## Control Structures

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

### if Conditionals

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

#### Arithmetic if

The first FORTRAN version from 1956 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Arithmetic `if` is now outdated and has been removed from Fortran 2018. 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
print *, sum
end if
``````

This conditional was introduced in FORTRAN 77.

### select Switches

The `select` switch can often be used instead of block `if`:

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

case ('B')
case ('C')
print *, 'Well done'

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

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

case default
end select``````

You may want to use ranges inside `select` switches 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)

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

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

case default
print *, 'Invalid marks'
end select``````

### do Loop

The `do` construct loops over statements until an `exit` occurs:

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

It can also be used like a for loop in other programming languages. Set begin, end, and step size in the head of the loop:

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

The loop variable `i` has to be declared a priori. The step size is optional, default is `1`. It is possible to name loops:

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

The `cycle` statement skips to the next iteration:

``````do i = 1, 10
if (i == 2) &
cycle
print *, i
end do``````

#### Implicit do

Implicit `do` loops can be used for input and output:

``````integer :: i

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

Arrays can be initialised implicitly as well:

``````integer, parameter :: n = 10
integer            :: i
integer            :: values(n) = [ (i * 2, i = 1, n) ]``````

The array `values` will be filled with `2`, `4`, …, `20`.

### do while Loops

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 Loops

The `forall` loop has been introduced with Fortran 95. 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))
``````

### where Statements

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``````