Cout and Cin
To interact with the user we need a way to communicate results and take inputs respectively to and from the outside world.
To do that we use the Command Prompt with the help of two predefined variables located in the
iostream library, called cout and cin.
Cout
The cout is used when we want to print some text or a value to the console as an output.
The world itself stands for "character output".
We already encountered this command in the previous chapter, when talking about printing "Hello World!". This command will be used a lot in our programs.
Syntax
To print something using cout we have to write two angular brackets << (insertion operator)
after that command, and the text between double quotation marks/quotes "", like as follows:
// ... linking section and main()
cout << "some text";
In C and in C++ double quotes "" are used for string variables (objects that represent
sequences of characters). Technically a string is a one-dimensional array of characters,
called string literal. Don't use single quotes '' for more than one character, since single
quotes can only represent 1 single character (char data type), which is called character literal.
So 'a' is a single character literal, while "a" is a string literal containing an 'a'
and a null terminator \0 (that is a 2 char array).
We can go deeper in the explanation on the caution note above. Try to print a string with single quotes, like here:
#include <iostream>
using namespace std;
int main() {
cout << 'text' << endl;
return 0;
}
The output will be 1952807028. Why on earth does this number turn up?
That decimal number is a multi-character literal. It corresponds to the hexadecimal
0x74657874 or the binary 01110100011001010111100001110100. You can easily convert it
here if you want to
check.
If we separate this long binary number into groups of 8 bits (1 byte) excluding the
first "0" on the left for each group (so 7 bits) and then search for
the corresponding number by comparing it with the
ASCII table,
we find t (01110100),
e (01100101),
x (01111000) and
t (01110100)
again, just like the message we have put between single quotes!
That's because computer programs (in most cases) use the ASCII (American Standard Code for Information Interchange) code to represent characters, and each of them has a number that uniquely identifies it. If you still haven't understood how to convert from binary to ASCII code you can watch this video explanation.
We can also print values or variables, and in this case the double quotation marks "" mustn't
be used.
int a = 5;
cout << a;
We will see what a variable is in a subsequent lesson.
In addition to that, we can also concatenate more than one thing on the same line using the
insertion operator << as many times as we want to print multiple pieces of output.
int a = 5;
cout << "The value stored in a is: " << a;
I've inserted a space before and after the insertion operator <<, and the same for the equal
sign = one line up. For both of these cases the space is optional, choose whether to put it
or not according to your preferences. Remember to be always consistent with your choice, though.
Little calculations can also be done here (keeping in mind the order of operations).
int a = 5;
cout << a+2*(6-3);
However, it's better to do calculations outside of cout.
Endl
If we want to print two or more outputs to the console into separate lines, we need the endl
command. As in the case of the word "cout", the word itself stands for “end line”.
cout << "new line" << endl;
Alternatively, to put a line break we can also use the escape sequence \n. When it is used
by itself to move the cursor to the next line, the single quotes are needed, but when embedded
into text that is already double-quoted, the single quotes aren't needed.
cout << "new line\n";
cout << "new line" << '\n';
You can put freely both endl and \n before and/or after the text or variable that you
want to print, but be aware that changing the order will result in a different behavior.
Escape sequences
As I said, \n is an escape sequence, but it's not the only one. The other ones are listed in the
table1 below:
| Escape sequence | Character represented |
|---|---|
\a | Alert (Beep, Bell) (added in C89) |
\b | Backspace |
\e | Escape character |
\f | Formfeed Page Break |
\n | Newline (Line Feed) |
\r | Carriage Return |
\t | Horizontal Tab |
\v | Vertical Tab |
\\ | Backslash |
\' | Apostrophe or single quotation mark |
\" | Double quotation mark |
\? | Question mark (used to avoid trigraphs) |
\0 | Null character, with value zero |
Cin
Whereas cout prints data to the console using the insertion operator <<, cin reads
input from the keyboard using the extraction operator >>. Cin stands for "character input".
To do that, the input inserted by the user must be stored in a variable to be used. If a value is already stored in that variable, it will be replaced.
Every time you use cin and ask for an input, the program will pause and wait for the input
and the ENTER/RETURN ⏎ key to confirm.
Try this program yourself:
#include <iostream>
using namespace std;
int main() {
int x; // declare an empty variable x to hold user input
cout << "Enter a number: "; // ask the user for a number and go to a new line
cin >>x ; // get number from keyboard and store it in x
cout << "You entered " << x << endl;
return 0;
}
When you run the program it will print “Enter a number: “, then wait for you to enter an input. Once you enter a number (and press enter), the number you enter will be assigned to variable x. Finally, on line 10, the program will print “You entered ” followed by the number you just entered.
Of course this program it's not very useful, but it shows how cin works and how it can be
combined with cout to interact with the user.
Multiple cin can also be concatenated one after the other just like we do with cout:
#include <iostream>
using namespace std;
int main() {
int a, b, c;
int sumOfValues;
cout << "Insert 3 values: " << endl;
cin >> a >> b >> c; // the order must be respected
sumOfValues = a+b+c;
float average = float(sumOfValues/3);
cout << "The average is: " << average << endl;
return 0;
}