Constants

 now we understand Constants in detail :::::::::::
Constants are expressions with a fixed value.
Literals
Literals are used to express particular values within the source code of a program. We have already used these
previously to give concrete values to variables or to express messages we wanted our programs to print out, for
example, when we wrote:
a = 5;
the 5 in this piece of code was a literal constant.
Literal constants can be divided in Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean
Values.
Integer Numerals
1776
707
-273
They are numerical constants that identify integer decimal values. Notice that to express a numerical constant we
do not have to write quotes (") nor any special character. There is no doubt that it is a constant: whenever we
write 1776 in a program, we will be referring to the value 1776.
In addition to decimal numbers (those that all of us are used to use every day) C++ allows the use as literal
constants of octal numbers (base 8) and hexadecimal numbers (base 16). If we want to express an octal number
we have to precede it with a 0 (zero character). And in order to express a hexadecimal number we have to precede
it with the characters 0x (zero, x). For example, the following literal constants are all equivalent to each other:
75 // decimal
0113 // octal
0x4b // hexadecimal
All of these represent the same number: 75 (seventy-five) expressed as a base-10 numeral, octal numeral and
hexadecimal numeral, respectively.
Literal constants, like variables, are considered to have a specific data type. By default, integer literals are of type
int. However, we can force them to either be unsigned by appending the u character to it, or long by appending l:
75 // int
75u // unsigned int
75l // long
75ul // unsigned long
In both cases, the suffix can be specified using either upper or lowercase letters.
Floating Point Numbers
They express numbers with decimals and/or exponents. They can include either a decimal point, an e character
(that expresses "by ten at the Xth height", where X is an integer value that follows the e character), or both a
decimal point and an e character:
3.14159 // 3.14159
6.02e23 // 6.02 x 10^23
1.6e-19 // 1.6 x 10^-19
3.0 // 3.0
These are four valid numbers with decimals expressed in C++. The first number is PI, the second one is the
number of Avogadro, the third is the electric charge of an electron (an extremely small number) -all of them
approximated- and the last one is the number three expressed as a floating-point numeric literal.
The default type for floating point literals is double. If you explicitly want to express a float or long double
numerical literal, you can use the f or l suffixes respectively:
3.14159L // long double
6.02e23f // float
Any of the letters that can be part of a floating-point numerical constant (e, f, l) can be written using either lower
or uppercase letters without any difference in their meanings.
Character and string literals
There also exist non-numerical constants, like:
'z'
'p'
"Hello world"
"How do you do?"
The first two expressions represent single character constants, and the following two represent string literals
composed of several characters. Notice that to represent a single character we enclose it between single quotes (')
and to express a string (which generally consists of more than one character) we enclose it between double quotes
(").
When writing both single character and string literals, it is necessary to put the quotation marks surrounding them
to distinguish them from possible variable identifiers or reserved keywords. Notice the difference between these
two expressions:
x
'x'
x alone would refer to a variable whose identifier is x, whereas 'x' (enclosed within single quotation marks) would
refer to the character constant 'x'.
Character and string literals have certain peculiarities, like the escape codes. These are special characters that are
difficult or impossible to express otherwise in the source code of a program, like newline (\n) or tab (\t). All of
them are preceded by a backslash (\). Here you have a list of some of such escape codes:
\n newline
\r carriage return
\t tab
\v vertical tab
\b backspace
\f form feed (page feed)
\a alert (beep)
\' single quote (')
\" double quote (")
\? question mark (?)
\\ backslash (\)
For example:
'\n'
'\t'
"Left \t Right"
"one\ntwo\nthree"
Additionally, you can express any character by its numerical ASCII code by writing a backslash character (\)
followed by the ASCII code expressed as an octal (base-8) or hexadecimal (base-16) number. In the first case
(octal) the digits must immediately follow the backslash (for example \23 or \40), in the second case
(hexadecimal), an x character must be written before the digits themselves (for example \x20 or \x4A).
String literals can extend to more than a single line of code by putting a backslash sign (\) at the end of each
unfinished line.
"string expressed in \
two lines"
You can also concatenate several string constants separating them by one or several blank spaces, tabulators,
newline or any other valid blank character:
"this forms" "a single" "string" "of characters"
Finally, if we want the string literal to be explicitly made of wide characters (wchar_t), instead of narrow characters
(char), we can precede the constant with the L prefix:
L"This is a wide character string"
Wide characters are used mainly to represent non-English or exotic character sets.
Boolean literals
There are only two valid Boolean values: true and false. These can be expressed in C++ as values of type bool by
using the Boolean literals true and false.