1

Introduction to Fortran

Ed Kornkven

PHYS F693

Spring 2004

Resources

• http://www.csl.mtu.edu/cs2911/www/FortranReview.htm

– The source of these notes - some bugs• http://www.fortran.com/fortran/tutorials.html

– List of Fortran Tutorials

– The Manchester Computer Centre materials are anice set of notes but unfortunately in PostScriptformat

• Fortran 90 for Engineers and Scientists, Larry Nyhoff andSanford Leestma, Prentice-Hall, 1997

2

Source Form

• Max. line length of 132 characters

• Case insensitive

• Identifiers

– Begin with alpha

– Contain alphanumeric and underscore

– Max. length of 31 characters

• Exclamation begins comment

Source Form (cont.)

• Ampersand (&) at end of lineindicates continuation

• Semicolon separates multiplestatements on a line

• Statement labels are integers

3

Source Form (cont.)

PROGRAM name ! Exclamation mark indicates comment

IMPLICIT NONE ! Don’t implicitly declare variables ! declarations follow and must precede use:CONTAINS ! Internal subroutines and functions follow:END PROGRAM name

Prototype CodePROGRAM main

IMPLICIT NONEREAL :: a=6.0, b=30.34, c=98.98REAL :: mainsummainsum = add()

CONTAINSFUNCTION add()

REAL :: add ! a,b,c defined in 'main'add = a + b + c

END FUNCTION addEND PROGRAM main

4

Declaration of Primitive Types

INTEGER :: i, j = 2 ! do not forget the double :: without spaces REAL :: a, b, c = 1.2 LOGICAL, PARAMETER :: debug = .true.

! Parameter indicates a constant CHARACTER(20) :: name = "John"

IMPLICIT NONE ! always use

Assignment Statement

• variable = expression

I = 3**2 J = MOD(15, 2)

A = 'Quotes delineate character literal' B = "Can also use double quotes.”

5

Operators and Their Priority

Left-to-right+ -Addition and

subtraction

Left-to-right* /Multiplication

and division

Right-to_left**Exponentiation

AssociativitySymbolArithmeticOperator

Some Intrinsic Functions

REALREALLOG(x)

REALREALexEXP(x)

REALREALTangent of x radiansTAN(x)

REALREALCosine of x radiansCOS(x)

REALREALSine of x radiansSIN(x)

REALREALSquare root of xSQRT(x)

REALREAL

INTEGERINTEGERAbsolute value of xABS(x)

RETURNTYPEARG TYPEDESCRIPTIONFUNCTION

6

Conversion Functions

INTEGERINTEGERMax of x1,.. xnMAX(x1,.., xn)

REALINTEGERConverts x to REALREAL(x)

INTEGERINTEGERx - INT( x/y) * yMOD(x,y)

REALREAL

INTEGERINTEGERMin of x1,.. xnMIN(x1,.., xn)

REALREAL

INTEGERREALFractional part of xFRACTION(x)

INTEGERREALGreatest integer < or =

to xFLOOR(x)

INTEGERREALNearest integer to xNINT(x)

INTEGERREALInteger part of xINT(x)

RETURN

TYPEARG TYPEDESCRIPTIONFUNCTION

Input/Output

• READ(integer_unit_number, format-format_line)– Fortran “unit number” functions like a file

descriptor in C– Formats are powerful and complex like they are in

C• READ(*,*) a, b, c ! asterisks for default values

– Traditional default unit numbers are• 5 = stdin• 6 = stdout

• PRINT format,• PRINT *, 'hi’

– Shortcut for WRITE(*,*)

7

Input/Output

• Full treatment of I/O is important forscientific computing but not possiblehere -- e.g.:– Binary and text files– Sequential and direct access– On-the-fly conversions between different binary

formats– Setting record lengths, block sizes, etc.– Special instructions for asynchronous I/O

Logical Operators andPrecedence

Left-to-right.EQV. .NEQV.

Left-to-right.OR.

Left-to-right.AND.

Right-to-left.NOT.Logical

None.LT. .LE. .GT..GE. .EQ. .NE.

(old style)

< <= > >= == /=Relational

ASSOCIATIVITYOPERATORTYPE

8

IF Statements

• Single line formIF ( logical-expr ) statement

• Multiple statement formIF ( logical-expr ) THEN statementsEND IF

• If-else formIF ( logical-expr ) THEN statementsELSE statementsEND IF

• Else-if formIF ( logical-expression )

THEN statementsELSE IF (logical-expression)

THEN statementsELSE IF (logical-expression)

THEN statementsELSE statementsEND IF

Selection• SELECT CASE StatementSELECT CASE (selector)

CASE (label-list-1)

statements-1

CASE (label-list-2)

statements-2

CASE (label-list-3)

statements-3

...........

CASE (label-list-n)

statements-n

CASE DEFAULT

statements-DEFAULT

END CASE

9

SELECT Labels

• A label has one of the following fourforms:

Label Meaning

:x all values less or equal x

x: all values greater or equal to x

x:y all values in the range of x and y

x the values of x itself

SELECT Example

SELECT CASE(index) CASE(:0) print *, "index is equal or less then zero" CASE(1: maxIndex) print *, "index is in range" CASE( int(maxIndex/2) ) print *, "index is at mean" CASE(maxIndex+1:) print *, "index is greater the max"END CASE

10

Iteration

• General DO-Loop w/ EXITDO Statements-1 IF (Logical-Expr) EXIT Statements-2END DO

• Nested DO-loop:Outer: DO IF (expression-1) EXIT Outer Statements-1 Inner: DO IF (expression-2) EXIT Inner Statements-2 END DO Inner Statements-3END DO Outer

• Counting LoopDO var=init-val,final-val,step-size StatementsEND DO

• Default step-size is 1DO var=initial-value, final-value StatementsEND DO

• CYCLE: jump up to the topof loop and increment (likecontinue in C)

Iteration Examples

• Classic example

Sum = 0DO Count = 1, N

READ*, InputSum = Sum + Input

END DOAverage = REAL(Sum) / REAL(N)

• Another example

READ(*,*) NFactorial = 1DO I = 1, N

Factorial = Factorial * IEND DO

11

Obsolescent/RedundantLoops

• Fortran 77 DO loops

DO 100 I=1, Nstatements

100 CONTINUE

• Redundant WHILE loop

DO WHILE(logical-expr)statements

END DO

• Equivalent toDO

IF (logical_expr) EXITstatements

END DO

Subprograms

• Subroutines– Don’t return a value

except by modifyingarguments

– Therefore, not typed andnot declared

– Arguments are passedby reference

– Invoked by CALLstatement

• Functions– Conceptually return a

value, don’t modifyarguments; but this is notenforced!

– Typed by return value; mustbe declared

– Arguments are passed byreference

– Assign return value tofunction name

– Invoked by name reference

12

Subroutine Example

SUBROUTINE swap(a,b)IMPLICIT NONE ! Good habitINTEGER, INTENT(INOUT):: a, b ! INTENT is optionalINTEGER:: tmp ! localtmp = aa = bb = tmpEND SUBROUTINE swap

! Call with:CALL swap(x,y) ! Call by reference!

Function ExampleREAL FUNCTION fact(k)IMPLICIT NONEINTEGER, INTENT (IN) :: kREAL :: fINTEGER :: i

IF (k .lt. 1) THEN fact = 0.0ELSE f = 1.0 DO i = 1, k f = f * i END DO fact = fEND IFEND FUNCTION fact

13

More Fun With Functions• Look, Ma! Fortran can do recursion!

– Just declare the routine to be recursive andspecify the result value

RECURSIVE REAL FUNCTION fact3(k) RESULT (fact)IMPLICIT NONEINTEGER, INTENT (IN) :: k

IF (k .lt. 1) THEN fact = 0.0ELSE IF (k .eq. 1) THEN fact = 1.0ELSE fact = k * fact3(k-1)END IFEND FUNCTION fact3

More Fun With Functions• By default, variables declared inside a

subprogram have– Local scope– “Automatic” lifetime

• A local variable can be given a “static”lifetime by either– Initializing it

INTEGER :: keeper = 0– Or giving it the SAVE attribute when declaring it

INTEGER, SAVE :: keeper

14

Expressions & Assignments

• Data types

– Implicit typing rules

• Operators

• Library functions

• Assignments

1-D Arrays

• Syntax– type, DIMENSION ( extent ) :: name-1, name-2, ...

– type, DIMENSION( lower : upper) :: list-array-names

• Array operands and operators

– InitializationA = (/ 1, 2, 3 /)

– Array expressions and assignmentsA = B + C ! These operations are done

A = B * 3.14 ! element-wise

15

Array ExampleREAL FUNCTION fact(k)INTEGER, INTENT (IN) :: kINTEGER, PARAMETER :: N = 8REAL :: f ! Don’t use “fact” on RHS!REAL :: precmp(N)=(/1.0,2.0,6.0,24.0,120.0,720.0,5040.0,40320.0/)IF (k .lt. 1) THEN fact = 0.0ELSE IF (k .le. N) THEN fact = precmp(k)ELSE f = precmp(N) DO i = N+1, k f = f * i END DO fact = fEND IFEND FUNCTION fact

Some Array Functions

Maximum value in array AMAXVAL(A)

Product of the elements in APRODUCT(A)

Sum of the elements in ASUM(A)

Number of elements in ASIZE(A)

One Dimensional array of oneelement containing the location ofthe smallest element

MINLOC(A)

One Dimensional array of oneelement containing the location ofthe largest element

MAXLOC(A)

Minimum value in array AMINVAL(A)

RETURNSFUNCTION

16

Dynamic Array Allocation

• Syntax– type, DIMENSION(:), ALLOCATABLE :: list-of-array-names

– ALLOCATE(list, STAT = status-variable)

– DEALLOCATE( list, STAT= status-variable)

Dynamic Array Allocation

• Example

PROGRAM mainINTEGER, DIMENSION(:), ALLOCATABLE :: AINTEGER :: aStatus, NWRITE(*, '(1X, A)', ADVANCE = "NO") "Enter array size: "READ *, N ! Try 1 billion on your PC!ALLOCATE( A(N), STAT = aStatus )IF (aStatus /= 0) STOP "*** Not enough memory ***"PRINT*, ‘Array allocated with size ‘, N

DEALLOCATE(A)PRINT*, ‘Array deallocated…’END PROGRAM main

17

Multidimensional Arrays

• Syntax– type, DIMENSION( dim1,dim2,…) :: list-array-names

– type, DIMENSION(:,:,…), ALLOCATABLE :: list-array-names

– ALLOCATE(array-name( lower1: upper1, lower2: upper2) ,

STAT = status)

• Examples– INTEGER, DIMENSION(100,200) :: a

– INTEGER, DIMENSION(:,:), ALLOCATABLE :: a

Multidimensional Arrays• Column-major ordering

– Suppose we have the declarationINTEGER, DIMENSION(100,200) :: a

– The usual way of viewing a 2-D array reference is• A(row,column) - first dimension is “rows”, second is “columns”

– Of course, computer memory is 1-D. How is virtual 2-D array mapped to real memory?

• In Fortran, it is in column-major order -- I.e., column-by-column• NB: C is row-major order!

– Yes, there are situations in which we care!• Certain performance situations• Interfacing Fortran and C

18

Multi-D Array Functions

Array of one less dimension containing the

maximum values in array A along dimension D. If D

is omitted, maximum of the entire array is returned.

MAXVAL(A,D)

Number of elements in ASIZE(A)

Like MAXLOC() but for smallest elementMINLOC(A)

One Dimensional array of one element containing

the location of the largest elementMAXLOC(A)

Like MAXVAL() but for minimaMINVAL(A,D)

RETURNSFUNCTION

Multi-D Array Fns (cont.)

Matrix product of A and B (provided result is

defined)MATMUL(A,B)

Array of one less dimension containing the products

of the elements of A along dimension D. If D is

omitted, the product of the elements of the entire

array is returned.

PRODUCT(A)

Array of one less dimension containing the sums of

the elements of A along dimension D. If D is

omitted, the sum of the elements of the entire array

is returned.SUM(A,D)

RETURNSFUNCTION

19

Modules

• Modules - used to packagetogether

– Type declarations

– Subprograms

– Data type definitions

• Forms a library that can be usedin other program units

Module Syntax

• Module definitionMODULE module-name IMPLICIT NONE specification part

PUBLIC :: Name-1, Name-2, ... , Name-n PRIVATE :: Name-1, Name-2, ... , Name-n

CONTAINS internal-functionsEND MODULE

• Module use - use the USE to useUSE module-name

20

New Fortran Features ThatWe Can’t Cover

• Pointers

• User-defined types and operations

• INTERFACE blocks

• KIND

• Others -- can you say “C++”?

Obsolescent & RedundantFeatures

• Arithmetic IF

• PARAMETER

• DATA

• GO TO

• Computed GO TO

• COMMON blocks

• EQUIVALENCE

21

NEC/Cray SX-6Architecture Overview

SX-6 Architecture

22

SX-6 Overview• SX-6 Node

– 8 CPUs

– 64 GB Memory

• Performance

– 64 GFLOPS perNode

• SUPER-UXoperating system

– 32- and 64-bit IEEE

SX-6Languages /

ProgrammingModels

√Fortran 90

Co-array Fortran

UPC

√HPF

√C++ / C

SHMEM

√ ("almost all")MPI-2

√MPI

√OpenMP

√Auto par.

SX-6 ProgrammingEnvironment

23

CPU Performance Overview

12.828Peak perf.(GFLOPS)

800500500Clock (mHz)

34.13.631.9Mem. BW(GB/sec)

128 x 64 x 648 x 64 x 648 x 256 x 64Vector regs.

828Vector pipes

X1SV1exSX-6

Earth Simulator - 40 TFlops

• 640 SX-6 cabinets

• x 8 Pes each

• = 40960 GFLOPS

• > combined 5nearest competitors

“Real Performance”?

• SC 2002 Gordon Bell prize winner

• Sustained rates exceeding 25 TFLOPS

24

Compiling on SX-6

• Gateway machine

– rimegate.arsc.edu

– An SGI front end

– Fast compiles using SX cross-compilers

• SX-6

– rime.arsc.edu

– Limited disk space, no backups• http://www.arsc.edu/support/howtos/usingsx6.html

Compiling on SX-6 (cont.)

• Summary

– Give Fortran source files a suffix of .f90

– Rime compile: f90 file.f90

– Rimegate compile: sxf90 file.f90

– Execute (must be on Rime!): ./a.out

– For compiler options: man {sx}f90

– NQS batch system

25

Compiling on SX-6 (cont.)

• Important compiler options

– -ew : 64-bit

– -Wf,”-pvctl fullmsg -L fmtlist” : listing details

– -pi : automatic inlining

SX-6 Source File NameDefaults

.F90

.FTN.F

PreprocessorNot Invoked

.f90

.ftn.f

PreprocessorInvoked

Free FormatFixed Format