Fortran/Program flow control
Selection
editIf-then(-else) conditional
editConditional execution is done using the if
, then
and else
statements in the following construct:
if (logical_expression1) then
! Block of code
else if (logical_expression2) then
! Block of code
else
! Block of code
end if
You may have as many else if
statements as you desire.
The following operators can be used when making expressions:
Operation | Modern Fortran | Old FORTRAN |
---|---|---|
Less than | < | .LT. |
Greater than | > | .GT. |
Greater than/equal | >= | .GE. |
Less than/equal | <= | .LE. |
Equal | == | .EQ. |
Not equal | /= | .NE. |
Logical equivalent | .EQV. | |
Logical not equivalent | .NEQV. | |
Logical not | .NOT. | |
Logical and | .AND. | |
Logical or | .OR. |
Note: The Fortran standard mandates .EQ.
and .NEQ.
cannot be used with logicals but some compilers will not enforce the standard
To check more than one statement, use parentheses.
if ((a .gt. b) .and. .not. (a .lt. c)) then
The following program generates a random number between 0 and 1 and tests if it is between 0 and 0.3, 0.3 and 0.6, or between 0.6 and 1.0.
program xif
implicit none
real :: x
real, parameter :: x1 = 0.3, x2 = 0.6
call random_seed()
call random_number(x)
if (x < x1) then
print *, x, "<",x1
else if (x < x2) then
print *, x, "<", x2
else
print *, x, ">=", x2
end if
end program xif
There are two interesting archaic forms of IF
:
IF (<logical_expression>) GOTO <statement_label>
IF (<arithmetic_expression>) <first_label>, <second_label>, <third_label>
In the first form, things are pretty straightforward. In the second form, the arithmetic expression is evaluated. If the expression evaluates to a negative number, then execution continues at the first line number. If the expression evaluates to zero, then execution continues at the second line number. Otherwise, execution continues at the third line number.
case (switch)
edit- select case(...) case (...); ... end select
If an if block consists of repeated tests on a single variable, it may be possible to replace it with a select case construct. For example, the code
if (month=="January" .or. month=="December") then
num_days = 31
else if (month=="February") then
num_days = 28
else if (month=="March") then
num_days = 31
else
num_days = 30
end if
can be replaced by
select case (month)
case ("January", "December")
num_days = 31
case ("February")
num_days = 28
case ("March")
num_days = 31
case default
num_days = 30
end select
Fortran does not need a break statement.
Loops
edit- do i=1,10 ... end do
To iterate, Fortran has a do loop. The following loop prints the squares of the integers from 1 to 10:
do i=1,10
print *, i**2
end do
One can exit a loop early using exit, as shown in the code below, which prints the squares of integers until one of the squares exceeds 25.
do i=1,10
isquare = i**2
if (isquare > 25) exit
print *, isquare
end do
Loops can be nested. The following code prints powers 2 through 4 of the integers from 1 to 10
do i=1,10
do ipower=1,3
print *, i, ipower, i**ipower
end do
end do
In an archaic form of DO
, a line number on which the loop(s) end is used. Here's the same loop, explicitly stating that label 1
is the last line of each loop:
DO 1 i=1,10
DO 1 ipower=1,3
1 PRINT *, i, ipower, i**ipower
If using the archaic form, the loop must not end on an IF
or GO TO
statement. You may use a CONTINUE
statement as an anchor for a the 1
label.
There is also an optional increment argument when declaring a do loop. The following will count up by two's. 2, 4, 6, ...
do i=2,10,2
write (*,*) i
end do
Arguments to the do loop don't have to be numbers, they can be any integer that is defined elsewhere in the program. first, last, and increment can be any variable name.
do i=first,last,increment
! Code goes here
end do
Simple statements
editgoto statement_label
will jump to the specified statement number.
stop exit_code
will stop with the specified condition code or exit code. stop
may be coded without an argument. Note that on many systems, stop 0
is still a failure. Also note that pre-Fortran 2008, the condition code must be a constant expression and not a variable.
exit
will leave a loop.
continue
can be used to end an archaic DO
loop when it would otherwise end on an IF
.
cycle
will transfer the control of the program to the next end do
statement.
return
leaves a subroutine or function.