### 3.1.2.1 General Rules for Real Constants

A real constant approximates the value of a mathematical real number. The value of the constant can be positive, zero, or negative.

The following is the general form of a real constant with no exponent part:

  [s]n[n...][ _k]

A real constant with an exponent part has one of the following forms:

  [s]n[n...]E[s]nn...[_k]
  [s]n[n...]D[s]nn...
  [s]n[n...]Q[s]nn...
  s
  Is a sign; required if negative (-), optional if positive (+).
  n
  Is a decimal digit (0 through 9). A decimal point must appear if the real constant has no exponent part.
  k
  Is the optional kind parameter: 4 for REAL(4), 8 for REAL(8), or 16 for REAL(16) (VMS, U*X). It must be preceded by an underscore ( _ ).

Rules and Behavior

Leading zeros (zeros to the left of the first nonzero digit) are ignored in counting significant digits. For example, in the constant 0.00001234567, all of the nonzero digits, and none of the zeros, are significant. (See the following sections for the number of significant digits each kind type parameter typically has).

The exponent represents a power of 10 by which the preceding real or integer constant is to be multiplied (for example, 1.0E6 represents the value 1.0 * 10**6).

A real constant with no exponent part and no kind type parameter is (by default) a single-precision (REAL(4)) constant. You can change the default behavior by specifying the compiler option that controls the default real kind.

If the real constant has no exponent part, a decimal point must appear in the string (anywhere before the optional kind parameter). If there is an exponent part, a decimal point is optional in the string preceding the exponent part; the exponent part must not contain a decimal point.

The exponent letter E denotes a single-precision real (REAL(4)) constant, unless the optional kind parameter specifies otherwise. For example, -9.E2_8 is a double-precision constant (which can also be written as -9.D2).

The exponent letter D denotes a double-precision real (REAL(8)) constant.

On OpenVMS, Tru64 UNIX, and Linux systems, the exponent letter Q denotes a quad-precision real (REAL(16)) constant.

A minus sign must appear before a negative real constant; a plus sign is optional before a positive constant. Similarly, a minus sign must appear between the exponent letter (E, D, or Q) and a negative exponent, whereas a plus sign is optional between the exponent letter and a positive exponent.

If the real constant includes an exponent letter, the exponent field cannot be omitted, but it can be zero.

To specify a real constant using both an exponent letter and a kind parameter, the exponent letter must be E, and the kind parameter must follow the exponent part.

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