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<FONT SIZE=4> Section 1.4 </FONT><BR>
<FONT SIZE=5><B> Communication through console. </B></FONT>
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<p>
The <i>console</i> is the basic interface of computers, normally it is the set composed of
the keyboard and the screen. The keyboard is generally the standard <I>input</I> device and
the screen the standard <I>output</I> device.
<p>
In the <i>iostream</i> C++ library, standard <i>input</i> and <i>output</i> operations
for a program are supported by two data streams: <tt><b>cin</b></tt> for input and
<tt><b>cout</b></tt> for output. Additionally, <tt><b>cerr</b></tt> and <tt><b>clog</b></tt>
have also been implemented - these are two output streams specially
designed to show error messages. They can be redirected to the standard output
or to a log file.
<p>
Therefore <tt><b>cout</b></tt> (the standard output stream) is normally directed to the
screen and <tt><b>cin</b></tt> (the standard input stream) is normally assigned to the
keyboard.
<p>
By handling these two streams you will be able to interact with the user in your programs
since you will be able to show messages on the screen and receive his/her input
from the keyboard.
<p>
<h2>Output (<tt>cout</tt>)</h2>
The <tt><b>cout</b></tt> stream is used in conjunction with the overloaded operator
<tt><<</tt> (a pair of <i>"less than"</I> signs).
<blockquote><tt><pre>
cout << "Output sentence"; <FONT COLOR="green"><i>// prints Output sentence on screen</i></FONT>
cout << 120; <FONT COLOR="green"><i>// prints number 120 on screen</i></FONT>
cout << x; <FONT COLOR="green"><i>// prints the content of variable x on screen</i></FONT>
</pre></tt></blockquote>
The <tt><<</tt> operator is known as <I>insertion operator</I> since it inserts the
data that follows it into the stream that precedes it. In the examples above
it inserted the constant string <tt>Output sentence</tt>, the numerical constant
<tt>120</tt> and the variable <tt>x</tt> into the output stream <tt><b>cout</b></tt>.
Notice that the first of the two sentences is enclosed between double quotes (<tt>"</tt>)
because it is a string of characters. Whenever we want to use constant strings of
characters we must enclose them between double quotes (<tt>"</tt>) so that they can be
clearly distinguished from variables. For example, these two sentences are very
different:
<blockquote><tt><pre>
cout << "Hello"; <FONT COLOR="green"><i>// prints Hello on screen</i></FONT>
cout << Hello; <FONT COLOR="green"><i>// prints the content of Hello variable on screen</i></FONT>
</pre></tt></blockquote>
The <I>insertion</I> operator (<tt><<</tt>) may be used more than once in a same
sentence:
<blockquote><tt><pre>
cout << "Hello, " << "I am " << "a C++ sentence";
</pre></tt></blockquote>
this last sentence would print the message <tt><B>Hello, I am a C++ sentence</B></tt>
on the screen. The utility of repeating the insertion operator (<tt><<</tt>)
is demonstrated when we want to print out a combination of variables and constants
or more than one variable:
<blockquote><tt><pre>
cout << "Hello, I am " << age << " years old and my zipcode is " << zipcode;
</pre></tt></blockquote>
If we supose that variable <tt>age</tt> contains the number <tt>24</tt> and the variable
<tt>zipcode</tt> contains <tt>90064</tt> the output of the previous sentence would be:
<blockquote><tt>
Hello, I am 24 years old and my zipcode is 90064
</tt></blockquote>
It is important to notice that <tt><b>cout</b></tt> does not add a line break
after its output unless we explicitly indicate it, therefore, the following sentences:
<blockquote><tt>
cout << "This is a sentence.";<br>cout << "This is another sentence.";
</tt></blockquote>
will be shown followed in screen:
<blockquote><tt>
This is a sentence.This is another sentence.
</tt></blockquote>
even if we have written them in two different calls to <tt><b>cout</b></tt>.
So, in order to perform a line break on output we must explicitly order it by
inserting a new-line character, that in C++ can be written as <tt><b>\n</b></tt>:
<blockquote><tt>
cout << "First sentence.\n ";<br>
cout << "Second sentence.\nThird sentence.";
</tt></blockquote>
produces the following output:
<blockquote><tt>
First sentence.<br>Second sentence.<br>Third sentence.
</tt></blockquote>
Additionally, to add a new-line, you may also use the <TT><B>endl</B></TT>
manipulator. For example:
<blockquote><tt>
cout << "First sentence." << endl;<br>
cout << "Second sentence." << endl;
</tt></blockquote>
would print out:
<blockquote><tt>
First sentence.<br>Second sentence.
</tt></blockquote>
The <TT><B>endl</B></TT> manipulator has a special behavior when it is used with buffered streams: they
are flushed. But anyway <TT><B>cout</B></TT> is unbuffered by default.
<P>
You may use either the <tt><b>\n</b></tt> escape character or the
<tt><b>endl</b></tt> manipulator in order to specify a line jump to <TT><B>cout</B></TT>.
Notice the differences of use shown earlier.
<p>
<h2>Input (<tt>cin</tt>).</h2>
Handling the standard input in C++ is done by applying the overloaded operator of
<i>extraction</i> (<tt>>></tt>) on the <tt><b>cin</b></tt> stream. This must be
followed by the variable that will store the data that is going to be read.
For example:
<blockquote><tt>
int age;<br>
cin >> age;
</tt></blockquote>
declares the variable <tt>age</tt> as an <tt>int</tt> and then waits for an input
from <tt>cin</tt> (keyborad) in order to store it in this integer variable.
<p>
<TT><B>cin</B></TT> can only process the input from the keyboard once the <TT>RETURN</TT>
key has been pressed. Therefore, even if you request a single character
<tt><b>cin</b></tt> will not process the input until the user presses <TT>RETURN</TT>
once the character has been introduced.
<p>
You must always consider the <i>type</i> of the variable that you are using
as a container with <tt><b>cin</b></tt> extraction. If you request an integer you will get
an integer, if you request a character you will get
a character and if you request a string of characters you will get a string of characters.
<P>
<TABLE WIDTH=100% CELLPADDING=5 CELLSPACING=5><TR><TD BGCOLOR="#FFFFBF" WIDTH=50% VALIGN="top">
<TT><PRE><I>// i/o example</I>
#include <iostream.h>
int main ()
{
int i;
cout << "Please enter an integer value: ";
cin >> i;
cout << "The value you entered is " << i;
cout << " and its double is " << i*2 << ".\n";
return 0;
}
</PRE></TT>
</TD><TD BGCOLOR="silver" WIDTH=50% VALIGN="top">
<TT><B>Please enter an integer value: </B>702<BR>
<B>The value you entered is 702 and its double is 1404.</B>
</TT>
</TD></TR></TABLE>
<p>
The user of a program may be one of the reasons that provoke errors even in the simplest
programs that use <tt><b>cin</b></tt> (like the one we have just seen). Since if you
request an integer value and the user introduces a name (which is a string of characters),
the result may cause your program to misoperate since it is not what we were expecting
from the user. So when you use the data input provided by <tt><b>cin</b></tt> you will
have to trust that the user of your program will be totally cooperative and that he will
not introduce his name when an interger value is requested. Farther ahead, when we will see
how to use strings of characters we will see possible solutions for the
errors that can be caused by this type of user input.
<p>
You can also use <tt><b>cin</b></tt> to request more than one datum input from the
user:
<blockquote><tt>cin >> a >> b;</tt></blockquote>
is equivalent to:
<blockquote><tt>cin >> a;<br>cin >> b;</tt></blockquote>
In both cases the user must give two data, one for variable <tt><b>a</b></tt> and
another for variable <tt><b>b</b></tt> that may be separated by any valid blank
separator: a space, a tab character or a newline.
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