# Fortran 2003

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**Fortran 2003** is the latest standard of Fortran. This book will focus on modern Fortran, and it will not cover the obsolescent features of Fortran 77 and earlier standards.

## Contents

## Hello World[edit]

Here is a Hello World program in Fortran.

program hello implicit none write (*,*) "Hello, world." end program hello

The "implicit none" statement forces the programmer to declare all variables, which is considered good style. Fortran has integer, character, real, complex, and logical data types. The following program illustrates their use.

program data_types implicit none integer :: i real :: x logical :: tf complex :: z i = 3 x = 3.0 z = (3.0,3.0) tf = .true. write (*,*) "i =",i," x =",x," z =",z," tf = ",tf end program data_types

output: i = 3 x = 3. z = (3.,3.) tf = T

## arithmetic operators[edit]

Fortran has the arithmetic operators +, -, /,*, and ** (for exponentiation). The output of

program xx implicit none write (*,*) 2+3,2-3,2/3,4*3,2**3 end program xx

is

5 -1 0 12 8

## arrays[edit]

Fortran 90 and later versions have powerful functionality for arrays. The following program demonstrates some features of arrays. By default, array elements are numbered from 1, not 0, as in C or C++.

program xarray ! demonstrate array constructor and intrinsic functions implicit none integer, parameter :: n = 3 integer :: vec(n) vec = (/9,4,1/) ! set vec(1) to 9, vec(2) to 4, vec(3) to 1 write (*,*) "vec = ",vec ! print each element of vec write (*,*) "vec(1) = ",vec(1),", vec(3) =",vec(3) ! print the 1st and 3rd elements write (*,*) "size(vec), sum(vec), product(vec) = ", & size(vec), sum(vec), product(vec) write (*,*) "minval(vec), maxval(vec) = ",minval(vec),maxval(vec) vec = vec + 2 ! add 2 to each element of vec write (*,*) "vec = ",vec ! print each element of vec vec = vec**2 ! square each element of vec write (*,*) "vec = ",vec ! print each element of vec end program xarray

output:

vec = 9 4 1 vec(1) = 9 , vec(3) = 1 size(vec), sum(vec), product(vec) = 3 14 36 minval(vec), maxval(vec) = 1 9 vec = 11 6 3 vec = 121 36 9

## loops[edit]

Fortran uses do loops for iteration. For example, the program

program xloop implicit none integer :: i do i=1,3 write (*,*) i,i**2 end do write (*,*) "i=",i do i=1,4,2 write (*,*) i end do write (*,*) "i=",i end program xloop

gives output

1 1 2 4 3 9 i= 4 1 3 i= 5

because within the first loop, variable i takes on values between 1 and 3 with step size of 1, and in the second loop the step size is 2. After completing the loop the value of i is its last value before leaving the loop plus the step size.

### comparison operators[edit]

Fortran has the comparison operators < <= /= == >= > , where /= means "not equal" and the other operators have the usual meanings. The program

program xcompare implicit none write (*,*) 1<0,1<=0,1==0,1/=0,1>=0,1>0 end program xcompare

has output

F F F T T T

### do[edit]

One can have a DO loop without a counter variable, in which case an EXIT statement will be needed to leave the loop, as shown in the following program

program xfibonacci ! print Fibonacci numbers up to max_fib implicit none integer, parameter :: max_fib = 10 integer :: i,fib,fib1,fib2 i = 0 fib = 0 fib1 = 0 fib2 = 0 write (*,*) "Fibonacci numbers <= ",max_fib do if (fib > max_fib) exit write (*,*) fib i = i + 1 if (i > 1) then fib = fib1 + fib2 else fib = 1 end if fib2 = fib1 fib1 = fib end do end program xfibonacci

Declaring max_fib a parameter means that its value cannot be changed in the rest of the program.

### nested loops[edit]

Loops can be nested, as shown in the following program

program xnest implicit none integer :: i,j do i=1,3 do j=1,2 write (*,*) "i,j=",i,j end do end do end program xnest

which gives output

i,j= 1 1 i,j= 1 2 i,j= 2 1 i,j= 2 2 i,j= 3 1 i,j= 3 2

## functions and return values[edit]

A function can be used to return a value depending on zero or more arguments. The code below shows a function that converts from degrees Fahrenheit to degrees Celsius.

module convert_mod implicit none contains function cels_from_fahr(degrees_fahr) result(degrees_cels) real, intent(in) :: degrees_fahr real :: degrees_cels degrees_cels = (degrees_fahr-32)/1.8 end function cels_from_fahr end module convert_mod program xtemperature use convert_mod, only: cels_from_fahr real :: deg integer :: i write (*,"(2a10)") "degrees_F","degrees_C" do i=12,100,20 deg = real(i) write (*,"(2f10.1)") deg,cels_from_fahr(deg) end do end program xtemperature

Output:

degrees_F degrees_C 12.0 -11.1 32.0 0.0 52.0 11.1 72.0 22.2 92.0 33.3

## subroutines[edit]

A subroutine cannot be used in an expression and is invoked with a call statement, as demonstrated in the program below, which gives the same output as the program above.

module convert_mod implicit none contains subroutine cels_from_fahr(degrees_fahr,degrees_cels) real, intent(in) :: degrees_fahr real, intent(out) :: degrees_cels degrees_cels = (degrees_fahr-32)/1.8 end subroutine cels_from_fahr end module convert_mod program xtemperature use convert_mod, only: cels_from_fahr real :: deg_f,deg_c integer :: i write (*,"(2a10)") "degrees_F","degrees_C" do i=12,100,20 deg_f = real(i) call cels_from_fahr(deg_f,deg_c) write (*,"(2f10.1)") deg_f,deg_c end do end program xtemperature