# Fortran Constants

Constants or more formally literal constants are the tokens used to denote the value of a particular type. Fortran has five types of constants: integer, real, complex, logical, and character string.
• Integer Constants: a string of digits with an optional sign:
• Correct Examples: 0, -345, 768, +12345
• Incorrect Examples:
• 1,234 : comma is not allowed
• 12.0: no decimal point
• --4 and ++3: too many optional signs
• 5- and 7+: the optional sign must precede the string of digits
• Real Constants: There are two representations, decimal representation and exponential representation.
• Decimal Representation: A decimal point must be presented, but no commas are allowed. A real constant can have an optional sign.
• Correct Examples: 23.45, .123, 123., -0.12, -.12
• Incorrect Examples:
• 12,345.95: no comma is allowed
• 75: real constant must have a decimal point
• 123.5-: the optional sign must precede the number
• \$12.34: cannot use dollar sign \$
• Exponential Representation: It consists of an integer or a real number in decimal representation (the mantissa or fractional part), followed by the letter E or e, followed by an integer (the exponent).
• Correct Examples
• 12.3456E2 or 12.3456e2: this is equal to 1234.56
• -3.14E1 or -3.14e1: this is equal to -31.4
• -1.2E-3 or -1.2e-3: this is equal to -0.0012
• 12E3 or 12e3: this is equal to 12000.0
• 0E0 or 0e0: this is equal to 0.0
• Incorrect Examples
• 12.34E1.2: the exponential part must be an integer constant
• 12.34-5: there is no exponential sign E or e
• Complex: Will not be covered in this course
• Logical: See Chapter 3
• Character String: Character constants must be enclosed between double quotes or apostrophes (single quotes).
The content of a string consists of all characters, spaces included, between the single or quote quotes, while the length of the string is the number of characters of its content. The content of a string can be zero and in this case it is an empty string
• Correct Examples:
• 'John' and "John": content = John and length = 4
• ' ' and " ": content = a single space and length = 1
• 'John Dow #2' and "John Dow #2": content = John Dow #2 and length = 11
• '' and "": content = nothing and length = 0 (empty string)
• Incorrect Examples:
• 'you and me: the closing apostrophe is missing
• Hello, world': the opening apostrophe is missing
• 'Hi" and "Hi': the opening and closing quotes do not match.
If single quote is used in a string, then double quotes should be used to enclose the string:
```"Lori's apple"
```
This string has content Lori's apple and length 12. Alternatively, you can write the single quote twice as follows:
```'Lori''s apple'
```
The compiler will treat a pair of single quotes in the content of a string as one. Thus, the content of the above string is still Lori's apple.
• Correct Examples:
• 'What''s this?': content = What's this? and length = 11
• '''''': content = '' and length = 2
• Incorrect Examples:
• 'Tech's seminar': the single quote between h and s should be written twice.