9. The `DATA` statement

DATA supplies initial values at compile time — the value a variable or array holds before the first executable statement runs (§9). It is the way to set up constant tables, lookup arrays, and starting values without spending executable assignments on them.

Form

      DATA  nlist / clist / [ , nlist / clist / ] ...

nlist is a list of variables, array elements, arrays, or substrings; clist is the matching list of constants. The two lists are matched one constant per storage element, left to right.

      INTEGER N
      REAL    X, Y
      DATA    N /0/, X /1.5/, Y /2.5/

Repeat counts

A constant may be prefixed r* to repeat it r times — handy for filling an array:

      REAL V(5)
      DATA V /5 * 0.0/            ! all five elements set to 0.0
      DATA V /3 * 1.0, 2 * 9.0/   ! 1.0 1.0 1.0 9.0 9.0

Whole arrays and implied-DO

An unsubscripted array name takes one constant per element, in storage order (column-major, Chapter 5):

      INTEGER T(3)
      DATA    T /10, 20, 30/      ! T(1)=10 T(2)=20 T(3)=30

An implied-DO initializes selected elements with a loop-like list:

      REAL A(100)
      DATA (A(I), I = 1, 100) / 100 * 0.0 /     ! zero the whole array
      DATA (A(I), I = 1, 10, 2) / 5 * 1.0 /     ! A(1),A(3),A(5),A(7),A(9) = 1.0

Type rules

A numeric constant is converted to the type of the entity it initializes (as in assignment, Chapter 10).
A CHARACTER entity is initialized from a character constant: shorter is blank-padded on the right, longer is truncated.
A LOGICAL entity takes .TRUE./.FALSE..

      CHARACTER*5 TAG
      DATA        TAG /'AB'/      ! TAG = 'AB   '  (padded to 5)

What may (and may not) be initialized

May: local variables and arrays; named-common entities only inside a BLOCK DATA subprogram (Chapter 16).
May not: blank-common entities, and dummy arguments.

A DATA statement may appear anywhere after the specification statements (it is not required to be grouped with them), but it only sets the initial value — it is not re-executed if control passes it again.

forterp notes. DATA is implemented as specified — repeat counts, implied-DO, column-major array fill, and the character pad/truncate rule all behave per the standard. (A BLOCK DATA that initializes named common is Chapter 16, where forterp also enforces the "at most one unnamed block data" rule.)