Logical Expressions

For a computer to accomplish this selection it evaluates the “Truth Value” of a logical statement. Therefore, an expression will need to be evaluated to determine if it is True or False. To begin here are the common expression evaluators summarized in the table below.

For example, say we had the following expression

5 < 3

Is the above statement (five is less than three) True or False?

False! Five is greater than three.

Remember that there is such a thing as a logical variable in Fortran, so the above expression could be set to a variable.

      PROGRAM logical_test

      implicit none
      integer x, y
      logical test

      x = 5
      y = 3

      test = x .LT. y

      print *, "The statement is ",test

      END

The above program will yield the following output.

The statement is F

Go ahead and test if for yourself. Copy the above code to a new Fortran program, compile, and run it to see what you get. Change the logical expression to get different results.

Note

5 .LT. 5 is False

5 .LE. 5 is True