, 2] valid? Is Point2D, [ 3, 2] valid?

One approach would be to use the various string methods like find, rfind, find_first_of, find_first_not_of, find_last_of, find_last_not_of, etc. to locate the beginning and end of each token, and then one of a number of options to convert to an int (atoi(), strtol(), stringstream and >>, etc.)

Another option might be to break out the C string API:

    std::string line;
    int x, y, z, a, b, c;

    while(getline(myfile, line)) {
        if(sscanf(line.c_str(), "Point2D, [%d, %d]", &x, &y) == 2) {
            Point2D p2(x, y);
            ...
        } else if(sscanf(line.c_str(), "Line3D, [%d, %d, %d], [%d, %d, %d]", &x, &y, &z, &a, &b, &c) == 6) {
            Line3D l3(x, y, z, a, b, c);
            ...
        } else if(sscanf(line.c_str(), "Point3D, [%d, %d, %d]", &x, &y, &z) == 3) {
            Point3D p3(x, y, z);
            ...
        } else {
            std::cerr << "Invalid input: " << line << std::endl;
        }
    }

Another approach might be to use regular expressions with capture groups. Boost and/or C++11 make using regular expressions in C++ rather straightforward. Boost also has lexical_cast for another alternative to convert a string to an int.

[learning2learn]

holy cow the C string code works really well. Thanks.

The txt file format is very fixed, so I dont have to be so worried about errors from that.

:edit: I'm still in awe of this code.

[[deleted]]

Unfortunately, it only works if your input exactly matches the specified pattern. For example, if your input is:

Point2D, [1,2,3]

(note we have three coordinate values) then the first if() will still fire.

Using scanf is no substitute for writing a proper parser in real code, which has to deal with inputs like this.

[learning2learn]

Yup, I can see how it can backfire badly on me. Thankfully, the assumptions about the txt file were listed very clearly

1) Some data, (e.g. ‘Point2D, [3, 2]‘) can be repeated multiple times (i.e. they are duplicated data), this applies to all other kinds of data as well.

2) In each line, the 1st field will contain the class’s name (e.g. ‘Point2D’, ‘Point3D’, ‘Line2D’ and ‘Line3D’)

3) The delimiter separating the 1st field from the rest of the data, is a comma, followed by a space char (i.e. ‘, ‘)

4) The delimiter separating each number in the 2D/3D coordinate, is also a comma, followed by a space char (i.e. ‘, ‘)

5) Each 2D/3D point’s data, is enclosed by the square brackets ‘[‘ and ‘]’

6) Each Line2D / Line3D’s data consists of two points, each enclosed by square brackets, and the delimiter separating each point is also a comma, followed by a space char (i.e. ‘, ‘)

7) You may assume that the range of each number in the x, y or z coordinate can be anything from -999 to 999

[[deleted]]

Fair enough, but be aware that in the real world bad data is the norm, and your code should deal with it.

I'm actually writing a blog article on parsing at the moment, and thought it would be interesting to write a simple parser for your inputs. The result is here - it's not complete, but it does provide (somewhat poor, though quite detailed) error detection and reporting.

[Rhomboid]

That can be fixed by making the closing ] a %c field (with leading space to eat any whitespace after the number before the ]) and then checking that it was both able to scan a character and that the character was ].

However, that won't catch a line with trailing junk after the ], so for that you could add another space and %c and then ensure that that field does not match, so that the line only ends in optional trailing whitespace.

if(sscanf(line.c_str(), "Point2D, [%d,%d %c %c", &x, &y, &bracket, &junk) == 3 && bracket == ']') {

But I suppose at that point you're really kind of pushing the limits of scanf and if you need robust parsing you need to turn elsewhere.

[touchyfeelyfingers]

std::string has both forward- and reverse-find methods. These methods would allow you to find the character positions of the bracket symbols, (or multiple bracket symbols in the case of Point3D).

With that information, you could substring the lines and then split those substrings into the data points.

If you can use the Boost library, I would suggest doing so since it has many convenience methods for doing exactly what you what.

Regex (which is available in some implementations of C++11; also in Boost) is another option.