Logical and conditionals, character variables

Last updated on 2026-02-21 | Edit this page

Overview

Questions

  • What other types of variable does Fortran provide?
  • How can I direct a program’s flow down different paths?
  • Is it possible to store characters in a variable?

Objectives

  • Understand how to use logical variables.
  • Be able to control the flow of a program with if statements.
  • Write conditional expressions using logical and relational operators.
  • Be able to use a case construct.
  • Learn how to declare character variables.

Logical variables

Fortran has a logical type (cf. Boolean type in C); there are two relevant literal values, illustrated here:

FORTRAN 

  logical :: switch0 = .false.
  logical :: switch1 = .true.

It is possible to specify a kind type parameter for logical kinds, but you don’t see it very often. The default logical kind has kind type parameter kind(.false.).

Logical operators and expressions

Standard logical operators .or., .and. and .not. are available. The precedence is illustrated by, e.g.,

FORTRAN 

  q = i .or. j .and. .not. k    ! evaluated as i .or. (j .and. (.not. k))

Again, use parentheses to avoid ambiguity, or to add clarity.

Remember that .or. evaluates to .true. if either or both its operands are .true., while .and. requires both to be .true. in order to evaluate to .true.. The .not. operator inverts the value of its operand.

Relational operators

To form logical expressions from numeric or other expressions, we require relational operators. The are two forms in Fortran:

Relation Operator Older form For
Less than < .lt. integer real
Less than or equal to <= .le. integer real
Greater than > .gt. integer real
Greater than or equal to >= .ge. integer real
Equal to == .eq. integer real complex
Not equal to /= .neq. integer real complex

If you are coming from C/C++ don’t be tempted by != for inequality.

Logical equivalence

Equivalence between two logical expressions or variables is established via the logical operators .eqv. and .neqv.; do not be tempted to use == for logical equality, although some compilers may let you get away with it.

Logic and flow of control

if construct

Conditional statements are provided by the if construct, formally:

FORTRAN 

[if-name:]  if (logical-expression) then
              block
            [ else if (logical-expression) then
              block ] ...
            [ else
              block ]
            end if [if-name]

There may be zero or more else if blocks, but at most one else block. At most one block is executed. For example:

FORTRAN 

  if (i < j) then
    print *, "The smaller is: i ", i
  else if (i > j) then
    print *, "The larger is:  i ", i
  else
    print *, "i,j are the same: ", i
  end if

A single clause if statement is also available, for example:

FORTRAN 

  if (a >= 0.0) b = sqrt(a)

Example 1 (3 minutes)

Challenge

Using relational operators

The file example1.f90 contains a version of the code above. Check it works as expected. Can you replace the relational operators to use the older form given in the table above?

You should be able to compile the code and run it to obtain the output:

OUTPUT 

 The smaller is: i  1

To use the older forms of the operators > and <, change them respectively to .gt. and .lt.:

FORTRAN 

  condition1 = (i .lt. j)
  condition2 = (i .gt. j)

Construct names

A number of control constructs in Fortran include the option to specify names. This can be useful when highly nested structures are present and one needs to refer unambiguously to one or other. A construct name follows the same rules as for variable names.

A if construct with a name must have the matching name with the end if.

For example

FORTRAN 

highly_nested_if_construct: if (a < b) then
                              ! ... structured block ...
                            end if highly_nested_if_construct

As a matter of style, a leading name can be obtrusive, so one can put it on a separate line using the continuation character &, e.g.,

FORTRAN 

outer_if: &
  if (a < b) then
     ! ... structured block ...
  endif outer_if

The standard maximum line length in Fortran is 132 characters. Longer lines can be broken with the continuation character & to a maximum of 255 continuation lines (F2003).

Finally, note the use of endif without a space is not a typo; both forms with and without a space are acceptable (this is true of a number of Fortran statements). It’s probably preferable to stick to “end if” (at least be consistent).

case construct

This is an analogue of the C switch facility, and can be useful for actions conditional on a range or set of distinct values. Formally,

FORTRAN 

[case-name:]  select case (case-expression)
                [ case (case-value-range-list)
		    block ] ...
		[ case default
		    block ]
              end select [case-name]

The case-expression must be a scalar integer, logical, or character expression. The case-value-range-list is a comma-separated list of either individual values, or ranges.

For example:

FORTRAN 

   integer :: mycard = 1         ! Playing cards 1--13

   select case (mycard)
     case (1)
       ! Action for ace ...
     case (2:10)
       ! Action for other card ...
     case (11, 12, 13)
       ! Court card ...
     case default
       ! error...?
   end select

A range may be open-ended (e.g., 2: or :10). Note there is no break-like statement as in the C switch; only the relevant case block is executed.

Character variables

Character variables hold zero or more characters. Some examples are:

FORTRAN 

program example2

  implicit none
  character (len = *), parameter :: string1 = "don't"   ! 5 characters
  character (len = 5)            :: string2 = "Don""t"  ! 5 characters
  character (len = 6)            :: string3 = 'don''t'  ! 5 characters + blank

end program example2

The implementation must provide at least one type of character storage, with the default kind being kind('A'). In practice, kind type parameters are not often specified. However, there should be a len specifier.

There is, again, a certain elasticity in the form of the declaration, so you may see slightly different things.

Strings may be concatenated with the string concatenation operator //; a single character, or a subset of characters, can be extracted via use of an array-index like notation e.g., string(1:2).

We will return to character variables in more detail when we consider strings in a later section.

Exercise (1 minute)

Challenge

Character len

Compile and check the output of example2.f90 to see the result of the examples above. What happens if you change one of the len specifications to be too short?

Without modifications, the code should produce the following output:

OUTPUT 

 string1: don't
 string2: Don"t
 string3: don't
 Catenated:       don't Don"tdon't
 Substring:       Do
 kind('A'):        1

If we make string3 too short, e.g.:

FORTRAN 

  character (len = 2)            :: string3 = 'don''t'

then the variable will truncate at that length:

OUTPUT 

 string1: don't
 string2: Don"t
 string3: do
 Catenated:       doDon"tdon't
 Substring:       Do
 kind('A'):        1

Exercise (5 minutes)

Challenge

Solving a quadratic equation

Write a program which uses real data types to compute the two solutions to the quadratic equation:

\(ax^2 + bx + c = 0\)

for given values of a, b, and c. See Wikipedia’s page for some background. A template exercise1.f90 provides some instructions.

A solution to this exercise appears as a template for the first exercise in the episode on array expressions

Key Points
  • Fortran provides two non-numeric intrinsic data types: logical and character.
  • A program’s flow can be directed using the results of logical operations used in conjunction with if and case constructs.