fortran_fortran examples - wiki books, open books for an open world

8/3/2019 Fortran_Fortran Examples - Wiki Books, Open Books for an Open World

1/20

Fortran/Fortran examples

Part of the Fortran WikiBook

The following Fortran code examples or sample programs show different situations depending on the

compiler. The first set of examples are for the Fortran II, IV, and 77 compilers. The remaining examples can be

compiled and run with any newer standard Fortran compiler (see the end of the main Fortran article for lists of

compilers). Most modern Fortran compilers expect a file with a .f or .for extension (for FORTRAN 66 or

FORTRAN 77 source, although the FORTRAN 66 dialect may have to be selected specifically with a

command-line option) or .f90/.f95 extension (for Fortran 90/95 source, respectively).

FORTRAN II, IV, and 77 compilers

NOTE: Before FORTRAN 90, most FORTRAN compilers enforced fixed-format source code, a carryover

from IBM punch cards (http://en.wikipedia.org/wiki/Punch_card)

comments must begin with a * or C or ! in column 1

statement labels must occur in columns 1-5

continuation lines must have a non-blank character in column 6

statements must start in column 7

the line-length may be limited to 72 characters (derived from the 80-byte width of a punch-card, with last

8 characters reserved for (optional) sequence numbers)

If errors are produced when you compile your FORTRAN code, first check the column alignment. Some

compilers also offer free form source (http://en.wikipedia.org/wiki/Free-form_language) by using a compiler

flag

Area Of a Triangle program

Simple Fortran II program

One data card input

If one of the input values is zero, then the program will end with an error code of "1" in the job control card

listing following the execution of the program. Normal output will be one line printed with A, B, C, and AREA.

No specific units are stated.

C AREA OF A TRIANGLE - HERON'S FORMULA

C INPUT - CARD READER UNIT 5, INTEGER INPUT

C OUTPUT - LINE PRINTER UNIT 6, REAL OUTPUT

C INPUT ERROR DISPAY ERROR OUTPUT CODE 1 IN JOB CONTROL LISTING

INTEGER A,B,C

READ(5,501) A,B,C

501 FORMAT(3I5)

IF(A.EQ.0 .OR. B.EQ.0 .OR. C.EQ.0)STOP1

S =(A + B + C)/2.0

an/Fortran examples - Wikibooks, open books for an open world http://en.wikibooks.org/wiki/Fortran/Fortran_

0 10/28/2011


2/20

AREA =SQRT( S *(S - A)*(S - B)*(S - C))

WRITE(6,601) A,B,C,AREA

601 FORMAT(4H A= ,I5,5H B= ,I5,5H C= ,I5,8H AREA= ,F10.2,12HSQUARE

STOP

END

Simple Fortran IV program

Multiple data card input

This program has two input checks: one for a blank card to indicate end-of-data, and the other for a zero value

within the input data. Either condition causes a message to be printed.


C INPUT - CARD READER UNIT 5, INTEGER INPUT, ONE BLANK CARD FOR END-OF-


C INPUT ERROR DISPAY ERROR MESSAGE ON OUTPUT

501 FORMAT(3I5)

601 FORMAT(4H A= ,I5,5H B= ,I5,5H C= ,I5,8H AREA= ,F10.2,12HSQUARE

602 FORMAT(10HNORMAL END)

603 FORMAT(23HINPUT ERROR, ZERO VALUE)

INTEGER A,B,C

10 READ(5,501) A,B,C

IF(A.EQ.0 .AND. B.EQ.0 .AND. C.EQ.0)GOTO50

IF(A.EQ.0 .OR. B.EQ.0 .OR. C.EQ.0)GOTO90

S =(A + B + C)/2.0


WRITE(6,601) A,B,C,AREA GOTO10

50 WRITE(6,602)

STOP

90 WRITE(6,603)

STOP

END

Simple Fortran 77 program

Multiple data card input

This program has two input checks in the READ statement with the END and ERR parameters, one for a blank

card to indicate end-of-data; and the other for zero value along with valid data. In either condition, a message

will be printed.


C INPUT - CARD READER UNIT 5, INTEGER INPUT, NO BLANK CARD FOR END OF D



0 10/28/2011


3/20

C INPUT ERROR DISPAYS ERROR MESSAGE ON OUTPUT

501 FORMAT(3I5)

601 FORMAT(" A= ",I5," B= ",I5," C= ",I5," AREA= ",F10.2,"SQUARE U

602 FORMAT("NORMAL END")

603 FORMAT("INPUT ERROR OR ZERO VALUE ERROR")

INTEGER A,B,C

10 READ(5,501,END=50,ERR=90) A,B,C

IF(A=0.OR. B=0.OR. C=0)GOTO90

S =(A + B + C)/2.0


WRITE(6,601) A,B,C,AREA

GOTO10

50 WRITE(6,602)

STOP

90 WRITE(6,603)

STOP

END

"Retro" FORTRAN IV

A retro example of a FORTRAN IV (later evolved into FORTRAN 66) program deck is available on the IBM

1130 page, including the IBM 1130 DM2 JCL required for compilation and execution. An IBM 1130 emulator

is available at IBM 1130.org (http://ibm1130.org/) that will allow the FORTRAN IV program to be compiled

and run on a PC.

Hello, World program

In keeping with computing tradition, the first example presented is a simple program to display the words"Hello, world" on the screen (or printer).

FORTRAN 66 (also FORTRAN IV)

C FORTRAN IV WAS ONE OF THE FIRST PROGRAMMING

C LANGUAGES TO SUPPORT SOURCE COMMENTS

WRITE (6,7)

7 FORMAT(13H HELLO, WORLD)

STOP

END

This program prints "HELLO, WORLD" to Fortran unit number 6, which on most machines was the line printer

or terminal. (The card reader or keyboard was usually connected as unit 5). The number 7 in the WRITE

statement refers to the statement number of the corresponding FORMAT statement. FORMAT statements may be

placed anywhere in the same program or function/subroutine block as the WRITE statements which reference

them. Typically a FORMAT statement is placed immediately following the WRITE statement which invokes it;

alternatively, FORMAT statements are grouped together at the end of the program or subprogram block. If

execution flows into a FORMAT statement, it is a no-op; thus, the example above has only two executable


0 10/28/2011


4/20


5/20

PROGRAM EUCLID

PRINT *, 'A?'

READ *, NA

IF(NA.LE.0)THEN

PRINT *, 'A must be a positive integer.'

STOP

ENDIF

PRINT *, 'B?'

READ *, NB

IF(NB.LE.0)THEN

PRINT *, 'B must be a positive integer.'

STOP

ENDIF

PRINT *, 'The GCD of', NA, ' and', NB, ' is', NGCD(NA, NB), '.'

STOP

END

FUNCTION NGCD(NA, NB)

IA = NAIB = NB

1 IF(IB.NE.0)THEN

ITEMP = IA

IA = IB

IB =MOD(ITEMP, IB)

GOTO1

ENDIF

NGCD = IA

RETURN

END

The above example is intended to illustrate the following:

The PRINT and READ statements in the above use '*' as a format, specifying list-directed formatting.

List-directed formatting instructs the compiler to make an educated guess about the required input or

output format based on the following arguments.

As the earliest machines running Fortran had restricted character sets, FORTRAN 77 uses abbreviations

such as .EQ., .NE., .LT., .GT., .LE., and .GE. to represent the relational operators =, , , , and ,

respectively.

This example relies on the implicit typing mechanism to specify the INTEGER types ofNA, NB, IA, IB,

and ITEMP.

In the function NGCD(NA, NB), the values of the function arguments NA and NB are copied into the local

variables IA and IB respectively. This is necessary as the values ofIA and IB are altered within the

function. Because argument passing in Fortran functions and subroutines utilize call by reference by

default (rather than call by value, as is the default in languages such as C), modifying NA and NB from

within the function would effectively have modified the corresponding actual arguments in the main

PROGRAM unit which called the function.

The following shows the results of compiling and running the program.


0 10/28/2011


6/20


7/20

EXP() corresponds to the exponential function ex. In FORTRAN 77, this is a generic function, meaning

that it accepts arguments of multiple types (such as REAL and, in this example, COMPLEX). In FORTRAN

66, a specific function would have to be called by name depending on the type of the function arguments

(for this example, CEXP() for a COMPLEX-valued argument).

When applied to a COMPLEX-valued argument, REAL() and AIMAG() return the values of the argument's

real and imaginary components, respectively.

Incidentally, the output of the above program is as follows (see the article on Euler's formula for the geometricinterpretation of these values as eight points spaced evenly about a unit circle in the complex plane).

$ cmplxd

e**(j*0*pi/4) = 1.0000000 + j0.0000000

e**(j*1*pi/4) = 0.7071068 + j0.7071068

e**(j*2*pi/4) = 0.0000000 + j1.0000000

e**(j*3*pi/4) = -0.7071068 + j0.7071068

e**(j*4*pi/4) = -1.0000000 - j0.0000001

e**(j*5*pi/4) = -0.7071066 - j0.7071069

e**(j*6*pi/4) = 0.0000000 - j1.0000000e**(j*7*pi/4) = 0.7071070 - j0.7071065

Error can be seen occurring in the last decimal place in some of the numbers above, a result of the COMPLEX data

type representing its real and imaginary components in single precision. Incidentally, Fortran 90 also made

standard a double-precision complex-number data type (although several compilers provided such a type even

earlier).

Fortran 90/95 examples

Summations with a DO loop

In this example of Fortran 90 code, the programmer has written the bulk of the code inside of a DO loop. Upon

execution, instructions are printed to the screen and a SUM variable is initialized to zero outside the loop. Once

the loop begins, it asks the user to input any number. This number is added to the variable SUM every time the

loop repeats. If the user inputs 0, the EXIT statement terminates the loop, and the value of SUM is displayed on

screen.

Also apparent in this program is a data file. Before the loop begins, the program creates (or opens, if it has

already been run before) a text file called "SumData.DAT". During the loop, the WRITE statement stores any

user-inputted number in this file, and upon termination of the loop, also saves the answer.

! sum.f90

! Performs summations using in a loop using EXIT statement

! Saves input information and the summation in a data file

program summation

implicitnone

integer:: sum, a


0 10/28/2011


8/20

print*, "This program performs summations. Enter 0 to stop."

open(unit=10, file="SumData.DAT")

sum =0

do

print*, "Add:"

read*, a

if(a ==0)then

exit

else

sum = sum + a

endif

write(10,*) a

enddo

print*, "Summation =", sum

write(10,*)"Summation =", sumclose(10)

end

When executed, the console would display the following:

This program performs summations. Enter 0 to stop.

Add:

1

Add:

2

Add:

3

Add:

0

Summation = 6

And the file SumData.DAT would contain:

1

2

3

Summation = 6

Calculating cylinder area

The following program, which calculates the surface area of a cylinder, illustrates free-form source input and


0 10/28/2011


9/20

other features introduced by Fortran 90.

program cylinder

! Calculate the surface area of a cylinder.

!

! Declare variables and constants.

! constants=pi! variables=radius squared and height

implicitnone ! Require all variables to be explicitly declared

integer:: ierr

character(1):: yn

real:: radius, height, area

real, parameter:: pi =3.141592653589793

interactive_loop:do

! Prompt the user for radius and height

! and read them.

write (*,*)'Enter radius and height.'

read (*,*,iostat=ierr) radius,height

! If radius and height could not be read from input,

! then cycle through the loop.

if(ierr /=0)thenwrite(*,*)'Error, invalid input.'

cycle interactive_loop

endif

! Compute area. The ** means "raise to a power."

area =2* pi *(radius**2+ radius*height)

! Write the input variables (radius, height)

! and output (area) to the screen.

write (*,'(1x,a7,f6.2,5x,a7,f6.2,5x,a5,f6.2)')&

'radius=',radius,'height=',height,'area=',area

yn =' '

yn_loop:do

write(*,*)'Perform another calculation? y[n]'

read(*,'(a1)') yn

if(yn=='y'.or. yn=='Y')exit yn_loop

if(yn=='n'.or. yn=='N'.or. yn==' ')exit interactive_loop


0 10/28/2011


10/20

enddo yn_loop

enddo interactive_loop

endprogram cylinder

Dynamic memory allocation and arrays

The following program illustrates dynamic memory allocation and array-based operations, two features

introduced with Fortran 90. Particularly noteworthy is the absence ofDO loops and IF/THEN statements in

manipulating the array; mathematical operations are applied to the array as a whole. Also apparent is the use of

descriptive variable names and general code formatting that comport with contemporary programming style.

This example computes an average over data entered interactively.

program average

! Read in some numbers and take the average

! As written, if there are no data points, an average of zero is return

! While this may not be desired behavior, it keeps this example simple

implicitnone

integer:: number_of_points

real, dimension(:), allocatable:: points

real:: average_points=0., positive_average=0., negative_average=0.

write (*,*)"Input number of points to average:"

read (*,*) number_of_points

allocate(points(number_of_points))

write (*,*)"Enter the points to average:"

read (*,*) points

! Take the average by summing points and dividing by number_of_points

if(number_of_points > 0) average_points = sum(points)/number_of_point

! Now form average over positive and negative points only

if(count(points > 0.) > 0) positive_average = sum(points, points > 0.

/count(points > 0.)

if(count(points < 0.) > 0) negative_average = sum(points, points < 0.

/count(points < 0.)

deallocate(points)

! Print result to terminal

write (*,'(''Average = '', 1g12.4)') average_points

write (*,'(''Average of positive points = '', 1g12.4)') positive_aver

write (*,'(''Average of negative points = '', 1g12.4)') negative_aver


20 10/28/2011


11/20


12/20

! dot_product(a,v)=a'b

tol_max =max((abs(x(i)- xk)/(1. +abs(xk)))**2, abs(A(i, i)

x(i)= xk

enddo iteration_loop

enddo convergence_loop

if(present(actual_iter)) actual_iter = iter

endfunction gauss_sparse

Note that an explicit interface to this routine must be available to its caller so that the type signature is known.

This is preferably done by placing the function in a MODULE and then USEing the module in the calling routine.

An alternative is to use an INTERFACE block, as shown by the following example:

program test_gauss_sparse

implicitnone

! explicit interface to the gauss_sparse function

interface

function gauss_sparse(num_iter, tol, b, A, x, actual_iter)resu

real:: tol_max

integer, intent(in):: num_iter

real, intent(in):: tol

real, intent(in), dimension(:):: b, A(:,:)

real, intent(inout):: x(:)

integer, optional, intent(out):: actual_iter

endfunction

endinterface

! declare variables

integer:: i, N =3, actual_iter

real:: residue

real, allocatable:: A(:,:), x(:), b(:)

! allocate arrays

allocate(A(N, N), b(N), x(N))

! Initialize matrix

A =reshape([(real(i), i =1, size(A))], shape(A))

! Make matrix diagonally dominant

do i =1, size(A, 1)

A(i,i)= sum(A(i,:))+1

enddo

! Initialize b

b =[(i, i =1, size(b))]


20 10/28/2011


13/20

! Initial (guess) solution

x = b

! invoke the gauss_sparse function

residue = gauss_sparse(num_iter =100, &

tol = 1E-5, &

b = b, &

A = a, &

x = x, &

actual_iter = actual_iter)

! Output

print '(/ "A = ")'

do i =1, size(A, 1)

print '(100f6.1)', A(i,:)

enddo

print '(/ "b = " / (f6.1))', b

print '(/ "residue = ", g10.3 / "iterations = ", i0 / "solution = "

residue, actual_iter, x

endprogram test_gauss_sparse

Writing subroutines

In those cases where it is desired to return values via a procedure's arguments, a subroutine is preferred over a

function; this is illustrated by the following subroutine to swap the contents of two arrays:


20 10/28/2011


14/20

subroutine swap_real(a1, a2)

implicitnone

! Input/Output

real, intent(inout):: a1(:), a2(:)

! Locals

integer:: i

real:: a

! Swap

do i =1, min(size(a1), size(a2))

a = a1(i)

a1(i)= a2(i)

a2(i)= a

enddo

endsubroutine swap_real

As in the previous example, an explicit interface to this routine must be available to its caller so that the type

signature is known. As before, this is preferably done by placing the function in a MODULE and then USEing the

module in the calling routine. An alternative is to use a INTERFACE block.

Internal and Elemental Procedures

An alternative way to write the swap_real subroutine from the previous example, is:


implicitnone

! Input/Output


! Locals

integer:: N

! Swap, using the internal subroutine

N =min(size(a1), size(a2))

call swap_e(a1(:N), a2(:N))

contains

elemental subroutine swap_e(a1, a2)

real, intent(inout):: a1, a2

real:: a

a = a1


20 10/28/2011


15/20

a1 = a2

a2 = a

endsubroutine swap_e


In the example, the swap_e subroutine is elemental, i.e., it acts upon its array arguments, on an element-

by-element basis. Elemental procedures must be pure (i.e., they must have no side effects and can invoke only

pure procedures), and all the arguments must be scalar. Since swap_e is internal to the swap_real subroutine, no

other program unit can invoke it.

The following program serves as a test for any of the two swap_real subroutines presented:

program test_swap_real

implicitnone

! explicit interface to the swap_real subroutine

interface




endinterface

! Declare variables

integer:: i

real:: a(10), b(10)

! Initialize a, b

a =[(real(i), i =1, 20, 2)]b = a +1

! Output before swap

print '(/"before swap:")'

print '("a = [", 10f6.1, "]")', a

print '("b = [", 10f6.1, "]")', b

! Call the swap_real subroutine

call swap_real(a, b)

! Output after swapprint '(// "after swap:")'

print '("a = [", 10f6.1, "]")', a

print '("b = [", 10f6.1, "]")', b

endprogram test_swap_real

Pointers and targets methods


20 10/28/2011


16/20

In Fortran, the concept of pointers differs from that in C-like languages. A Fortran 90 pointer does not merely

store the memory address of a target variable; it also contains additional descriptive information such as the

target's rank, the upper and lower bounds of each dimension, and even strides through memory. This allows a

Fortran 90 pointer to point at submatrices.

Fortran 90 pointers are "associated" with well-defined "target" variables, via either the pointer assignment

operator (=>) or an ALLOCATE statement. When appearing in expressions, pointers are always dereferenced; no

"pointer arithmetic" is possible.

The following example illustrates the concept:

module SomeModule

implicitnone

contains

elemental function A(x)result(res)

integer:: res

integer, intent(IN):: x

res = x +1

endfunctionendmodule SomeModule

program Test

use SomeModule, DoSomething => A

implicitnone

!Declare variables

integer, parameter:: m =3, n =3

integer, pointer:: p(:)=>null(), q(:,:)=>null()

integer, allocatable, target:: A(:,:)

integer:: istat =0, i, j

character(80)::fmt

! Write format string for matrices

! (/ A / A, " = [", 3( "[",3(i2, 1x), "]" / 5x), "]" )

write (fmt, '("(/ A / A, "" = ["", ", i0, "( ""["",", i0, "(i2, 1x),

allocate(A(m, n), q(m, n), stat = istat)

if(istat /=0)stop'Error during allocation of A and q'

! Matrix A is:! A = [[ 1 4 7 ]

! [ 2 5 8 ]

! [ 3 6 9 ]

! ]

A =reshape([(i, i =1, size(A))], shape(A))

q = A

write(*, fmt)"Matrix A is:", "A", ((A(i, j), j =1, size(A, 2)), i


20 10/28/2011


17/20

! p will be associated with the first column of A

p => A(:, 1)

! This operation on p has a direct effect on matrix A

p = p **2

! This will end the association between p and the first column of A

nullify(p)

! Matrix A becomes:

! A = [[ 1 4 7 ]

! [ 4 5 8 ]

! [ 9 6 9 ]

! ]

write(*, fmt)"Matrix A becomes:", "A", ((A(i, j), j =1, size(A, 2))

! Perform some array operation

q = q + A

! Matrix q becomes:

! q = [[ 2 8 14 ]

! [ 6 10 16 ]

! [12 12 18 ]

! ]

write(*, fmt)"Matrix q becomes:", "q", ((q(i, j), j =1, size(A, 2))

! Use p as an ordinary array

allocate(p(1:m*n), stat = istat)

if(istat /=0)stop'Error during allocation of p'

! Perform some array operation

p =reshape(DoSomething(A + A **2), shape(p))

! Array operation:

! p(1) = 3

! p(2) = 21

! p(3) = 91

! p(4) = 21

! p(5) = 31

! p(6) = 43

! p(7) = 57

! p(8) = 73

! p(9) = 91

write(*, '("Array operation:" / (4x,"p(",i0,") = ",i0))')(i, p(i),

deallocate(A, p, q, stat = istat)

if(istat /=0)stop'Error during deallocation'

endprogram Test


20 10/28/2011


18/20

Module programming

A module is a program unit which contains data definitions, global data, and CONTAINed procedures. Unlike a

simple INCLUDE file, a module is an independent program unit that can be compiled separately and linked in its

binary form. Once compiled, a module'spublic contents can be made visible to a calling routine via the USE

statement.

The module mechanism makes the explicit interface of procedures easily available to calling routines. In fact,modern Fortran encourages every SUBROUTINE and FUNCTION to be CONTAINed in a MODULE. This allows the

programmer to use the newer argument passing options and allows the compiler to perform full type checking

on the interface.

The following example also illustrates derived types, overloading of operators and generic procedures.

module GlobalModule

! Reference to a pair of procedures included in a previously compiled

! module named PortabilityLibrary use PortabilityLibrary, only: GetLastError, & ! Generic procedure

Date ! Specific procedure

! Constants

integer, parameter:: dp_k =kind(1.0d0) ! Double precision ki

real, parameter:: zero =(0.)

real(dp_k), parameter:: pi =3.141592653589793_dp_k

! Variables

integer:: n, m, retint

logical::status, retlog

character(50):: AppName

! Arrays

real, allocatable, dimension(:,:,:):: a, b, c, d

complex(dp_k), allocatable, dimension(:):: z

! Derived type definitions

type ijk

integer:: i

integer:: j

integer:: k

endtype ijk

type matrix

integer m, n

real, allocatable:: a(:,:) ! Fortran 2003 feature. For Fortran 9

endtype matrix

! All the variables and procedures from this module can be accessed

! by other program units, except for AppName

public


20 10/28/2011


19/20

private:: AppName

! Generic procedure swap

interface swap

moduleprocedure swap_integer, swap_real

endinterface swap

interface GetLastError ! This adds a new, additional procedure to t

! generic procedure GetLastError

moduleprocedure GetLastError_GlobalModule

endinterface GetLastError

! Operator overloading

interfaceoperator(+)

moduleprocedure add_ijk

endinterface

! Prototype for external procedure

interface function gauss_sparse(num_iter, tol, b, A, x, actual_iter)result(

real:: tol_max

integer, intent(in):: num_iter

real, intent(in):: tol

real, intent(in), dimension(:):: b, A(:,:)

real, intent(inout):: x(:)

integer, optional, intent(out):: actual_iter

endfunction gauss_sparse

endinterface

! Procedures included in the module contains

! Internal function

function add_ijk(ijk_1, ijk_2)

type(ijk) add_ijk, ijk_1, ijk_2

intent(in):: ijk_1, ijk_2

add_ijk = ijk(ijk_1%i + ijk_2%i, ijk_1%j + ijk_2%j, ijk_1%k + ijk_

endfunction add_ijk

! Include external files

include 'swap_integer.f90'! Comments SHOULDN'T be added on include

include 'swap_real.f90'

endmodule GlobalModule

Retrieved from "http://en.wikibooks.org/w/index.php?title=Fortran/Fortran_examples&oldid=2065810"

This page was last modified on 6 March 2011, at 03:15.

Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may

apply. See Terms of Use for details.


20 10/28/2011


20/20


fortran_fortran examples - wiki books, open books for an open world

Documents