________________Basic Input/Output_____________________


Standard Output (cout):

By default, the standard output of a program is 
the screen, and the C++ stream object defined 
to access it is cout.

cout is used in conjunction with the 
insertion operator, which is written as << (two "less than" signs).



cout << "Output sentence"; // prints Output sentence on screen
cout << 120;               // prints number 120 on screen
cout << x;                 // prints the content of x on screen 


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cout << "Hello";  // prints Hello
cout << Hello;    // prints the content of Hello variable 


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The insertion operator (<<) may be used more than 
once in a single statement:

  	

cout << "Hello, " << "I am " << "a C++ statement";


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cout << "Hello, I am " << age << " years old and my zipcode is " << zipcode;



If we assume the age variable to contain the 
value 24 and the zipcode variable to contain 90064 
the output of the previous statement would be:

  	

Hello, I am 24 years old and my zipcode is 90064 



It is important to notice that cout does not add 
a line break after its output unless we explicitly indicate it, therefore, the following statements:


cout << "This is a sentence.";
cout << "This is another sentence."; 



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cout << "First sentence.\n ";
cout << "Second sentence.\nThird sentence."; 



This produces the following output:


First sentence.
Second sentence.
Third sentence.


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Additionally, to add a new-line, you may also use 
the endl manipulator. For example:
	

cout << "First sentence." << endl;
cout << "Second sentence." << endl; 



would print out:


First sentence.
Second sentence.


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Standard Input (cin)

The standard input device is usually the keyboard. Handling the standard input in C++ is done by 
applying the overloaded operator of extraction (>>)
 on the cin stream. The operator must be followed 
by the variable that will store the data that is 
going to be extracted from the stream. For example:
	

int age;
cin >> age; 



The first statement declares a variable of type 
int called age, and the second one waits for an 
input from cin (the keyboard) in order to store 
it in this integer variable.

cin can only process the input from the keyboard 
once the RETURN key has been pressed. Therefore, 
even if you request a single character, the 
extraction from cin will not process the input 
until the user presses RETURN after the character
 has been introduced.

You must always consider the type of the variable 
that you are using as a container with cin extractions. 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.

	

// i/o example

#include <iostream>
using namespace std;

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;
}

output:	

      Please enter an integer value: 702
      The value you entered is 702 and its double is 1404.


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The user of a program may be one of the factors 
that generate errors even in the simplest programs that use cin (like the one we have just seen). 
Since if you request an integer value and the 
user introduces a name (which generally 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 cin extractions you will 
have to trust that the user of your program will 
be cooperative and that he/she will not introduce his/her name or something similar when an integer value is requested. A little ahead, when we see 
the stringstream class we will see a possible 
solution for the errors that can be caused
by this type of user input.

You can also use cin to request more than one 
datum input from the user:

  	

cin >> a >> b;



is equivalent to:


cin >> a;
cin >> b;



In both cases the user must give two data, one 
for variable a and another one for variable b that 
may be separated by any valid blank separator: a space, a tab character or a newline.


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cin and strings
We can use cin to get strings with the extraction operator (>>) as we do with fundamental data type variables:

  	

cin >> mystring;



However, as it has been said, cin extraction 
stops reading as soon as if finds any blank 
space character, so in this case we will be able
to get just one word for each extraction. 
This behavior may or may not be what we want; 
for example if we want to get a sentence from 
the user, this extraction operation would not
be useful.

In order to get entire lines, we can use the function getline, which is the more recommendable way to get user input with cin:

	

// cin with strings
#include <iostream>
#include <string>
using namespace std;

int main ()
{
      string mystr;

      cout << "What's your name? ";

      getline (cin, mystr);

      cout << "Hello " << mystr << ".\n";

      cout << "What is your favorite team? ";

      getline (cin, mystr);

      cout << "I like " << mystr << " too!\n";
   
      return 0;
}

output:	

       What's your name? Jessica
       Hello Jessica.
       What is your favorite team? The Isotopes
       I like The Isotopes too!


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stringstream


The standard header file <sstream> defines a 
class called stringstream that allows a string
-based object to be treated as a stream. This 
way we can perform extraction or insertion 
operations from/to strings, which is especially
useful to convert strings to numerical values and
vice versa. For example, if we want to extract 
an integer from a string we can write:


string mystr ("1204");
int myint;
stringstream(mystr) >> myint;



This declares a string object with a value of "1204", and an int object. Then we use stringstream's constructor to construct an object of this type 
from the string object. Because we can use stringstream objects as if they were streams, 
we can extract an integer from it as we would 
have done on cin by applying the extractor 
operator (>>) on it followed by a variable of 
type int.

After this piece of code, the variable myint will contain the numerical value 1204.


// stringstreams
#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int main ()
{
      string mystr;
      float price=0;
      int quantity=0;

      cout << "Enter price: ";

      getline (cin,mystr);

      stringstream(mystr) >> price;

      cout << "Enter quantity: ";

      getline (cin,mystr);

      stringstream(mystr) >> quantity;

      cout << "Total price: " << price*quantity <<  endl;

  return 0;
}

output:	

       Enter price: 22.25
       Enter quantity: 7
       Total price: 155.75



In this example, we acquire numeric values from 
the standard input indirectly. Instead of 
extracting numeric values directly from the 
standard input, we get lines from the standard 
input (cin) into a string object (mystr), and 
then we extract the integer values from this 
string into a variable of type int (quantity).

Using this method, instead of direct extractions 
of integer values, we have more control over what happens with the input of numeric values from the user, since we are separating the process of 
obtaining input from the user (we now simply ask 
for lines) with the interpretation of that input. Therefore, this method is usually preferred to 
get numerical values from the user in all 
programs that are intensive in user input.