The <sstream> header contains classes which allow you to associate a stream with a string in memory, in the same way that the classes in <fstream> allow you to associate a stream with a file. Looking at the class hierarchy below, you can see that istringstream is a kind of istream, (just as ifstream is), while ostringstream is a kind of ostream, just like ofstream is.

To use a string stream for output, follow these three steps:

1. Create an ostringstream object.
2. Write to the stream object.
3. Collect the results using the stream's str() member function.

Let's look at an example, where string strings can be very useful. The standard library has a to_string() function (starting in C++11) which works for all types of numbers. Unfortunately, for floating-point numbers, you have no control over the output format, which isn't very useful. Let's fix that.

Here's a short function with two arguments: a double for the value, and the number of decimal digits to display The function returns the value as a formatted string. (Note the default argument, which makes the function easier to use.)

string format(double value, int digits=2)
{
    ostringstream out;
    out << fixed << setprecision(digits) << value;
    return out.str();
}

You can use the format() function like this:

cout << format(2.456) << endl;      // 2.46
cout << format(1.0 / 3.0, 5) << endl;     // .33333
    

The C++11 string class introduced several new functions, like stoi() and stod(), which convert string values like 42 to int or double. Prior to C++11, you used the istringstream class.

istringstream in("January 3, 2020");

The istringstream (or input string stream) variable named in allows you to parse all of the pieces of the date that has been supplied, and, in a much easier manner than using the string functions like find() and substr():

string month; int day, year;
in >> month >> day; // read month and day
in.get();           // consume comma
in >> year;         // read and convert year

For complex values, like dates, the istringstream technique is the most efficient.

Using an input string stream is the easiest way to parse the individual parts of a line of text. Let's solve a problem which puts this to work. Click on the "running man" to open the starter code in CodeCheck.

For example, if input contains the following text:

"Beware the Jabberwock, my son,
the jaws that bite, the claws that catch,
Beware the JubJub bird and shun
the frumious bandersnatch."

Then the output should be:

Line 1 has 5 tokens (longest = 11)
Line 2 has 8 tokens (longest = 6)
Line 3 has 6 tokens (longest = 6)
Line 4 has 3 tokens (longest = 14)
Longest line: the jaws that bite, the claws that catch,

When you tackle a complex problem like this, you should always tackle it one piece at a time. Let's start with this:

1. Reading the entire input file, line-by-line
2. Finding the longest line
3. Printing the longest line to the output file

Take a look at the starter code for the inputStats function. Reading the documentation comment, you can see that the two arguments it expects are an input stream and an output stream.

string line;
string longest_line;
while (getline(in, line))
{
    if (line.size() > longest_line.size())
   { 
        longest_line = line;
   } 
}
out << "Longest line: " << longest_line << endl;

To read line-by-line, use getline() and the data-loop pattern:

string line;
while (getline(in, line)) . . .

To find the longest line, you must:

1. Create a variableto hold the longest line (before the loop)
2. Compare the current line size to the longest line size (in the loop)
3. Print the longest line (after the loop)

Here's the pseudocode for this part of the problem. Once you've turned it into C++, check your work with the code at the top of the page.

longest line : string
while getline line
    if line size > longest line size then
        longest line = line
print the longest line

Note: make sure that you read from your input stream, and write to your output stream. You should not use cin or cout in this problem; use your parameters.

string longest_line;
string line;
int line_number{0};
while (getline(in, line))
{
    ++line_number;
    if (line.size() > longest_line.size()) { 
        longest_line = line; 
   } 
    out << "Line " << line_number << " has ..." << endl;
}
out << "Longest line: " << longest_line << endl;

To process and count the lines, you must:

1. Create a variable to hold the line number before the while loop.
2. Increment the variable every time a line is read
3. Print Line <line number> at the end of the while loop

Here's the pseudocode to do that with the new lines highlighted. Try it out and then check your work with the code I've supplied.

longest line : string
line number <- 0
while getline line
    line number = line number + 1
    if line size > longest line size then
        longest line = line
    Print "Line <line number> has . . ."
Print "Longest line: " longest line

void inputStats(istream& in, ostream& out)
{
   string longest_line;
   string line;
   int line_number = 0;
   while (getline(in, line))
   {
       line_number++;
       if (line.size() > longest_line.size()) {
          longest_line = line;
     } 
       istringstream str_in(line);
       int num_tokens0{};
       string token;
       int longest_token = 0;
       while (str_in >> token)  {
          ++num_tokens; 
          if (token.size() > longest_token) {
             longest_token = token.size();
        } 
     } 
       out << "Line " << line_number << " has " 
           << num_tokens << " tokens (longest = " 
           << longest_token << ")" << endl;
  } 
   out << "Longest line: " << longest_line << endl;
}

To process and count the tokens, you must re-read each line, token-by-token. To do that you must:

1. Create an input string stream variable using the line.
2. Read (and count) each token from the string stream.