1. Overview & program structure

FORTRAN 77 is a language for numerical and scientific computing, standardized as ANSI X3.9-1978. It is compact and close to the machine of its era: fixed-format source, a handful of data types, arrays, and a rich formatted-I/O system. This chapter shows you a whole program, names its parts, and explains how it runs — the details of each part follow in later chapters.

Your first program

      PROGRAM AREA
C     Area of a circle of radius 2.
      REAL R, A
      R = 2.0
      A = 3.14159 * R * R
      WRITE (6, 10) R, A
   10 FORMAT (' radius =', F5.2, '   area =', F7.3)
      END

Running it prints:

 radius = 2.00   area = 12.566

Reading it top to bottom:

PROGRAM AREA names the main program. The name is for your benefit; it is optional (a program with no PROGRAM line is still a valid main program).
C in column 1 marks a comment line — the whole line is ignored.
REAL R, A is a declaration: it tells the compiler that R and A hold real (floating-point) numbers. Declarations describe data; they do not do anything when the program runs.
R = 2.0 and A = 3.14159 * R * R are assignments — they compute a value and store it in a variable. These are executable statements: they happen, in order, when the program runs.
WRITE (6, 10) R, A sends R and A to output unit 6 (the standard output), formatted according to the FORMAT statement labelled 10.
10 FORMAT (...) describes how the numbers are laid out as text. F5.2 means "a real number in a 5-character field with 2 decimals"; the quoted pieces are printed literally. The 10 in columns 1–5 is its statement label, referenced by the WRITE.
END marks the physical end of the program unit. Reaching END in a main program stops the program.

Notice the indentation: real code starts in column 7. The first six columns are reserved (for comment markers, statement labels, and line continuation). That fixed layout is the one quirk that surprises newcomers; Chapter 3 covers it fully. For now, just start each statement at column 7 and put any label in columns 1–5.

What a program is made of

A complete FORTRAN program is one executable program, built from one or more program units. There are four kinds:

Program unit	Purpose	Begins with
Main program	where execution starts; exactly one per program	`PROGRAM` (optional)
Subroutine	a reusable action, invoked with `CALL`	`SUBROUTINE`
Function	a reusable computation that returns a value, used in an expression	`[type] FUNCTION`
Block data	supplies initial values for `COMMON` blocks; does nothing at run time	`BLOCK DATA`

Subroutines and functions are collectively called subprograms or procedures. Here is a program with a function alongside the main program:

      PROGRAM SQ
      INTEGER N
      N = ISQ(7)
      WRITE (6, *) N
      END

      INTEGER FUNCTION ISQ(K)
      ISQ = K * K
      RETURN
      END

→ prints 49. (WRITE (6, *) is list-directed output — "format it sensibly for me"; see Chapter 12.)

Each program unit is independent: it has its own variables and its own statement labels. Units share data only through argument lists (Chapter 15) or COMMON (Chapter 8). The program units may appear in any order in the source; the one that runs first is always the main program, regardless of where it sits.

How a program runs

Execution begins at the first executable statement of the main program and proceeds one statement after another, in source order, until something redirects it:

a control statement (GO TO, an IF, a DO loop — Chapter 11) jumps or loops,
a CALL or function reference runs a subprogram and returns,
STOP (or reaching the main program's END) ends the program.

Nonexecutable statements — declarations, FORMAT, DATA, PARAMETER, and so on — are not "reached" in this flow; they inform the compiler and may appear before the executable statements (the exact ordering rules are in Chapter 7).

Running these examples

Every example in this manual runs on forterp. The shortest way:

import forterp
forterp.run_source(src, dialect=forterp.F77, printer=print)

or from the command line, forterp --std f77 prog.for. See the Python API guide for the dialect switches, targets, and tunable knobs, and Command-line tools for the command line.

forterp notes. forterp implements the full F77 language (not the subset level). The F77 dialect is one of three forterp offers — F66 (the 1966 base), FORTRAN10 (the DEC FORTRAN-10 superset), and F77 — selected with dialect=forterp.F77. Standard output (unit 6) defaults to a terminal model under F77, matching modern compilers; the classic line-printer carriage control is the default under F66/FORTRAN10 and is tunable (see Chapter 13). Where a program does something the standard prohibits, forterp's response is described in the relevant chapter's notes and summarized in Appendix D.