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Input and Output

Input and output use the input stream cin and the output stream cout. The input stream cin is usually associated with the keyboard and the output stream cout is usually associated with the monitor.

The following statement waits for a number to be entered from the keyboard and assigns it to the variable number:

cin >> number;
The general form of a statement to perform input using the input stream cin is:
cin input-list;
where input-list is a list of identifiers, each identifier preceded by the input operator >>. Thus
cin >> n1 >> n2;
would take the next two values entered by the user and assign the value of the first one to the variable n1 and the second to the variable n2.

The program must read a value for each variable in the input-list before it executes any more statements. The order in which the values are entered must correspond to the order of the variables in the input-list and they must be of the same type as the corresponding variable. They should be separated by spaces. Normally, the C++ system will not pass any values to the variables in the input-list until a complete line of input has been read, i.e. until the return or enter key has been pressed. If more values are supplied than are required to give each variable in the input-list a value, the unused values will be used for any subsequent input statements using cin. For example given the following declarations and input statement:

int count, n;
float value;
cin >> count >> value >> n;
the user could enter
  23  -65.1 3
to assign 23 to count, -65.1 to value and 3 to n. There is no indication in the data of which value is to be associated with which variable; the order of the data items must correspond to the order of the variables in the input list. The data items on input should be separated by spaces or new lines. Any number of these will be skipped over before or between data items. Thus the input above could equally well have been entered as:
     23
-65.1        3

The following statement outputs the current value of the variable count to the output stream cout, which is usually associated with the monitor. The value will be printed on the current line of output starting immediately after any previous output.

cout << count;
The general form of a statement to perform output using the output stream cout is:
cout output-list;
where output-list is a list of variables, constants, or character strings in quotation marks, each preceded by the output operator <<. The output operator displays the value of the item that follows it. The values are displayed in the order in which they appear in the output-list. A new line is taken if the special end-of-line character endl is output. If an endl is not output, the output line will either be chopped off at the right hand edge of the screen or it may wrap round on to the next line. Do not rely on either behaviour as different computer systems may do things differently. Thus
cout << "Hello there" << endl;
will print Hello there on the current output line and then take a new line for the next output. The statements:
float length, breadth;
cout << "Enter the length and breadth: ";
cin >> length >> breadth;
cout << endl << "The length is " << length;
cout << endl << "The breadth is " << breadth << endl;
will display, if the user enters 6.51 and 3.24 at the prompt, the following output:
The length is 6.51
The breadth is 3.24

Note that a value written to cout will be printed immediately after any previous value with no space between. In the above program the character strings written to cout each end with a space character. The statement

cout << length << breadth;
would print out the results as
6.513.24
which is obviously impossible to interpret correctly. If printing several values on the same line remember to separate them with spaces by printing a string in between them as follows:
cout << length << " " << breadth;


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