FORTRAN

FORmula TRANslator

-Anand Trivedi

HISTORY

Designed and written from scratch in 1954-57 by an IBM team lead by John W. Backus as the first ever High Level Language

Direct competition with assembler compelled it to have a fast, well optimized code

INTRODUCTION

Fortran is a general purpose programming language, mainly intended for engineering & scientific computation

Browse over its most popular version Fortran –77(in 1977)

LEXICAL ASPECTS

Input format : Formerly Punch cards Not a free format language Column position rules :

Col. 1 : Blank, or a "c" or "*" for comments Col. 2-5 : Statement label (optional) Col. 6 : Continuation of previous line (optional) Col. 7-72 : Statements Col. 73-80: Sequence number (optional, rarely used today)

Delimiters : End of the line Blank space ignored Variable names of 1-6 characters (A-Z, 0-9). The first character must be

a letter.

EXPRESSIONS

Arithmetic Operators : **, /, *, -, +

Relational Operators :.LT., .LE., .GE., .GT., .EQ., and .NE.

Logical Operators : .NOT., .AND., .OR., .EQV. , .NEQV. eg: logical a, b

a = .TRUE.

b = a .AND. 3 .LT. 5/2

Arithmetic expressions are evaluated first, then relational operators, and finally logical operators

DATA TYPES-I

Six data types are explicitly permitted: INTEGER (0,25,+25,-25) REAL (-1.5,3E5, +.123E-3) DOUBLE PRECISION (1D2, 6.89D-8) COMPLEX ((-10,5), (.4E2,-.31E-1) LOGICAL(.TRUE., .FALSE.) CHARACTER(‘Don’’t’, ’A1 PLC+/’)

DATA TYPES-II

Each variable has to be declared explicitly Implicit rule : All variables starting with the

letters i-n are integers and all others are real CONSTANT : By using PARAMETER

statement eg. parameter (pi = 3.14159)

SAMPLE EXAMPLE- I

1234567890123456789012345678901234567890 program circle real r, area pi parameter (pi = 3.14159)C write & read statements for I/p O/p

write (*,*) 'Give radius r:' read (*,*) r area = pi*r*r write (*,*) 'Area = ', area stop end

DATA TYPES-III

Supports multiple assignments :

eg: data m,n/10,20/, x,y/2*2.5/

or data m/10/, n/20/, x/2.5/, y/2.5/

DATA TYPES : ARRAYS

The only complex data structure Index starts from 1 onwards : INTEGER i(10) , REAL a(12), REAL b(*) However these are also valid :

REAL b(0:19), REA: weird(-162:237) Allows arrays of up to seven dimensions REAL a(3,5), REAL b(2,0:3) By default values are not Zero. Array values are not checked before being used.

CONTROL STATEMENTS-I

GOTO statement : GOTO label IF statement :

Arithmetic if : IF (e)s1,s2,s3 Eg. IF((A+B)*2)100,200,300

Logical if : IF(e)statement Eg.IF(A.LT.0.)a=0.0

IF-THEN-ELSE Statement :If (e) THEN [statements]

Else [statements] END IF

Nested IF allowed

CONTROL STATEMENTS-II

CONTINUE Just one type of loop : DO loop eg integer i, n, sum n = 10 DO 10 i = 0, n,2 write(*,*) 'i =', i 10 CONTINUE No recursion (static allocation)

FUNCTIONS

Inbuilt functions like : abs, sin, cos etc Define own functions :

real function r(m,t)

real t,m

r = 0.1*t * (m**2 + 14*m)

if (r .LT. 0) r = 0.0

return

end

SUBROUTINES

Makes language modular No global variables. So subroutines helps to

pass it. Eg : subroutine iswap (a, b) integer a, bc Local variables integer tmp  tmp = a a = b b = tmp  return end

CALL BY REFERENCE PARADIGM

program callex integer m, n m = 1 n = 2   call iswap(m, n) write(*,*) m, n  stop end

subroutine iswap (a, b) integer a, bc Local variables integer tmp  tmp = a a = b b = tmp  return end

Fortran follows call by reference paradigm. Eg.

FORMAT STATEMENT

Used for particular input or output format The most common format code letters are:

A - text string D - double precision numbers, exponent notation E - real numbers, exponent notation F - real numbers, fixed point format I - integer X - horizontal skip (space) / - vertical skip (newline)

THINGS NOT COVERED

Input and Output concepts

Input and Output statements (READ, WRITE, PRINT, OPEN,CLOSE,INQUIRE..)

Format specifications (Numeric editing, Logical editing, Character editing..)

PRESENT APPLICATIONS

Cosmology, fusion research, surface physics, molecular dynamics….

Nasa’s Anisotropy probe (flown in 2000) used some legacy f-77 though mostly f-90

US geological survey still uses f-77!...

PRESENT & FUTURE

F-90 has free format, dynamic allocation and pointers, user defined data type, modules, recursive functions, built-in arrays & operator overloading.

Fortran 2000 (delayed to 2004) hopes to have object orientation, interoperability with C, asynchronous I/o and lot more

REFERENCES

http://personal.cfw.com/~terry2/tutorial http://www.ibiblio.org/pub/languages/fortran/unfp.html http://physics.weber.edu/ostlie/phsx2300/future.pdf http://macams1.bo.infn.it/tutorial/format.html http://sunsite.univalle.edu.co/fortran/ch2-3.html Fortran-77

- Harry Katzan

Structured Fortran77 Programming - Seymour Pollack