Preparation
• Log in to mirage
o ssh –l username mirage[0-4].ucar.edu
o Use CryptoCard or Yubikey
• Obtain the example source code
o cp –r /glade/home/siliu/f90_1 your-own-directory
• Slides and handouts are available at:
www2.cisl.ucar.edu/training/CSG_winter_2011
• In case you are not working on mirage:
o download the tar ball
o tar –xf f90_1.tar
Introduction to
Fortran 90
Consulting Services Group
Si Liu (Presenter)
Feb 14, 2011
NCAR/CISL/OSD/USS
Syllabus
Introduction
Basic syntax
Arrays
Control structures
Basic I/O
Introduction
History
Objectives
Major new features
Other new features
Availability of compilers
History of Fortran
FORTRAN is an acronym for FORmula TRANslation
IBM Fortran (1957)
Fortran 66 standard (1966)
Fortran 77 standard (1978)
Fortran 90 standard (1991)
Fortran 95 standard (1996)
Fortran 2003 standard (2004)
Fortran 2008 standard (2010)
Objectives
 Language evolution
Obsolescent features
 Standardize vendor extensions
Portability
 Modernize the language
•
Ease-of-use improvements through new features such as free
source form and derived types
• Space conservation of a program with dynamic memory
allocation
• Modularization through defining collections called modules
• Numerical portability through selected precision
Objectives, continued
 Provide data parallel capability
Parallel array operations for better use of vector and parallel
processors
 Compatibility with Fortran 77
Fortran 77 is a subset of Fortran 90
 Improve safety
Reduce risk of errors in standard code
 Standard conformance
Compiler must report non standard code and obsolescent
features
Major new features
Array processing
Dynamic memory allocation
Modules
Procedures:
• Optional/Keyword Parameters
• Internal Procedures
• Recursive Procedures
Pointers
Other new features
Free-format source code
Specifications/Implicit none
Parameterized data types (KIND)
Derived types
Operator overloading
New control structures
New intrinsic functions
New I/O features
Available Fortran 90 compilers











Intel® Fortran Compiler: Ifort
IBM XL Fortran Compiler: XLF
GNU Fortran compiler: gfortran
PGI Fortran Compiler: pgf90
Cray CF90
EKOPATH (PathScale)
DEC Fortran 90
EPC Fortran 90
Microway
NA Software F90+
Pacific Sierra VAST-90
First Fortran program
Syntax Example1: syntax_ex1.f90
PROGRAM HelloWorld
! Hello World in Fortran 90 and 95
WRITE(*,*) "Hello World!"
END PROGRAM
Compile and run
ifort syntax_example1.f90 -o syntax_example1.exe
./syntax_ex1.exe
Free source form
 Lines up to 132 characters
 Lowercase letters permitted
 Names up to 31 characters (including underscore)
 Semicolon to separate multiple statements on one
line
 Comments may follow exclamation (!)
 Ampersand (&) is a continuation symbol
 Character set includes + < > ; ! ? % - “ &
 New relational operators: ‘<’, ‘<=’, ‘==’,’/=‘,’>=‘,’>’
Example: Source form
free_source_form.f90
PROGRAM free_source_form
! Long names with underscores
! No special columns
IMPLICIT NONE
! upper and lower case letters
REAL :: tx, ty, tz ! trailing comment
! Multiple statements per line
tx = 1.0; ty = 2.0; tz = tx * ty;
! Continuation on line to be continued
PRINT *, &
tx, ty, tz
• END PROGRAM free_source_form
Specifications
type [[,attribute]... ::] entity list
type can be INTEGER, REAL, COMPLEX,
LOGICAL, CHARACTER or TYPE with optional
kind value:
• INTEGER [(KIND=] kind-value)]
• CHARACTER ([actual parameter list])
([LEN=] len-value and/or [KIND=] kind-value)
• TYPE (type name)
Specifications, continued
type [[,attribute]... ::] entity list
attribute can be
PARAMETER,
ALLOCATABLE,
INTENT(inout),
OPTIONAL,
INTRINSIC
PUBLIC,
PRIVATE,
POINTER, TARGET,
DIMENSION (extent-list),
SAVE,
EXTERNAL,
Can initialize variables in specifications
Example: Specifications
spcifiacation.f90
 ! Integer variables:
INTEGER :: ia, ib
 ! Parameters:
INTEGER, PARAMETER :: n=100, m=1000
 ! Initialization of variables:
REAL :: a = 2.61828, b = 3.14159
 !Logical variables
LOGICAL :: E=.False.
 ! Character variable of length 20:
CHARACTER (LEN = 20) :: ch
IMPLICIT NONE
In Fortran 77, implicit typing permitted use of
undeclared variables. This has been the cause
of many programming errors.
IMPLICIT NONE forces you to declare the type
of all variables, arrays, and functions.
IMPLICIT NONE may be preceded in a program
unit only by USE and FORMAT.
It is recommended to include this statement in all
program units.
Intrinsic Logical Operations
Relational Operators
•
•
•
•
•
•
•
•
•
•
•
.NOT.
.AND.
.OR.
.EQV.
.NEQV.
.GT.
.GE.
.LE.
.LT.
.NE.
.EQ.
Numeric Operations
• **: exponentiation
• * and / : multiply and divide
• + and - : plus and minus
>
>=
<=
<
/=
==
Character operations
string1.f90
Intrinsic Character Operations
substring
Intrinsic Character Operations
Concatenation
Character(Len=20) :: ch
Ch=“specification”
Character(Len=5), Parameter :: s1 = &
“abcde”
Character(Len=20) :: s2=s1//”fg”
•
•
ch(1:1) is ‘s’
ch(10:13) us “tion”
•
ch(1) error
PRINT *, “hello ”//”world ”//”again”
PRINT *, s2
Kind Values
 5 intrinsic types: REAL, INTEGER, COMPLEX,
CHARACTER, LOGICAL
 Each type has an associated non negative integer value
called the KIND type parameter
 A processor must support at least 2 kinds for REAL and
COMPLEX, and 1 for INTEGER, LOGICAL and
CHARACTER
 Many intrinsics for enquiring about and setting kind
values
 Useful feature for writing portable code requiring
specified precision
Example: Kind Values
INTEGER(8) :: I
REAL(KIND=4) :: F
INTEGER :: IK=SELECTED_INT_KIND(9)
INTEGER :: IR=SELECTED_REAL_KIND(3,10)
Kind values: INTEGER
INTEGER (KIND = wp) :: ia
INTEGER(wp) :: ia
ia=3.0_wp
! or
! or
 Integers usually have 16, 32, or 64 bit
 16 bit integer normally permits -32768 < i < 32767
 Kind values are system dependent
• An 8 byte integer variable usually has kind value 8 or 2
• A 4 byte integer variable usually has kind value 4 or 1
Kind values: INTEGER, continued
 To declare integer in system independent way, specify
kind value associated with range of integers required:
INTEGER, PARAMETER :: &
i8 =SELECTED_INT_KIND(8)
INTEGER (KIND = i8) :: ia, ib, ic
ia, ib and ic can have values between -108 and +108 at
least (if permitted by processor).
•
Kind example: kind1.f90
INTEGER, PARAMETER :: k2=SELECTED_INT_KIND(2)
INTEGER, PARAMETER :: k4=SELECTED_INT_KIND(4)
INTEGER(kind=k2) :: i2
INTEGER(kind=k4) :: i4
print *,"k2= ",k2
print *,"k4= ",k4
print *,"huge(i2)= ", huge(i2)
print *,"huge(i4)= ", huge(i4)
Kind values: REAL
REAL (KIND = wp) :: ra
REAL(wp) :: ra
ra= 3.14_wp
! or
! or
 Declare a real variable, ra, whose precision is
determined by the value of the kind parameter, wp
 Kind values are system dependent
• An 8 byte (64 bit) real variable usually has kind value 8 or 2.
• A 4 byte (32 bit) real variable usually has kind value 4 or 1.
Kind values: REAL,continued
 To declare real in system independent way, specify kind
value associated with precision and exponent range
required:
INTEGER, PARAMETER :: &
i10 = SELECTED_REAL_KIND(10, 200)
REAL (KIND = i10) :: a, b, c
a, b and c have at least 10 decimal digits of precision
and the exponent range 200.
Kind example: kind2.f90
INTEGER, PARAMETER :: &
i10 = SELECTED_REAL_KIND(10, 200)
REAL (KIND = i10) :: a
PRINT *, RANGE(a), PRECISION(a), KIND(a)
This will print the exponent range, the decimal digits of
precision and the kind value of a.
Syntax Example 2
syntax_ex2.f90
Program Triangle
implicit none
real :: a, b, c, Area
print *, 'Welcome, please enter the &
&lengths of the 3 sides.'
read *, a, b, c
print *, 'Triangle''s area: ', Area(a,b,c)
end program Triangle
Syntax Example 2 , continued
Function Area(x,y,z)
implicit none
! function type
real :: Area
real, intent (in) :: x, y, z
real :: theta, height
theta = acos((x**2+y**2-z**2)/(2.0*x*y))
height = x*sin(theta)
Area = 0.5*y*height
end function Area
Derived Types (structures)
Defined by user
Can include different intrinsic types and
other derived types
Components accessed using percent (%)
Only assignment operator (=) is defined
for derived types
Can (re)define operators
Example: Derived Types
 Define the form of derived type
TYPE acar
CHARACTER (LEN = 10) :: model
INTEGER :: year
CHARACTER (LEN = 3) :: style
END TYPE acar
 Create the structures of that type
TYPE (acar) :: mycar1, mycar2
 Assigned by a derived type constant
mycar1 = acar(’Nissan’, 2000, ’ABC’)
 Use % to select a component of that type
mycar2%model = ’Mazda’
Example: Derived Types
 Arrays of derived types:
TYPE (acar), DIMENSION (n) :: mycars
 Derived type including derived type:
TYPE household
CHARACTER (LEN = 30) :: name
CHARACTER (LEN = 50) :: address
TYPE (acar) :: mycar
END TYPE household
TYPE (household) :: myhouse
myhouse%mycar%model = ’Lexus’
Control Structures
Three block constructs
• IF
• DO and DO WHILE
• CASE
 All can be nested
 All may have construct names to help
readability or to increase flexibility
Control structure: IF
[name:]IF (logical expression) THEN
block
[ELSE IF (logical expression) THEN
[name] block]...
[ELSE [name]
block]
END IF [name]
Example: IF
IF (i < 0) THEN
CALL negative
ELSE IF (i == 0) THEN
CALL zero
ELSE selection
CALL positive
END IF
see if_ex1.f90
Control Structure: Do
[name:] DO [control clause]
block
END DO [name]
Control clause may be:
• an iteration control clause
count = initial, final [,inc]
• a WHILE control clause
WHILE (logical expression)
• or nothing (no control clause at all)
Example: DO
Iteration control clause:
rows: DO i = 1, n
cols: DO j = 1, m
a(i, j) = i + j
END DO cols
END DO rows
Example: DO
WHILE control clause:
true: DO WHILE (i <= 100)
...
body of loop
...
END DO true
Use of EXIT and CYCLE
exit from loop with EXIT
transfer to END DO with CYCLE
EXIT and CYCLE apply to inner loop by
default, but can refer to specific, named
loop
Example: Do
outer: DO i = 1, n
middle: DO j = 1, m
inner: DO k = 1, l
IF (a(i,j,k) < 0.0) EXIT outer
IF (j == 5) CYCLE middle
IF (i == 5) CYCLE
...
END DO inner
END DO middle
END DO outer
! leave loops
! set j = 6
! skip rest of inner
Example: DO
No control clause:
DO
READ (*, *) x
IF (x < 0) EXIT
y = SQRT(x)
...
END DO
Notice that this form can have the same effect as a DO
WHILE loop.
Control Structures: CASE
Structured way of selecting different
options, dependent on value of single
Expression
Replacement for
• computed GOTO
• or IF ... THEN ... ELSE IF ... END IF
constructs
Control Structure: CASE
General form:
[name:] SELECT CASE (expression)
[CASE (selector) [name]
block]
...
END SELECT [name]
Control Structure: CASE
expression - character, logical or integer
selector - DEFAULT, or one or more
values of same type as expression:
• single value
• range of values separated by : (character or
integer only), upper or lower value may be
absent
• list of values or ranges
Example: CASE
hat: SELECT CASE (ch)
CASE (’C’, ’D’, ’G’:’M’)
color = ’red’
CASE (’X’)
color = ’green’
CASE DEFAULT
color = ’blue’
END SELECT hat
Array
Arrays
Terminology
Specifications
Array constructors
Array assignment
Array sections
Arrays, continued
Whole array operations
WHERE statement and construct
Allocatable arrays
Array intrinsic procedures
Terminology
 Column majored
 Rank: Number of dimensions
 Extent: Number of elements in a dimension
 Shape: Vector of extents
 Size: Product of extents
 Conformance: Same shape
REAL, DIMENSION :: a(-3:4, 7)
REAL, DIMENSION :: b(8, 2:8)
REAL, DIMENSION :: d(8, 1:8)
Specifications
type [[,DIMENSION (extent-list)] [,attribute]... ::] entity-list
where:
 type - INTRINSIC or derived type
 DIMENSION - Optional, but required to define default dimensions
 (extent-list) - Gives array dimension:
• Integer constant
• “:”, if array is allocatable or assumed shape
 attribute - as given earlier
 entity-list - list of array names optionally with dimensions and initial
values.
REAL, DIMENSION(-3:4, 7) :: ra, rb
INTEGER, DIMENSION (3) :: ia = (/ 1, 2, 3 /), ib = (/ (i, i = 1, 3) /)
Array Constructor
 Specify the value of an array by listing its elements
p = (/ 2, 3, 5, 7, 11, 13, 17 /)
 DATA
DATA SS /6*0/
 Reshape
REAL, DIMENSION (3, 2) :: ra
ra = RESHAPE( (/ ((i + j, i = 1, 3), j = 1, 2) /), &
SHAPE = (/ 3, 2 /) )
Array sections
A sub-array, called a section, of an array may be
referenced by specifying a range of subscripts, either:
 A simple subscript
• a (2, 3) ! single array element
 A subscript triplet
• [lower bound]:[upper bound] [:stride]
a(1:3,2:4)
• defaults to declared bounds and stride 1
 A vector subscript
iv =(/1,3,5/)
rb=ra(iv)
Whole array operations
Arrays for whole array operation must be
conformable
Evaluate element by element, i.e.,
expressions evaluated before assignment
Scalars broadcast
Functions may be array valued
Whole array operations, continued
Fortran 77:
Fortran 90:
REAL a(20), b(20), c(20)
…
DO 10 i = 1, 20
a(i) = 0.0
10 CONTINUE
…
DO 20 i = 1, 20
a(i) = a(i) / 3.1 + b(i) *SQRT(c(i))
20 CONTINUE
…
REAL, DIMENSION (20) :: a, b, c
...
a = 0.0
...
…
a = a / 3.1 + b * SQRT(c)
...
Array examples
Use –fixed or –FI option to compile the
code in fixed format
Array example 1
Array example 2
Array assignment
Operands must be conformable
REAL, DIMENSION (5, 5) :: ra, rb, rc
INTEGER :: id
...
ra = rb + rc * id
! Shape(/ 5, 5 /)
ra(3:5, 3:4) = rb(1::2, 3:5:2) + rc(1:3, 1:2)
! Shape(/ 3, 2 /)
ra(:, 1) = rb(:, 1) + rb(:, 2) + rb(:, 3)
! Shape(/ 5 /)
Array Intrinsic Functions
ALL(mask, dim)
.true. if all elements are true
ANY(mask, dim)
.true. if any elements are true
COUNT(mask, dim)
Number of true elements
SUM(array, dim, mask)
Sum of elements x
PRODUCT(array, dim, mask) Product of elements
MAXVAL(array, dim, mask) Maximum value in array
MINVAL(array, dim, mask)
Minimum value in array
DOT_PRODUCT(va, vb)
Dot product of two vectors
MATMUL(mata, matb)
Matrix multiplication (or matrix X vector)
TRANSPOSE(matrix)
Transpose of 2-d array
MAXLOC(array, mask)
Location of maximum element
MINLOC(array, mask)
Location of minimum element
Where statement
Form:
WHERE (logical-array-expr)
array-assignments
ELSEWHERE
array-assignments
END WHERE
REAL DIMENSION (5, 5) :: ra, rb
WHERE (rb > 0.0)
ra = ra / rb
ELSEWHERE
ra = 0.0
END WHERE
Another example: where_ex.f90
Allocatable arrays
 A deferred shape array which is declared with the
ALLOCATABLE attribute
 ALLOCATE(allocate_object_list [, STAT= status])
 DEALLOCATE(allocate_obj_list [, STAT= status])
 When STAT= is present,
status = 0 (success) or status > 0 (error).
When STAT= is not present and an error occurs, the
program execution aborts
...
array_ex3.f90
REAL, DIMENSION (:, :), ALLOCATABLE :: ra
INTEGER :: status
READ (*, *) nsize1, nsize2
ALLOCATE (ra(nsize1, nsize2), STAT = status)
IF (status > 0) STOP ’Fail to allocate meomry’
...
IF (ALLOCATED(ra)) DEALLOCATE (ra)
Basic I/O
Namelist
Gather set of variables into group to simplify I/O
General form of NAMELIST statement:
NAMELIST /namelist-group-name/ variable-list
Use namelist-group-name as format
instead of io-list on READ and WRITE
Input record has specific format:
&namelist-group-name var2=x, var1=y, var3=z
Variables optional and order unimportant
Example: Namelist
see example: nml_ex.f90
...
INTEGER :: size = 2
CHARACTER (LEN = 5) :: &
color(3) = (/ ’red ’, ’pink ’, ’blue ’ /)
NAMELIST /clothes/ size, color
WRITE(*, NML = clothes)
...
outputs:
&CLOTHES
SIZE =
2,
COLOR = red pink blue
/
Example: Formatted I/O
see example: io_ex1.f90
INTEGER :: I,J
REAL:: A,B
PRINT *,"Enter two integers, I and J:"
READ *, I,J
PRINT *,"Enter two Reals, A and B:"
READ *,A,B
PRINT *,"I= ",I,"J= ",J
PRINT *,"A= ",A,"B= ",B
Example: Formatted I/O
see example: io_ex2.f90
DO I=-3,3
WRITE(*,100) I
END DO
100 FORMAT(1X,5I5)
WRITE(*,200) (J,J=-3,3)
200 FORMAT(1X,7I5)
Example: Formatted I/O
see example: io_ex3.f90
Fw.d
w: the number of positions to be used
d: the number of digits to the right of the decimal point
PROGRAM io_ex3
IMPLICIT NONE
REAL A,B,C
WRITE(*,*) "Please enter 3 real numbers:"
READ(*,10)A,B,C
WRITE(*,*) "These 3 real numbers are:"
PRINT 20,A,B,C
10 FORMAT(3(F6.2,1X))
20 FORMAT('A= ',F6.2,' B= ',F6.2,' C= ', F6.2)
END PROGRAM io_ex3
Recommended text
Full text on Books 24x7 in NCAR Library
References
 Fortran 90: A Conversion Course for Fortran 77
Programmers OHP Overviews
F Lin, S Ramsden, M A Pettipher, J M Brooke, G S
Noland, Manchester and North HPC T&EC
 An introduction to Fortran 90 and Fortran 90 for
programmers
A Marshall, University of Liverpool
 Fortran 90 for Fortran 77 Programmers
Clive page, University of Leicester
Acknowledgments
•
•
•
•
•
•
•
Siddhartha Ghosh
Davide Del Vento
Rory Kelly
Dick Valent
Other colleagues from CISL
Manchester and North HPC T&EC
University of Liverpool for examples
Resources
Forums for the Fortran 90 course
http://sea.ucar.edu/forum
CSG will provide Fortran 90 support.
Walk-in: ML suite 51/55
Phone: (303)-497-1200
Email: cislhelp@ucar.edu