How to test whether stringstream operator>> has parsed a bad type and skip it

Question

I am interested in discussing methods for using stringstream to parse a line with multiple types. I would begin by looking at the following line:

"2.832 1.3067 nana 1.678"

Now lets assume I have a long line that has multiple strings and doubles. The obvious way to solve this is to tokenize the string and then check converting each one. I am interested in skipping this second step and using stringstream directly to only find the numbers.

I figured a good way to approach this would be to read through the string and check if the failbit has been set, which it will if I try to parse a string into a double.

Say I have the following code:

string a("2.832 1.3067 nana 1.678");

 stringstream parser;
 parser.str(a);

 for (int i = 0; i < 4; ++i)
 {
     double b;
     parser >> b;
     if (parser.fail())
     {
         std::cout << "Failed!" << std::endl;
         parser.clear();
     }
     std::cout << b << std::endl;
 }

It will print out the following:

2.832
1.3067
Failed!
0
Failed!
0

I am not surprised that it fails to parse a string, but what is happening internally such that it fails to clear its failbit and parse the next number?

Answer 1

The following code works well to skip the bad word and collect the valid double values

istringstream iss("2.832 1.3067 nana 1.678");
double num = 0;
while(iss >> num || !iss.eof()) {
    if(iss.fail()) {
        iss.clear();
        string dummy;
        iss >> dummy;
        continue;
    }
    cout << num << endl;
}

Here's a fully working sample.

---

Your sample almost got it right, it was just missing to consume the invalid input field from the stream after detecting it's wrong format

 if (parser.fail()) {
     std::cout << "Failed!" << std::endl;
     parser.clear();
     string dummy;
     parser >> dummy;
 }

In your case the extraction will try to read again from "nana" for the last iteration, hence the last two lines in the output.

Also note the trickery about iostream::fail() and how to actually test for iostream::eof() in my 1st sample. There's a well known Q&A, why simple testing for EOF as a loop condition is considered wrong. And it answers well, how to break the input loop when unexpected/invalid values were encountered. But just how to skip/ignore invalid input fields isn't explained there (and wasn't asked for).