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Modern Programming Languages: Fortran90/95/2003/2008
Modern Programming Languages:
Fortran90/95/2003/2008
Why we need modern languages (Fortran/C++)
How to write code in modern Fortran
Lars Koesterke
Texas Advanced Computing Center
The University of Texas at Austin
November 10, 2011

This is an Intermediate Class
• You know already one computer language
• You understand the very basic concepts:
– What is a variable, an assignment, function call, etc.?
– Why do I have to compile my code?
– What is an executable?
• You (may) already know some Fortran
• You are curious about what comes next
• What are the choices?
• How to proceed from old Fortran (or C), to much more modern
languages like Fortran2003/2008 (and C++)?
2

Outline
Outline
• Motivation
• Modern Fortran
• Object-Oriented Programming: (Very) Short Version
3

Motivation
Why do we (have to) learn advanced languages?
Basic features (BASIC)
• Variables — Data containers for Integers, Reals, Characters ,Logicals
Arrays: Vectors ,Matrices
• Basic operators — arithmetic (+, −, *, /) logical, lexical, etc.
• Control constructs — if/else-if, case/switch, goto, ...
• Loops — do/for, while/repeat, etc.
• I/O — All languages provide sophisticated mechanisms for I/O
(ASCII, binary, streams): Not covered!
Is that enough to write code?
My answer: No!
Subprograms: subroutines and functions
enables us to repeat operations on different data
enables us to savoid code replication
5

Motivation
Starting with: Fortran77
• basic language (BASIC): allows to write 500 lines of code
• w/ subprograms: we can do much, much better
Old Fortran (Fortran77) provides only the absolute Minimum!
And these languages (Fortran77 and C) have flaws:
• Fortran77: No dynamic memory allocation (on the heap)
– common blocks, equivalence statements
old & obsolete constructs
clunky style, missing blanks
old (legacy) code is usually cluttered
• C: Call by value, no multidimensional arrays
– Pointer (de)referencing everywhere, for no good reason
Fortran77 and C are simple languages
and they are (kind-of) easy to learn
7

Motivation
If Fortran77 and C are so simple,
Why is it then so difficult to write good code?
Is simple really better?
• Using a language allows us to express our thoughts (on a computer)
• A more sophisticated language allows for more complex thoughts
• I argue: Fortran77 and plain C are (way) too simple
• Basics+1 plus the flaws are not enough!
We need better tools!
• The basics without flaws
– Language has to provide new (flawless) features
– User has to avoid old (flawed) features
• more language elements to get organized
=) Fortran90/95/2003 and C++
9

Motivation
So, these languages (Fortran77 and C) are easy to learn?
... are you kiddin’? They are not!
We want to get our science done! Not learn languages!
How easy/difficult is it really to learn Fortran77 and C?
The concept is easy:
Variables, Arrays, Operators, If, Do, Subroutines/Functions
• I/O
• Syntax
• Rules & regulations, the fine print
• Conquering math, developing algorithms,
the environment: OS, compiler, hardware, queues, etc.
• I/O details
– print to screen
– read/write from/to files
– from ASCII to binary
– from basic to efficient to
parallel
• parallel computing: MPI, OpenMP, cudA, ...
• ... and the flaws =) simple things will be complicated
Invest some time now, gain big later!
Remember: so far, we have only the Basics + Functions/Subroutines
11

Modern Fortran
Overview
Modern Fortran starts here!
• Modern style
– Free format
– Attributes
– implicit none
– do, exit, cycle, case
– Single and double precision
• Fixing the flaws
– Allocatable arrays
– Structures, derived types
• Module-oriented Programming
– internal subprograms
– private, public, protected
– contains
– use
– Explicite interfaces
– Optional arguments & intent
• Formula translation
– Array syntax,
where and forall statement
– Extended & user-defined operators
– Functions: elemental, inquiry,
mathematical
• Odds and Ends
– Fortran pointers (References)
– Command line arguments
– Environment variables
– Preprocessor
– Interoperability with C (binding)
• Performance considerations
• Object oriented programming
13

Modern Fortran
Style
Free Format
• Statement may start at the first column (0−132 characters)
• Exclamation mark (!) starts a comment (not in literal strings)
• Blanks are significant: Not allowed in keywords or variables
• Continuation with an ampersand (&) as the last character
• Mulitple statements in one line separated by a semicolon (;)
Style example
program style
print *, ’This statement starts in column 1’
i = 5; j = 7 ! Two statements in one line
! Comment with an exclamation mark
i = & ! Line with continuation
j * j + j
end
15

Modern Fortran
Style
Blanks, blank lines, and comments
• Use blanks, blank lines, and comments freely
• Use indentation
Good
program square
! This program calculates ...
implicit none
real :: x, x2
x = 5.
x2 = x * x
if (x == 13.) print *, ’Lucky’
end
Bad
program square
x=5.
x2=x*x
if(x.eq.13)print*,’Lucky’
end
17

Modern Fortran
Style
Good
program square
! This program calculates ...
implicit none
integer :: i
real :: x, x2
do i=1, 20
x = real(i)
x2 = x * x
if (x == 13.) print *, Lucky
enddo
end
Bad
program square
do 100 i=1,20
x=i
x2=x*x
if(x.eq.13)print*,...
100 continue
end
19

Modern Fortran
Style
Attributes
Style example
program style
integer :: i, j
real :: x
real, parameter :: pi = 3.1415
real, dimension(100) :: array
real, dimension(:,:), allocatable :: dyn_array_2d
• General form
integer :: name
real, <attributes> :: name
• attributes are:
parameter, dimension, allocatable, intent, pointer, target, optional,
private, public, value, bind, etc.
21

Modern Fortran
Style
Implicit none
Implicit type declaration
program implicit
implicit none ! use to disable the default
• Default type of undeclared variables:
All variables starting with the letter i, j, k, l, m, n are integers
All other variables are real variables
• Turn default off with: implicit none
• Strongly recommended (may not be right for everybody, though)
23

Modern Fortran
Style
Loops: do, while, repeat
do-Loop
do i=1, 100, 8 ! No label
! loop-variable, start, increment
...
enddo
while-Loop
i = 0
do
if (i > 20) exit
i = i + 1
enddo
repeat-Loop
i = 0
do
i = i + 1
if (i > 20) exit
enddo
• Use the exit statement to “jump” out of a loop
25

Modern Fortran
Style
Loops: exit and cycle
Exit anywhere
do i=1, 100
x = real(i)
y = sin(x)
if (i > 20) exit
z = cos(x)
enddo
Skip a loop iteration
do i=1, 100
x = real(i)
y = sin(x)
if (i > 20) cycle
z = cos(x)
enddo
• exit: Exit a loop
• cycle: Skip to the end of a loop
• Put exit or cycle anywhere in the loop body
• Works with loops with bounds or without bounds
27

Modern Fortran
Style
Nested loops: exit and cycle
Exit Outer Loop
outer: do j=1, 100
inner: do i=1, 100
x = real(i)
y = sin(x)
if (i > 20) exit outer
z = cos(x)
enddo inner
enddo outer
Skip an outer loop iteration
outer: do j=1, 100
inner: do i=1, 100
x = real(i)
y = sin(x)
if (i > 20) cycle outer
z = cos(x)
enddo inner
enddo outer
• Constructs (do, if, case, where, etc.) may have names
• exit: Exit a nested loop
• cycle: Skip to the end of an outer loop
• Put exit or cycle anywhere in the loop body
• Works with loops with bounds or without bounds
29

Modern Fortran
Style
Case
integer :: temp_c
! Temperature in Celsius!
select case (temp_c)
case (:-1)
write (*,*) ’Below freezing’
case (0)
write (*,*) ’Freezing point’
case (1:20)
write (*,*) ’It is cool’
case (21:33)
write (*,*) ’It is warm’
case (34:)
write (*,*) ’This is Texas!’
end select
• case takes ranges (or one
element)
• works also with characters
• read the fine-print
31

Modern Fortran
Style
Variables of different kind values
integer :: i, my_kind
real :: r
! Selection based on
! precision
print *, kind(i), kind(r) ! prints 4 4 (most compilers)
my_kind = selected_real_kind(15) ! select a real that has
! 15 significant digits
print *, my_kind ! prints 8
integer, parameter :: k9 = selected_real_kind(9)
real(kind=k9) :: r
r = 2._k9; print *, sqrt(r) ! prints 1.41421356237309
33

Modern Fortran
Style
Variables of different kind values: The sloppy way
• There are only 2(3) kinds of reals: 4-byte, 8-byte (and 16-byte)
• The kind-numbers are 4, 8, and 16 (most compilers!)
• Kind number may not be byte number!
• Selection based on the number of bytes
real*8 :: x8 ! Real with 8 bytes (double precision)
real(kind=8) :: y8 ! same, but not completely safe
real*4 :: x4 ! Real with 4 bytes (single precision)
integer*4 :: i4 ! Integer single precision
integer*8 :: i8 ! Integer double precision
x8 = 3.1415_8 ! Literal constant in double precision
i8 = 6_8 ! same for an integer
• real*8, real*4: works well with MPI Real8 and MPI Real4
35

Modern Fortran
Style
Variables of different kind values
• Do not use ’double’ in your definition
• double refers to something; it’s double of what?
• double precision, dble(...)
• Select appropriate precision at compile time: ifort -r4, ifort -r8
• Compiler flag also elevates the unnamed constants
real*8 :: x8, y8
real*4 :: x4, y4
integer :: i
y8 = 3.1415 ! 3.1415 is an unnamed constant
! with -r8: 8 bytes
x4 = real(i)
x8 = dble(i) ! Old style, using dble
x8 = real(i, kind=8) ! New style using the kind parameter
37

Modern Fortran
Fixing the Flaws
Fixing the Flaws
Allocatable arrays
• flexible size
• allocated on the heap
– The size of the stack is severely limited (default: 2 GB)
– Remedies are problematic (Intel: -mcmodel=medium -intel-shared)
• Always allocate large arrays on the heap!
– Large arrays always have to be allocatable (heap) arrays,
even if you do not need the flexibility to avoid problems with the
limited size of the stack
Structures and derived types
• Organize your data
• Compound different variables into one type
39

Modern Fortran
Fixing the Flaws
Allocatable Arrays
• Variables live on the heap (vs. stack for scalars and static arrays)
• Declaration and allocation in 2 steps
• Declare an array as allocatable,
use colons (:) as placeholders
• allocate/deallocate in the executable part
• Allocation takes time. Do not allocate too often.
program alloc_array
real, dimension(:), allocatable :: x_1d ! Attribute
real, dimension(:,:), allocatable :: x_2d ! allocatable
...
read n, m
allocate(x_1d(n), x_2d(n,m), stat=ierror) ! Check the
if (ierror /= 0) stop ’error’ ! error status!
...
deallocate(x) ! optional
41

Modern Fortran
Fixing the Flaws
Structures and Derived Types
• Declaration specifies a list of items (Derived Type)
• A Structure (a variable of a derived type) can hold
– variables of simple type (real, integer, character, logical, complex)
– arrays: static and allocatable
– other derived types
– A structure can be allocatable
program struct
type my_struct ! Declaration of a Derived Type
integer :: i
real :: r
real*8 :: r8
real, dimension(100,100) :: array_s ! stack
real, dimension(:), allocatable :: array_h ! heap
type(other_struct), dimension(5) :: os ! structure
end type my_struct
43

Modern Fortran
Fixing the Flaws
Declaration of a Structure
Variables of Derived Type
program struct
type my_struct ! Declaration of a Derived Type
...
end type my_struct
! Structures (Variables) of the the derived type my_struct
type(my_struct) :: data
type(my_struct), dimension(10) :: data_array
45

Modern Fortran
Fixing the Flaws
Example: Structures
program people
type person
character(len=10) :: name
real :: age
character(len=6) :: eid
end type person
type(person) :: you
type(person), dimension(10) :: we
you%name = ’John Doe’ ! Use (%)
you%age = 34.2 ! to access
you%eid = ’jd3456’ ! elements
we(1)%name = you%name
we(2) = you
! Old style
! name, age, eid: arrays
call do_this(name,age,eid)
! Reduce parameter list
! to one structure
call do_this_smart(we)
• Need more data =)
add a component to the
derived type
47

Modern Fortran
Module-oriented Programming
From Functions to Modules
Let’s step back for a second:
Why do we use Subprograms (Functions/Subroutines)?
Subroutines and Functions serve mainly 3 purposes:
• Re-use code blocks
• Repeat operations on different datasets
call do this(data1)
call do this(data2)
call do this(data3)
• Hide local variables, so that the names can be re-used
subroutine do this(data)
integer :: i, j ! Local variables,
real :: x, y, z ! not accessible outside of the
! subprogram
49

Modern Fortran
Modules are another, more flexible tool to Hide
Content
Modules may contain all kind of things
• Derived Type declarations
• Variables and Arrays, etc.
– Parameters (named constants)
– Variables
– Arrays
– Structures
• Subprograms
– Subroutines, Functions
– other Modules
• Objects
Fortran 2008: Modules may contain Submodules.
Will make using Modules even nicer.
(Not implemented in Intel 12, yet)
51

Modern Fortran
Example: Constants and Variables
module mad_science
real, parameter :: pi = 3. &
c = 3.e8 &
e = 2.7
real :: r
end module mad_science
program go_mad
! make the content of module available
use mad_science
r = 2.
print *, ’Area = ’, pi * r**2
end program
53

Modern Fortran
Example: Type Declarations
module mad_science
c = 3.e8 &
e = 2.7
real :: r
type scientist
logical :: mad
real :: height
end type scientist
55

Modern Fortran
Example: Subroutines and Functions
module mad_science
real, parameter :: pi = 3.
type scientist
real :: height
logical :: mad
end type scientist
contains
subroutine set_mad(s)
type(scientist) :: s
s%mad = .true.
program go_mad
use mad_science
type(scientist) :: you
type(scientist), &
dimension(10) :: we
you%name = ’John Doe’
call set_mad(you)
we(1) = you
we%mad = .true.
you%height = 5.
area = you%height * pi
• Subprograms after the contains statement
57

Modern Fortran
Example: Public, Private Subroutine
module mad_science
contains
subroutine set_mad(s)
type(scientist) :: s
call reset(s)
s%mad = .true.
private
subroutine reset(s)
s%name = ’undef’
s%mad = .false.
• A module becomes accessible when
the module is used
• Even more control: public and
private components
• Default is public: all public content
can be used from the outside of the
module, i.e. by subprograms that
use the module
• private items are only accessible
from within the module
• Example: subroutine reset is only
accessible by subroutine set mad
59

Modern Fortran
Example: Public, Private Variables
module mad_science
c = 3.e8 &
e = 2.7
private
real, dimension(100) :: scratch
real, public :: p var
contains
subroutine swap(x, y)
real, dimension(100) :: x, y
scratch(1:100) = x(1:100)
x(1:100) = y(1:100)
y(1:100) = scratch(1:100)
• Default: public
• Private items not visible outside
of the module
• private array scratch not
accessible from outside of the
module
• Keywords private or public can
stand alone, or be an attribute
61

Modern Fortran
Example: Protected Variables
module mad_science
c = 3.e8 &
e = 2.7
integer, protected :: n
real, dimension(:), private &
allocatable :: scratch
contains
subroutine alloc()
n = ... ! n defined in the module
allocate (scratch(n))
• protected variables are visible
on the outside
• protected variables cannot be
modified outside the module
• protected variables may be
modified inside of the module
• variable n is set in the module
subroutine alloc
• n is visible to all subprograms
that use the module
• n cannot by change outside of
the module
63

Modern Fortran
Example: Rename Components of a Module
module mad_science
real, parameter :: pi = 3.
end module
program t
use mad_science, mad_pi => pi
real, parameter :: pi = 3.1415
print *, ’mad_pi = ’, mad_pi
print *, ’ pi = ’, pi
end program
• Use module mad science
• change the name of pi (so that
you can declare your own and
correct pi)
• mad pi => pi: Refer to pi from
the module as mad pi
• renaming works with function
names, too
prints mad pi = 3
prints pi = 3.1415
65

Modern Fortran
Interfaces: Implicit =) Explicit
• Implicit interface: matching positions
subroutine s(a, b, c, n, ...)
...
call s(x, y, z, m, ...)
• The subroutine may be compiled separately (separate file)
from the other routine(s) or the main program that calls the
subroutine
• The position is the only information available
67

Modern Fortran
Interfaces: Implicit =) Explicit
• Explicit interface which does not solely rely on positional information
module my_module
contains
subroutine s(a, b, c, n, ...)
...
subroutine upper_level
use my_module
call s(x, y, z, m, ...)
• Modules have to be compiled first
• Compilation of a module results in a .mod file
• At compile time (Subr. upper level), the (content of the) module
(my module) is known through the .mod file (my module.mod)
• Benefits:
– Allows consistency check by the compiler
– Assume-shape arrays, optional parameters, etc.
69

Modern Fortran
Passing an array
• Traditional scheme: Shapes of the actual and the dummy array
(may) have to agree
integer, parameter :: n = 100
real, dimension(n) :: x
call sub(x, n)
subroutine sub(y, m)
integer :: m
real, dimension(m) :: y
• You can, of course, play some games here
• The shape and the size do not have to match, but you have to
explicitle declare the shape and size in the subroutine
71

Modern Fortran
Passing Assumed-shape arrays
module my_module
contains
subroutine sub(x)
real, dimension(:) :: x
print *, size(x) ! prints 100
subroutine upper_level ! calls the subroutine ‘‘sub’’
use my_module
real, dimension(100) :: y
call sub(y)
• Variable y is declared as an array in subroutine upper level
• The subroutine (sub), “knows” the shape of the array
73

Modern Fortran
Example: Assumed-shape and Automatic Arrays
subroutine swap(a, b)
real, dimension(:) :: a, b
real, dimension(size(a)) :: work ! Scratch array
! work is an automatic array on the Stack
work = a ! uses Array syntax
a = b ! Inquire with
b = work ! lbound, ubound
end subroutine swap ! shape, size
• swap has to be in a module (explicit interface)
• calling routine has to use the module containing the subroutine swap
• No need to communicate the shape of the array
• size(a) returns the size of a, used to determine the size of work
• Automatic array work appears and disappears automatically
75

Modern Fortran
Intent: In, Out, InOut
• Formalize if a parameter is
– Input: intent(in)
– Output: intent(in)
– Both: intent(inout)
subroutine calc(result, a, b, c, d)
! This routine calculates ...
! Input: a, b, c
! Output: result
! d is scratch data: Input and Output
real, intent(out) :: result
real, intent(in) :: a, b, c
real, intent(inout) :: d ! Default
• You would put this information in the comment anyway.
• Improves maintainability
• Compiler will check for misuse
77

Modern Fortran
Optional Arguments
• Optional arguments require an explicit interface
• Optional arguments may not be changed, if they are not passed
module my_module
subroutine calc(a, b, c, d)
real :: a, b, c
real, optional :: d
real :: start
if (present(d)) then
start = d
d = d_new
else
start = 0.
endif
use my_module
call calc( 1., 2., 3., 4.)
call calc( 1., 2., 3.)
call calc(a=1., b=2., c=3., d=4.)
call calc(b=2., d=4., a=1., c=3.)
call calc( 1., 2., 3., d=4.)
call calc( 1., 2., d=4., c=3)
• Positional arguments first, then keyword arguments
79

Modern Fortran
Optional Arguments
• Optional arguments require an explicit interface
• Optional arguments may not be changed, if they are not passed
module my_module
subroutine calc(a, b, c, d)
real :: a, b, c
real, optional :: d
real :: start
if (present(d)) then
start = d
d = d_new
else
start = 0.
endif
use my_module
call calc( 1., 2., 3., 4.)
call calc( 1., 2., 3.)
call calc(a=1., b=2., c=3., d=4.)
call calc(b=2., d=4., a=1., c=3.)
call calc( 1., 2., 3., d=4.)
call calc( 1., 2., d=4., c=3)
• Positional arguments first, then keyword arguments
BREAK!
79

Modern Fortran
Complaints Department
This just in from the Complaints Department
• Isn’t it really easy to screw up in these advanced languages
(Fortran2003 and C++)?
• If modern Fortran is so much like C++,
Do I have to write Object-Oriented code in Fortran?
• Isn’t C++ (supposed to be) quite ugly? Will my Fortran code be
ugly, too?
• C++ does this name-mangling. That’s hideous! Does Fortran do
the same?
• There are so many features, do I need to master all of them to write
good code?
• I’m new to Fortran. How much of the old stuff do I need to know?
• What is the bear minimum to get started?
81

Modern Fortran
A more complex language can create more confusion!
We all deal with that every day ...
83

Modern Fortran
A more complex language can create more confusion!
We all deal with that every day ...
... because as we know, there are known knowns;
there are things we know we know.
We also know there are known unknowns;
that is to say, we know there are some things we do not know.
But there are also unknown unknowns,
the ones we don’t know we don’t know ...
some politician
Perfectly valid point, but the presentation is lacking
83

Modern Fortran
Do I have to write Object-Oriented code?
No, but you have to learn (sooner or later) how to write module-oriented
code.
Writing Object-Oriented code for access control is actually pretty nice!
If you problem/algorithm requires, you may add Object-Oriented code
exploiting Polymorphism (supported in Fortran2003 & 2008).
Learn later, how to write Object-Oriented code in Fortran without
performance penalty; Access control only.
85

Modern Fortran
Isn’t C++ code (supposed to be) ugly?
Will my Fortran2003 code be ugly, too?
Write clean code
Clean code is not ugly (in any language: C++ and/or modern Fortran)
• Use blanks, blank lines, indentation
• Comment your code
• Use modern constructs
• Use the language in a clear, unambigious manner
87

Modern Fortran
C++ does name-mangling
Does Fortran do the same?
It’s not a bug, it is a feature!
• It protects against misuse
• The objects (.o files) in your library (.a files) contain ”protected”
names
• If you do it right, name mangling causes no problems (see also
chapter on Interoperability with C)
89

Modern Fortran
There are so many features.
Do I have to master all of them?
Here is how you get started:
• Do not use common blocks or equivalence statements!
If you find yourself in a situation where you think they are needed, please
revisit the modern constructs
• Use Heap arrays: allocate and deallocate (2 slides)
• Use structures to organzie your data (3 slides)
=) Heap arrays + structures:
There is Absolutely! no need for common blocks and equivalence
statements
• Use Modules: start writing module-oriented code (2 slides)
91

Modern Fortran
Here is how you get started: cont’d
Use Modules: start writing module-oriented code
• What to put in a Module:
1. Constants (parameters)
2. Derived type declarations
avoid repeating parameter and derived type definitions. Sometimes
physical constants are put in an include file. This should be done
using a module.
3. Variables (probably not?)
4. Functions and Subroutines,
move on by using the public, private and protected attributes
5. Write Object-Oriented code without performance penalty
6. Use Inheritance and Polymorphism with care
What about learning old Fortran (F77 and older)?
• Don’t bother, if you don’t have to
• Learn how to read code, assume that the code works correctly
93

Modern Fortran
Formula Translation
Formula Tranlation
• Array syntax
• where construct
• forall construct
• Case study: Stencil Update
• User defined Operators
• Elemental Functions
• Inquiry Functions
• Odds and Ends
95

Modern Fortran
Formula Translation
Simple Array Syntax
real :: x
real, dimension(10) :: a, b
real, dimension(10,10) :: c, d
a = b
c = d
a(1:10) = b(1:10)
a(2:3) = b(4:5)
a(1:10) = c(1:10,2)
a = x
c = x
a(1:3) = b(1:5:2) ! a(1) = b(1)
! a(2) = b(3)
! a(3) = b(5)
• Variables on the left and the
right have to be conformable
• Number of Elements have to
agree
• Scalars are conformable, too
• Strides can be used, too
97

Modern Fortran
Formula Translation
Array constructor
real, dimension(4) :: x = [ 1., 2., 3. 4. ]
real, dimension(4) :: y, z
y = [ -1., 0., 1., 2. ] ! Array constructor
z(1:4) = [ (sqrt(real(i)), i=1, 4) ] ! with implicit
! loop
real, dimension(:), &
allocatable :: x
...
x = [ 1, 2, 3 ]
print *, size(x)
x = [ 4, 5 ]
print *, size(x)
prints 3
prints 2
99

Modern Fortran
Formula Translation
Derived Type constructor
type person
real :: age
character :: name
integer :: ssn
end type person
type(person) :: you
you = [ 17., ’John Doe’, 123456789 ]
101

Modern Fortran
Formula Translation
Arrays as Indices
real, dimension(5) :: &
a = [ 1, 3, 5, 7, 9 ]
i = [ 2, 4 ]
print *, a(i) prints 3. 7.
• Variable i is an array (vector)
• a(i) is [ a(i(1)), a(i(2)), ... ]
103

Modern Fortran
Formula Translation
where statement
x = [ -1, 0, 1, 2 ] &
a = [ 5, 6, 7, 8 ]
...
where (x < 0)
a = -1.
end where
where (x /= 0)
a = 1. / a
elsewhere
a = 0.
end where
• arrays must have the same
shape
• code block executes when
condition is true
• code block can contain
– Array assignments
– other where constructs
– forall constructs
105

Modern Fortran
Formula Translation
where statement
real :: v
real, dimension(100,100) :: x
...
call random_number(v) ! scalar
call random_number(x) ! array
where (x < 0.5)
x = 0.
end where
• Distinction between scalar and
array vanishes
call to random number()
• Subroutine random number
accepts scalars and arrays
• see also slides on elemental
functions
107

Modern Fortran
Formula Translation
any statement
real, dimension(n,n) :: a, b, c1, c2
c1 = my_matmul(a, b) ! home-grown function
c2 = matmul(a, b) ! built-in function
if (any(abs(c1 - c2) > 1.e-4)) then
print *, ’There are significant differences’
endif
• matmul (also dot product) is provided by the compiler
• abs(c1 - c2): Array syntax
• any returns one logical
109

Modern Fortran
Formula Translation
Example: Stencil Update Ai = (Ai−1 + Ai+1)/2.
real, dimension(n) :: v
real :: t1, t2
...
t2 = v(1)
do i=2, n-1
t1 = v(i)
v(i) = 0.5 * (t2 + v(i+1))
t2 = t1
enddo
111

Modern Fortran
Formula Translation
Example: Stencil Update Ai = (Ai−1 + Ai+1)/2.
real, dimension(n) :: v
real :: t1, t2
...
t2 = v(1)
do i=2, n-1
t1 = v(i)
v(i) = 0.5 * (t2 + v(i+1))
t2 = t1
enddo
v(2:n-1) = 0.5 * (v(1:n-2) + v(3:n))
• Traditional scheme requires scalar variables
• Array syntax: Evaluate RHS, then “copy” the result
111

Modern Fortran
Formula Translation
Stencil Update Ai,j = (Ai−1,j + Ai+1,j + Ai,j−1 + Ai,j+1)/4.
real, dimension(n,n) :: a, b
do j=2, n-1
do i=2, n-1
b(i,j) = 0.25 *
(a(i-1,j) + a(i+1,j) + a(i,j-1) + a(i,j+1))
enddo
enddo
do j=2, n-1
do i=2, n-1
a(i,j) = b(i,j)
enddo
enddo
• Two copies required: b = f(a); a = b
113

Modern Fortran
Formula Translation
a(2:n-1,2:n-1) = 0.25 *
(a(1:n-2,2:n) + a(3:n,2:n) + a(2:n,1:n-2) + a(2:n,3:n))
• No copy required (done internally)
115

Modern Fortran
Formula Translation
a(2:n-1,2:n-1) = 0.25 *
(a(1:n-2,2:n) + a(3:n,2:n) + a(2:n,1:n-2) + a(2:n,3:n))
• No copy required (done internally)
Now with the forall construct
forall (i=2:n-1, j=2:n-1) &
a(i,j) = 0.25 *
(a(i-1,j) + a(i+1,j) + a(i,j-1) + a(i,j+1))
• Fortran statement looks exactly like the original formula
115

Modern Fortran
Formula Translation
Detached Explicit Interfaces
• Enables User-defined Operators and Generic Subprograms
• The interface can be detached from the routine
• Only the interface may reside in the module (like in a C header file)
• Comes in handy, when a large number of people (n>1) work on one
project
module my_interfaces
interface
subroutine swap(a, b)
real, dimension(:) :: a, b
real, dimension(size(a)) :: work ! Scratch array
end subroutine
end interface
• Any subprogram that calls swap has to use the module my interfaces
117

Modern Fortran
Formula Translation
Generic Interfaces — Function/Subroutine Overload
Motivation: Write code that allows to swap two variables of type real
and two variables of type integer
• Subroutine 1: swap real()
• Subroutine 2: swap integer()
module mod_swap
contains
subroutine swap_real(x, y)
real :: x, y, t
t = x; x = y; y = t
end subroutine
subroutine swap_integer(i, j)
real :: i, j, k
k = i; i = j; j = k
end subroutine
end module
program p_swap
use mod_swap
real :: a, b
integer :: i1, i2
! Get a, b, i1 and i2 from
! somewhere
call swap_real(a, b)
call swap_integer(i1, i2)
end program
119

Modern Fortran
Formula Translation
• Add a generic interface (swap) to both routines
• swap with real arguments ! swap real
• swap with integer arguments ! swap integer
module mod_swap
public swap
private swap_real, swap_integer
interface swap
module procedure &
swap_real, swap_integer
end interface
contains
real :: x, y, t
t = x; x = y; y = t
end subroutine
subroutine swap_integer(i, j)
real :: i, j, k
k = i; i = j; j = k
end subroutine
end module
121

Modern Fortran
Formula Translation
module mod_swap
public swap
private swap_real, swap_integer
interface swap
module procedure &
swap_real, swap_integer
end interface
contains
...
program p_swap
use mod_swap
call swap(a, b) ! swap_real
call swap(i1, i2) ! swap_integer
call swap_real(a, b) ! Does NOT
! compile!
end program
• Interface swap is public
• Inner workings (swap real,
swap integer) are private
• User of module mod swap cannot
access/mess-up ”inner” routines
123

Modern Fortran
Formula Translation
• Anything distinguishable works
• real, integer, real(8), ...
• Only one argument may differ
module mod_swap
public swap
private swap_real, swap_real8
interface swap
module procedure &
swap_real, swap_real8
end interface
contains
real :: x, y, t
t = x; x = y; y = t
end subroutine
subroutine swap_real8(x, y)
real(8) :: x, y, t
t = x; x = y; y = t
end subroutine
end module
125

Modern Fortran
Formula Translation
User-defined Operators
module operator
public :: operator(.lpl.)
private :: log plus log
interface operator(.lpl.)
module procedure log plus log
end interface
contains
function log plus log(x, y) &
result(lpl result)
real, intent(in) :: x, y
real :: lpl result
lpl_result = log(x) + log(y)
end function
end module
program op
use operator
print *, 2. .lpl. 3.
end program
• prints 1.791759
• .lpl. is the new operator
(defined public)
• rest of the definition is private
– interface
– function log plus log
• .lpl. is defined as
log(x) + log(y)
• log(2.) + log(3.) = 1.791759
127

Modern Fortran
Formula Translation
Elemental Functions
module e_fct
elemental function sqr(x) &
result(sqr_result)
real, intent(in) :: x
real :: sqr_result
sqr_result = x * x
end function
end module
• Write function for scalars
• add elemental
• routine will also accept arrays
program example
use e_fct
real :: x = 1.5
real, dimension(2) :: a = &
[ 2.5, 3.5 ]
print *, ’x = ’, sqr(x)
print *, ’a = ’, sqr(a)
end program
• prints a = 2.25
• prints x = 6.25 12.25
• allows to extend array syntax to
more operations
129

Modern Fortran
Formula Translation
where/any in combination with elemental functions
module e_fct
elemental function log_sqr(x) &
result(ls_result)
real, intent(in) :: x
real :: ls_result
ls_result = log(sqr(x))
end function
end module
• Put an elemental function in a
module
subroutine example(x, y)
use e_fct
real, dimension(100) :: x, y
where (log_sqr(x) < 0.5)
y = x * x
end where
if (any(log_sqr(x) > 10.)) then
print *, ’... something ...’
endif
end program
• Use elemental function with
where and any
131

Modern Fortran
Formula Translation
Inquiry Functions
• digits(x): numer of significant digits
• epsilon(x): smallest with 1 + 1
• huge(x): largest number
• maxexponent/minexponent: largest/smallest exponent
• tiny(x): smallest positive number (that is not 0.)
• ubound, lbound, size, shape, ...
• input unit, output unit, error unit
• file storage size (Good when you use the Intel compiler!)
• character storage size, numeric storage size
• etc.
133

Modern Fortran
Formula Translation
Mathematical Functions
• sin, cos, tan, etc.
• New in Fortran 2008: Bessel fct., Error-fct., Gamma-fct., etc.
135

Modern Fortran
Odds and Ends
Fortran pointers (Aliases)
real, dimension(n*n), target :: data
real, dimension(:), pointer :: ptr, diag
real, dimension(:), allocatable,
pointer :: ptr_alloc
...
ptr = data
diag = data(1: :1001) ! start, end, stride
allocate(ptr_alloc(100))
• Pointer asscociation : “Pointing to”
• Pointer is of the same type as the target
• Target has the target attribute (needed for optimization)
• Pointers can have memory allocated by themselves (ptr alloc in C)
• Pointers are usefull to create “linked lists” (not covered here)
137

Modern Fortran
Odds and Ends
real, dimension(n,n), target :: data
real, dimension(:), pointer :: row, col
...
row = data(4,:) ! 4th row
col = data(:,2) ! 2nd column
print *, row, col ! Use pointer like a variable
• Pointers col and row are pointing to a colum/row of the 2-dim array
data
• Memory is not contigous for row
• When you pass row to a subroutine, a copy-in/copy-out may be
necessary
• What is ’=’ good for? Referencing and de-referencing is
automatic, so a special symbol is needed for pointing
139

Modern Fortran
Odds and Ends
real, dimension(100), target :: array1, array2, temp
real, dimension(:), pointer :: p1, p2, ptmp
...
temp = array1 ! Copy the whole array 3 times
array1 = array2 ! Very costly!
array2 = temp
...
ptmp = p1 ! Move the Pointers
p1 = p2 ! Very cheap!
p2 = ptmp
• Avoid copying data
• Switch the pointers
• Use the pointers as of they were normal variables
141

Modern Fortran
Odds and Ends
Command Line Arguments
command_argument_count() ! Function: returns
! number of arguments
call get_command argument(number, value, length, status)
! input: number
! output: value, length, status
! (all optional)
call get_command(command, length, status)
! output: command, length, status
Example:
./a.out option X
character(len=16) :: command
call get_command(command)
print command ! prints: ./a.out option X
143

Modern Fortran
Odds and Ends
Environment Variables
call get_environment_variable(name, value)
! Input : name
! Output: value
character(len=16) :: value
call get_environment_variable(’SHELL’, value)
print value ! prints /bin/bash
145

Modern Fortran
Odds and Ends
Fortran Prepocessor
• same as in C (#ifdef, #ifndef, #else, #endif)
• compile with -fpp
• use option -Dvariable to set variable to true
• Example: ifort -Dmacro t.f
#ifdef macro
x = y
#else
x = z
#endif
147

Modern Fortran
Odds and Ends
Interoperability with C (Name Mangling)
• Variables, Functions and Subroutines, etc., that appear in modules
have mangled names
• This enables hiding the components from misuse
• No naming convention for the mangled names
file t.f
module operator
real :: x
contains
subroutine s()
return
end subroutine
end
compile with:
ifort -c t.f
result is t.o
nm t.o prints this:
(nm is a Unix command)
T _operator_mp_s_
C operator_mp_x_
149

Modern Fortran
Odds and Ends
Give Objects (in object file) a specific Name
• Use intrinsic module (iso c binding) to use pass strings (not shown
here)
file t.f
module operator
real, bind(C) :: x
contains
subroutine s()
bind(C, name=’_s’)
return
end subroutine
end
compile with:
ifort -c t.f
result is t.o
nm t.o prints this:
T _s
C _x
151

Modern Fortran
Odds and Ends
Use C-compatible variable types
• Use variables of a special kind
• c float, c double, c int, c ptr, etc.
• works with characters, too
module operator
real, bind(C) :: x
type, bind(C) :: c_comp
real(c_float) :: data
integer(c_int) :: i
type(c_ptr) :: ptr
end type
contains
subroutine s()
bind(C, name=’_s’)
Arrays:
‘‘Fortran’’:
real(c float) :: x(5,6,7)
‘‘C’’:
float y[7][6][5]
153

Modern Fortran
Odds and Ends
Not Covered
• Floating-point Exception Handling
• Linked-Lists, Binary Trees
• Recursion
• I/O (Stream Data Access)
• Object-Oriented Programming, but see introduction in the next
chapter
155

Modern Fortran
History
History of Fortran
1954 1960 1965 1970 1975 1980 1985 1990 1995 2000 2001 2002 2003 2004
1986 1990 1990 1991 1991 1993 1994 1995 1996 1996 1997 1997 2000 2001 2001 2003 2003 2004
History of Programming Languages
©2004 O’Reilly Media, Inc. O’Reilly logo is a registered trademark of O’Reilly Media, Inc. All other trademarks are property of their respective owners. part#30417
For more than half of the fifty years computer programmers have been
writing code, O’Reilly has provided developers with comprehensive,
in-depth technical information. We’ve kept pace with rapidly changing
technologies as new languages have emerged, developed, and
matured. Whether you want to learn something new or need
answers to tough technical questions, you’ll find what you need
in O’Reilly books and on the O’Reilly Network.
This timeline includes fifty of the more than 2500 documented
programming languages. It is based on an original diagram created
by Éric Lévénez (www.levenez.com), augmented with suggestions
from O’Reilly authors, friends, and conference attendees.
For information and discussion on this poster,
go to www.oreilly.com/go/languageposter.
www.oreilly.com
Fortran started in 1954; the first “line” in the diagram.
157

Modern Fortran
History
Fortran 90+: 90, 95, 2003, 2008
• Modern, efficient, and appropriate for
Number Crunching and High Performance Computing
• Upgrades every few years: 90, 95, 2003, 2008, ...
• Major upgrade every other release: 90, 2003
• Easy switch: F90 is fully compatible with F77
Where are we now?
• F2003 fully supported by Cray, IBM, PGI and Intel compilers
• F2008 is partially supported
159

Modern Fortran
The Future
Performance Considerations and Object-Oriented
Programming
• (Most of the) Language elements shown in this class do not have
(any/severe) performance implications
– Most of the module-oriented programming handles access
– Some array syntax may! be done better in explicit loops,
if more than one statement can be grouped into one loop
– Pointers that have non-contigous elements in memory may! require a
copy in/out, when passed to a subprogram
– Compiler can warn you (Intel: -check arg temp created)
– Use pointers (references) and ]em non-contigous data with care
• Fortran allows for an Object-Oriented Programming style
– Access control, really a great concept!
– Type extension, Polymorphic entities
– Use with care (may be slower),
– but use these features if you algorithm requires and the
implemenation benefits from it
161

Modern Fortran
The Future
Functions, Modules, Objects
• Use Functions and Subroutines to hide local Data
• Use Modules to hide Data, Functions and Subroutines
• Use Objects to hide Data and expose Methods
163

Modern Fortran
The Future
Book Recommendations
• Fortran 95/2003 for Scientists and Engineers by Chapman
Very! verbose, with many examples. Guides the programmer nicely
towards a good programming style. (International/cheaper edition
available)
• modern fortran explained by Metcalf, Reid and Cohen
Good to learn new features; more complete than the Guide (1), but
sometimes a bit confusing. Covers Fortran 2008
• Guide to Fortran 2003 Programming by Walter S. Brainerd
Good to learn the new features, clever examples
• The Fortran 2003 Handbook by Adams, Brainerd, et al.
Complete syntax and Reference
Some Guidance is definitely needed
• The same task may be accomplished in several ways
• What to use When?
165

Object-Oriented Programming: (Very) Short Version
OO Concept in 1 Slide
• Objects contain (have the properties):
Data — Instance Variables or Fields
Subr./Fct. — Instance Methods
Polymorphism/Inheritance — to allow for a lot of flexibility
• Data is only accessible through the methods
• OO-speak: Call of a Subr. (instance method) Sending a Message
• A Class is a blueprint for a Object
Similar to a Derived Type being a blueprint for a structure
type(data) :: structure containing variables
class(data plus fct) :: object containing variables and functions
• Classes are organized in Hierarchies and can inherit
instance variables and methods from higher level classes
• An object can have many forms (polymorphism), depending on
context
167

Example of an Object in Fortran2003
module my_mod
type, public :: person
character(len=8), private ::
name
integer, private ::
iage
contains
procedure, public :: set
procedure, public :: out
end type person
private; contains
• Interface is Public
• Subroutines are Private
subroutine set(p, name, iage)
class(person) :: p
character(len=*) :: name
integer :: iage
p%name = name
p%iage = iage
write (0,*) ’set’
end subroutine
subroutine out(p)
class(person) :: p
write (0,*) p%name, p%iage
end subroutine
end module
169

How to use the Class defined in my mod:
Non-polymorphic object
program op
use my_mod
! Non-polymorphic
type(person), allocatable :: x
type(person), pointer :: y
allocate(x, y)
call x%set(’J. Doe’, 25)
call x%out ! or call y%out
end
• Declare object as a type
• Non-polymorphic: No
performance penalty
• Access to the data only through
approved methods
• Object may be a pointer
Note:
x%set called with 2 arguments,
but Subroutine has 3 arguments
171

How to use the Class defined in my mod:
Polymorphic object
program op
use my_mod
! Polymorphic
class(person), pointer :: z
allocate(z)
call z%set(’J. Doe’, 25)
call z%out
end
• Declare object as a class
• Polymorphic: full OO
functionality
• Object must be a pointer
Note:
z%set called with 2 arguments,
but Subroutine has 3 arguments
173

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