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4. Data types & constants

4. Data types & constants

FORTRAN 77 has six data types (§4): integer, real, double precision, complex, logical, and character. This chapter shows how to write a literal value (a constant) of each type, and how a name takes on a type. Declaring variables of these types is Chapter 8; this chapter is about the values themselves.

How a name gets its type

A variable's type is fixed once, in one of three ways (§4.1.2), in order of authority:

  1. an explicit type-statement — REAL X, CHARACTER*8 NAME;
  2. an IMPLICIT rule — IMPLICIT INTEGER (A-H);
  3. otherwise the implicit default: a name beginning with I, J, K, L, M, N is integer; any other initial letter is real.
      REAL    MASS          ! explicit: MASS is real despite starting with M
      I = 3                 ! implicit: I is integer (I-N)
      X = 2.5               ! implicit: X is real

Integer constants

A whole number, optionally signed: 0, 42, -7, +100 (§4.3). No decimal point, no exponent. The magnitude is limited by the machine's integer size.

Real constants

A single-precision floating-point number (§4.4). It needs either a decimal point or an exponent (or both). All of these are valid reals:

      3.14          .5            5.            5E3
      5.0E3         -2.5          1.0E-10       +0.0
  • .5 (no integer part) and 5. (no fractional part) are both fine; you cannot omit both.
  • E introduces a base-10 exponent: 5E3 is 5000.0, 1.0E-10 is 10⁻¹⁰.
      N(1) = 5.        ! 5.0
      N(2) = .5        ! 0.5
      N(3) = 5E3       ! 5000.0

Double-precision constants

Like a real constant but with a D exponent instead of E, which makes it double precision (§4.5). Double precision carries more significant digits than real.

      DOUBLE PRECISION PI
      PI = 3.141592653589793D0
      X  = 2.0D0           ! 2.0, double precision

A D exponent is what distinguishes 2.0D0 (double) from 2.0E0 (single).

Complex constants

A pair (real_part, imaginary_part), each part an optionally-signed real or integer constant (§4.6). It occupies two storage units (real then imaginary).

      COMPLEX Z
      Z = (1.0, 2.0)       ! 1 + 2i
      Z = (3.0, -4.0)      ! 3 - 4i

Logical constants

Exactly two: .TRUE. and .FALSE. (§4.7) — note the surrounding dots.

      LOGICAL FLAG
      FLAG = .TRUE.

Character constants

A string of characters between apostrophes (§4.8). The string must be non-empty. Blanks inside are significant (they are part of the value). To put an apostrophe in the string, write two apostrophes:

      'HELLO'              ! 5 characters
      'O''CLOCK'           ! 7 characters: O'CLOCK   (the '' is one apostrophe)
      'A B'                ! 3 characters: A, blank, B

The length of a character constant is the number of characters it represents (each '' counts as one). The full CHARACTER story — declaration, padding, concatenation, substrings — is in Chapter 5 and Chapter 8.

Signed zero

Zero is neither positive nor negative: -0.0 has the same value as 0.0, and they compare equal (§4.1.3).

      X = -0.0
      IF (X .EQ. 0.0) ...      ! true

Named constants

You can give a constant a name with the PARAMETER statement, then use the name wherever a constant is allowed:

      REAL PI
      PARAMETER (PI = 3.14159)
      AREA = PI * R * R

PARAMETER is a specification statement; its full rules (the constant expression, ordering, typing) are in Chapter 8. A PARAMETER name is a constant, not a variable — you cannot assign to it.

forterp notes. Two points about the value model:

  • On forterp's default NATIVE target, both REAL and DOUBLE PRECISION are the host's 64-bit float, so double precision is not more precise than real (the standard requires it to be). This only matters for a program that depends on the precision difference; the faithful PDP10 target reproduces the genuine 36-bit single / two-word double split. (This is the one documented divergence that can affect a conforming program — see Appendix D.)
  • The standard requires a character constant to be non-empty; forterp rejects the empty constant '' on every dialect (a zero-length string is meaningless in F77 and is a Fortran-90 feature). A doubled apostrophe inside a string ('O''CLOCK') is unaffected — that is an embedded apostrophe, not an empty string.

The Hollerith constant (5HHELLO) of older FORTRAN was replaced by CHARACTER in F77 and is not part of the F77 dialect; it remains available under F66/FORTRAN10.