FORTRAN FORmula TRANslator -Anand Trivedi

HISTORY  Designed and written from scratch in 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 cardsPunch 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 : Statements Col : 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 = )

SAMPLE EXAMPLE- I  program circle real r, area pi parameter (pi = ) 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, b c 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, b c 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       Fortran-77 - Harry Katzan  Structured Fortran77 Programming - Seymour Pollack