D - Literals

Constant values that are typed in the program as a part of the source code are called literals.
Literals can be of any of the basic data types and can be divided into Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean Values.

Again, literals are treated just like regular variables except that their values cannot be modified after their definition.

Integer literals:

An integer literal can be a of the folowing types:

• Decimal uses the normal number represention with the first digit cannot be 0 as that digit is reserved for indicating the octal system.This does not include 0 on its own: 0 is zero.
• Octal uses 0 as prefix to number.
• Binary uses 0b or 0B as prefix
• Hexadecimal uses 0x or 0X as prefix.

An integer literal can also have a suffix that is a combination of U and L, for unsigned and long, respectively. The suffix can be uppercase or lowercase and can be in any order.
When you dont use a suffix the compiler itself chooses between int, uint, long and ulong based on the magnitude of the value.
Here are some examples of integer literals:

212         // Legal
215u        // Legal
0xFeeL      // Legal
078         // Illegal: 8 is not an octal digit
032UU       // Illegal: cannot repeat a suffix

Following are other examples of various types of Integer literals:

85         // decimal
0213       // octal
0x4b       // hexadecimal
30         // int
30u        // unsigned int
30l        // long
30ul       // unsigned long
0b001      // binary

Floating-point literals:

The floating point literals can be specified in either the decimal system as in 1.568 or in the hexadecimal system as in 0x91.bc.
In the decimal system, an exponent can be represented by adding the character e or E and a number after that. For example, 2.3e4 means "2.3 times 10 to the power of 4". A + character may be specified before the value of the exponent, but it has no effect. For example 2.3e4 and 2.3e + 4 are the same. The - character added before the value of the exponent changes the meaning to be "divided by 10 to the power of". For example, 2.3e-2 means "2.3 divided by 10 to the power of 2".
In the hexadecimal system, the value starts with either 0x or 0X. The exponent is specified by p or P instead of e or E. The exponent does not mean "10 to the power of", but "2 to the power of". For example, the P4 in 0xabc.defP4 means "abc.de times 2 to the power of 4".
Here are some examples of floating-point literals:

3.14159       // Legal
314159E-5L    // Legal
510E          // Illegal: incomplete exponent
210f          // Illegal: no decimal or exponent
.e55          // Illegal: missing integer or fraction
0xabc.defP4   // Legal Hexa decimal with exponent
0xabc.defe4   // Legal Hexa decimal without exponent.

By default, the type of a floating point literal is double. The f and F mean float, and the L specifier means real.

Boolean literals:

There are two Boolean literals and they are part of standard D keywords:

• A value of true representing true.
• A value of false representing false.

You should not consider the value of true equal to 1 and value of false equal to 0.

Character literals:

Character literals are enclosed in single quotes.
A character literal can be a plain character (e.g., 'x'), an escape sequence (e.g., '\t'), ASCII character (e.g., '\x21'), Unicode character (e.g., '\u011e') or as named character (e.g. '\©','\♥', '\€' ).
There are certain characters in D when they are preceded by a backslash they will have special meaning and they are used to represent like newline (\n) or tab (\t). Here, you have a list of some of such escape sequence codes:

Escape sequence	Meaning
\\	\ character
\'	' character
\"	" character
\?	? character
\a	Alert or bell
\b	Backspace
\f	Form feed
\n	Newline
\r	Carriage return
\t	Horizontal tab
\v	Vertical tab

Following is the example to show few escape sequence characters:

import std.stdio;

int main(string[] args)
{
   writefln("Hello\tWorld%c\n",'\x21');
   writefln("Have a good day%c",'\x21');
   return 0;
}

When the above code is compiled and executed, it produces the following result:

Hello   World!

Have a good day!

String literals:

String literals are enclosed in double quotes. A string contains characters that are similar to character literals: plain characters, escape sequences, and universal characters.
You can break a long line into multiple lines using string literals and separate them using whitespaces.
Here are some examples of string literals.

import std.stdio;

int main(string[] args)
{
    writeln(q"MY_DELIMITER
Hello World
Have a good day
MY_DELIMITER");

    writefln("Have a good day%c",'\x21');
    auto str = q{int value = 20; ++value;};
    writeln(str);

In the above example, you can find the use of q"MY_DELIMITER MY_DELIMITER" to represent multi line characters. Also, you can see q{} to represent an D language statement itself.