Control Structures

This section gives a brief introduction to the constructs of the Fortran programming language:

if Construct

Fortran features three different types of conditionals: arithmetic if (deprecated), logical if, and block if.

Arithmetic if

The first FORTRAN version from 1957 introduced the arithmetic if conditional that evaluates a numeric expression and then jumps to one of three labeled statements, depending on whether the expression is either negative, zero, or positive:

if (x * y) 100, 200, 300

If negative, the statement does a goto to the first label (100), if zero to the second (200), and if positive to the third (300). This is equivalent to:

if (e < 0) goto 100
if (e == 0) goto 200
goto 300

Arithmetic if is obsolescent since Fortran 90, and has been removed from the Fortran 2018 standard. Like the goto statement, it should not be used anymore.

Logical if

The logical if conditional was added to FORTRAN IV and allows the execution of a statement depending on a logical or arithmetic expression, using operators. Only a single statement may be declared:

if (x * y < 0) y = 1

A line-break following the expression is legitimate, but must be indicated by an ampersand:

if (x * y < 0) &
    y = 1

Block if

The block if allows the conditional execution of a group of statements, for example:

if (a == 0) then
    exit
else if (a < 0) then
    sum = 0
else
    sum = sum + a
end if

The conditional was introduced in FORTRAN 77.

select Switch

The select case switch is an alternative to block if:

select case (grade)
    case ('A')
        print *, 'Excellent!'

    case ('B')
    case ('C') ! Or: case ('B', 'C')
        print *, 'Well done'

    case ('D')
        print *, 'You passed'

    case ('F')
        print *, 'Better try again'

    case default
        print *, 'Invalid grade'
end select

You may want to use ranges inside select case with case (begin:end):

select case (marks)
    case (91:100)
        print *, 'Excellent!'

    case (81:90)
        print *, 'Very good!'

    case (71:80)
        print *, 'Well done!'

    case (61:70)
        print *, 'Not bad!'

    case (41:60)
        print *, 'You passed!'

    case (:40)
        print *, 'Better try again!'

    case default
        print *, 'Invalid marks'
end select

Only parameters can be evaluated in a select case construct.

select type

The select type construct was introduced in Fortran 2003, and lets us execute blocks depending on a type or class of a variable. The following type guard statements are used to match a selector expression:

type is
Matches the selector if the dynamic type and kind type parameter values are the same as those specified by the statement
class is
Matches the selector if the dynamic type is an extension of the type specified by the statement, and the kind type parameter values specified by the statement are the same as the corresponding type parameter values of the dynamic type of the selector.
class default
Executed if selector is not matched by any other type guard statement.

The name of the identifier that becomes associated with the selector can be choosen freely, but must be unique within the construct.

type :: vec_type
    real :: x, y
end type vec_type

type, extends(vec_type) :: vec3_type
    real :: z
end type vec3_type

type, extends(vec3_type) :: color_type
    integer :: color
end type color_type

type(vec_type),   target  :: v
type(vec3_type),  target  :: v3
type(color_type), target  :: c
class(vec_type),  pointer :: ptr

v  = vec_type(1.0, 2.0)
v3 = vec3_type(1.0, 2.0, 3.0)
c  = color_type(0.0, 1.0, 2.0, 9)

! Point to either v, v3, or c:
ptr => c

select type (a => ptr)
    class is (vec_type)
        print *, a%x, a%y

    type is (vec3_type)
        print *, a%x, a%y, a%z

    type is (color_type)
        print *, a%x, a%y, a%z, a%color
end select

do Loop

The do construct loops over statements until an exit occurs:

do
    a = a + 1
    print *, a
    if (a > 10) exit
end do

The do loop is further similiar to the for loop in other programming languages. Set begin, end, and step size in the head of the loop:

integer :: i

do i = 1, 10, 2
    print *, i
end do

The loop index variable has to be declared a priori. The step size is optional. Before Fortran 2018, it was further possible to name loops:

loop: do
    a = a + 1
    print *, a
    if (a > 10) exit loop
end do loop

This language feature has been declared obsolescent in favour of cycle. The cycle statement skips to the next iteration:

integer :: i

do i = 1, 10
    if (modulo(i, 2) == 0) cycle
    print *, i
end do

Implied do

Implied do loops return a number of items, using the syntax (item1, item2, …, itemn, var = initial, final, step), for example:

integer :: i

print *, ('Hi there. ', i = 1, 3)

The print statement will output the string three times. Arrays are often initialised using an implied do loop:

integer, parameter :: N = 10

integer :: i
integer :: values(N) = [ (i * 2, i = 1, N) ]

The array values is filled with 2, 4, …, 20. An implied do loop may be nested into another implied do loop. Starting with Fortran 2018, we can declare the index variable in the implied loop:

integer, parameter :: N = 10

integer :: values(N) = [ (i * 2, integer :: i = 1, N) ]

do concurrent

The Fortran 2008 standard added the do concurrent construct that allows easy loop parallelisation and is equivalent to forall:

integer :: i
real :: a(100)

do concurrent (i = 1:size(a))
    a(i) = sqrt(i**i)
end do

Inside a do concurrent loop, only calls to pure functions are allowed. Multiple indices may be declared at once:

integer :: x, y
real :: a(8, 16)

do concurrent (x = 1:size(a, 1), y = 1:size(a, 2))
    a(x, y) = real(x)
end do

Since Fortran 2018, the loop indices may be declared inside the contruct:

real :: a(8, 16)

do concurrent (integer :: x = 1:size(a, 1), y = 1:size(a, 2))
    a(x, y) = real(x)
end do

do while

The do while loop cycles through statements as long as a given condition is true:

do while (i < 5)
    i = i + 1
    print *, i
end do

forall Loop

The forall loop has been introduced with Fortran 95, and declared obsolescent in Fortran 2018 in favour of do concurrent. It selects elements in an array by index or index range, with an optional step size. In the following example, the loop statement changes the values of the first three elements of the array to 0:

integer :: a(5) = [ 1, 2, 3, 4, 5 ]
integer :: i

forall (i = 1:3) a(i) = 0

! with explicit step size of 1:
forall (i = 1:3:1) a(i) = 0

Furthermore, a mask can be added to the condition of the statement in order to select only specific value, for instance:

forall (i = 1:3, a(i) == 0) &
    a(i) = 1

The mask can be any pure function. To allow more than one statement, use the forall block statement:

forall (i = 3:5)
    a(i) = 1
    print *, a(i)
end forall

Inside the loop, you can assign pure functions to the elements:

forall (i = 1:3) &
    a(i) = my_func(a(i))

Since Fortran 2018, the forall indices can be declared in the loop contruct:

real :: a(8, 16)

forall (integer :: i = 1:3)
    a(i) = i
end forall

where Statement

The where statement is used for masked array assignments. Elements of an array will be modified directly upon given conditions:

integer :: a(5) = [ 1, 2, 3, 4, 5 ]

where (a >= 3) &
    a = 0

You can add else where statements to the block form of where:

where (a > 0 .and. a < 2)
    a = 0
else where (a >= 4)
    a = 1
end where

block Construct

The block construct is available since Fortran 2008, and defines an executable block that can contain declarations. A block may be exited prematurely with exit, allowing a control flow similar to using goto:

real :: a

a = 1024.0

block1: block
    real :: b

    if (a < 0) exit block1

    b = sqrt(a)
    print *, b
end block block1

The variable b declared inside block1 is not accessible from outside of the construct. The identifier block1 is optional.