C++ Programming

Chapter 15: Streams

Standard streams

C++ standard streams are cin (input) and cout (output). By default, input means that the user is providing information, and output means that the information is printed on the screen.

However, you can also direct files to standard streams. For example, when running a program like this in Linux, information is read from input.txt and not from the user:

./code < input.txt

Similarly, you can redirect printing to the output.txt file as follows:

./code > output.txt

It is also possible to combine these controls as follows:

./code < input.txt > output.txt

Reading

As information is read from the stream, there should be something blank (such as a space or a line break) between each readable item but it doesn't matter how much empty space there is. For example, code

int a, b;
cin >> a >> b;

will read two integers from both of the following feeds:

99 123

    99
  123

If the number of items to read is not known in advance, the following code is handy:

int x;
while (cin >> x) {
    cout << "number read " << x << "\n";
}

This code reads integers from the stream one at a time to x until they no longer exist (for example, if the stream is a file redirect and the file ends).

The getline function reads and entire line from a stream to a string, including spaces. For example, the following code reads a line from the stream into variable s:

string s;
getline(cin, s);

Writing

#include <iomanip>

You can influence the formatting of the output by sending manipulators to the stream before the item to be printed. For example, the manipulator setw specifies the width of the field to be printed next:

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= 5; j++) {
        cout << setw(4) << i*j;
    }
    cout << "\n";
}

   1   2   3   4   5
   2   4   6   8  10
   3   6   9  12  15
   4   8  12  16  20
   5  10  15  20  25

The setfill manipulator determines the character to fill in the beginning of the field. For example, the code below prints the time in the following format 08:30:00:

int h = 8, m = 30, s = 0;
cout << setfill('0');
cout << setw(2) << h << ":";
cout << setw(2) << m << ":";
cout << setw(2) << s << "\n";

Handy manipulators with floating points are fixed and setprecision. When the manipulator fixed is first sent to the stream, then use the setprecision manipulator can dictate how many decimal places the following the items are printed.

double x = 3.141592653589793;
cout << fixed;
cout << setprecision(0) << x << "\n"; // 3
cout << setprecision(2) << x << "\n"; // 3.14
cout << setprecision(5) << x << "\n"; // 3.14159

Note that setw only works on the next printable item, but the other manipulators described above affect all printing thereafter.

Working with files

#include <fstream>

The file stream ifstream is for reading data from a given file. For example, the following code reads two integers from input.txt:

ifstream f("input.txt");
int a, b;
f >> a >> b;
f.close();

Similarly, the file stream ofstream prints information to a given file. For example, the following code prints the "Hello!" to the file output.txt.

ofstream f("output.txt");
f << "Hello!\n";
f.close();

By default, the file will be cleared before output. However, the ios::app (append) lets you dictate that the new information is ouput to the end of the file:

ofstream f("output.txt", ios::app);

String streams

#include <sstream>

A string stream allows you to read information from a string into variables. For example, the following code creates a string stream and uses it to read two integers.

string s = "  99  123 ";
stringstream ss(s);
int a, b;
ss >> a >> b;
cout << a << " " << b << "\n"; // 99 123

The following code uses a string stream to form a string, with a floating point number displayed to two decimal places.

stringstream ss;
double x = 3.141592653589793;
ss << fixed << setprecision(2) << x;
string s = ss.str();
cout << s << "\n"; // 3.14

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