LBS                       USING THE FORTRAN COMPILER

A compiler is a program which converts your program written in a high
level language like FORTRAN to machine language.  The machine language
is what is actually executed on the computer.

To run a FORTRAN program you must a text editor such as 'ce' to create
a file containing your program.  The name of the file must end with '.f'.

This file containing the source code cannot be executed - it must first
be run through the compiler - to do this you use the command fort.

The FORTRAN compiler (fort) checks the 'syntax' of the program before it 
is actually compiled.  The syntax is the rules of the language.  For example
in FORTRAN statements must begin in column 7.  (Except for labels and 
continuation lines).  If you violate one of the rules of syntax you will get
an error message from the compiler and no machine language will be generated.

The fort command produces a file called 'a.out'.  This file contains 
the machine language.  This file is unreadable by humans.  It contains the
actual instructions executed by the Central Processing Unit.

Once there are no syntax errors the machine language produced may not be what
you want.   When the code you have written executes but gives incorrect
results - that is called a logic error.

The following is an example of writing a FORTRAN program on briggs1:

$ stty erase
$ ls
$ ce test.f

$ cat test.f
*
*  Test program - Show how the compiler works
*
*  Written by: Charles Severance 01/10/92
*
      integer age,newage
*
      print *,'Enter your age'
 read *,age
      print *,'Your old age is ',age
* Compute the new age
*
      newage = age + 2
      print *,'Your new age is ',age
      end
$ ls
test.f
$ fort test.f
File test.f:
    MAIN:
        test.f(9):      Illegal character in label field
        test.f(9):      Syntax error: "PRINT " followed by "*":
                *, age
                ^
2 errors diagnosed in file test.f

### "no implicit type for variable" means you did not declare it.

$ ls
test.f
$ ce test.f

$ cat test.f
*
*  Test program - Show how the compiler works
*
*  Written by: Charles Severance 01/10/92
*
      integer age,newage
*
      print *,'Enter your age'
      read *,age
      print *,'Your old age is ',age
*
* Compute the new age
*
      newage = age + 2
      print *,'Your new age is ',age
      end
$ fort test.f
File test.f:
    MAIN:
$ ls
a.out
test.f
test.f.bak
$ a.out
 Enter your age
10
 Your old age is  10 
 Your new age is  10 
$ ce test.f

$ cat test.f
*
*  Test program - Show how the compiler works
*
*  Written by: Charles Severance 01/10/92
*
      integer age,newage
*
      print *,'Enter your age'
      read *,age
      print *,'Your old age is ',age
*
* Compute the new age
*
      newage = age + 2
      print *,'Your new age is ',newage
      end
$ fort test.f
File test.f:
    MAIN:
$ a.out
 Enter your age
10
 Your old age is  10 
 Your new age is  12 
$