Fortran Tutorial

5. Variables, types, and declarations

Variable names

Variable names in Fortran consist of 1-6 characters chosen from the letters a-z and the digits 0-9. The first character must be a letter. (Note: Fortran 90 allows variable names of arbitrary length). Fortran 77 does not distinguish between upper and lower case, in fact, it assumes all input is upper case. However, nearly all Fortran 77 compilers will accept lower case. If you should ever encounter a Fortran 77 compiler that insists on upper case it is usually easy to convert the source code to all upper case.

Types and declarations

Every variable should be defined in a declaration. This establishes the type of the variable. The most common declarations are:

      integer   list of variables
      real      list of variables
      double precision  list of variables
      complex   list of variables
      logical   list of variables
      character list of variables

The list of variables should consist of variable names separated by commas. Each variable should be declared exactly once. If a variable is undeclared, Fortran 77 uses a set of implicit rules to establish the type. This means all variables starting with the letters i-n are integers and all others are real. Many old Fortran 77 programs uses these implicit rules, but you should not! The probability of errors in your program grows dramatically if you do not consistently declare your variables.

Integers and floating point variables

Fortran 77 has only one type for integer variables. Integers are usually stored as 32 bits (4 bytes) variables. Therefore, all integer variables should take on values in the range [-m,m] where m is approximately 2*10^9.

Fortran 77 has two different types for floating point variables, called real and double precision. While real is often adequat, some numerical calculations need very high precision and double precision should be used. Usually a real is a 4 byte variable and the double precision is 8 bytes, but this is machine dependent. Some non-standard Fortran versions use the syntax real*8 to denote 8 byte floating point variables.

The `parameter` statement

Some constants appear many times in a program. It is then often desirable to define them only once, in the beginning of the program. This is what the parameter statement is for. It also makes programs more readable. For example, the circle area program should rather have been written like this:

      program circle
      real r, area, pi
      parameter (pi = 3.14159)
 
c This program reads a real number r and prints
c the area of a circle with radius r.
 
      write (*,*) 'Give radius r:'
      read  (*,*) r
      area = pi*r*r
      write (*,*) 'Area = ', area
 
      stop
      end

The syntax of the parameter statement is

      parameter (name = constant, ... , name = constant)

The rules for the parameter statement are:

The "variable" defined in the parameter statement is not a variable but rather a constant whose value can never change
A "variable" can appear in at most one parameter statement
The parameter statement(s) must come before the first executable statement

Some good reasons to use the parameter statement are:

it helps reduce the number of typos
it is easy to change a constant that appears many times in a program

Exercises

Exercise A

Which of the following variable names are invalid, and why?

A5, 5A, VARIABLE, XY3Z4Q, AT&T, NUMBER1, NO1, NO 1, NO_1, STOP