Fortran

FORmula TRANslation

Developed at IBM (John Backus)

Development cycle: Write, compile, link, execute, test/debug, change

Fortran I

• Names up to six chars
• Post-test counting loop (do)
• Formatted I/O
• User-defined subprograms
• Arithmetic IF (3-way - IF negative, zero, positive)
• No data typing - i,j,k,l,m,n prefixed are int, others float
• Very fast for that era

Evolution

• Before 90, it required a fix format (historically due to the use of punch cards)

  • Line 1-5: Label (C indicates comment line)
  • Line 6: Any char => continuation line
  • Line 7-72: Statements
  • Line 73-80: Optional Line #s
• Compiled language
• Before 90, memory is allocated statically at compile time
• Before 90, no recursion support
• Before 90, all in capital
• Since 90, free format allowed, ! for comments

Structure

• Program units

  • Program
  • Function
  • Subroutine
  • Block data

Program units may contain internal functions and subroutines.

• Variables are typed (nowadays)

  • integer: integer :: value
  • real/double precision: real :: value
  • complex: complex :: value
  • character: character*20 :: value value = 'abcd'
  • logical: logical :: err err = .false.
• Case insensitive (var is same as VAR)

• Arrays: integer :: a dimension a(10)

  • Shortcut: real :: a(10, 10, 10)
  • Linear allocation of memory
  • Up to 7 dimensions
  • Index starting at 1

• Constants (parameter statement)

  • Old: PARAMETER (MAX=20)
  • New: integer, parameter :: max=20

• Control statements

  • Branching (goto)
  • Comparison (if)
  • Looping (do)
  • Subroutine invocation (call)
  • Do-nothing (continue)

Implicit typing: If variable is not declared when used, then it is created at compile time based on first character. Starting from 77, implicit none can be used to require explicit typing (good practice).

Subroutine

• Internal or external
• No return value
• All arguments are passed by reference

    subroutine mult(x,y,result)
    [...]
    end

    call mult(2,3,result)
    ! result contains 6

Function

• Internal or external
• Has return value

    function mult(x,y)
    [...]
    end

    result = mult(2,3)
    ! result contains 6

Block Data

• Named shared memory areas

    pogramm a
    common /myblock/ a,b,c

    programm b
    common /myblock/ a,b,c

Branching

• Computed goto can be used to create switch statements

    if (i .eq. 0) go to 100
    [...]
100 a = 1
    [...]
200 a = 2

I/O

• Formatted or unformatted/binary
• Sequential or random

Parallel programming

• Support via OMP