Free vs. Fixed Formats

In Fortran 95 the code layout is obeys the following rules, which describe the ``free source form''.

• statements can begin in any column;
• multiple statements on a line are allowed; they have to be separated by semicolon ``;''
• an exclamation mark ``!'' in any column is the beginning of a comment; the rest of the line is ignored by the compiler;
• a statement can be continued on the following line by appending a ``&'' sign on the current line.

For example, the above code fragment could be written in free source form as

language=[95]Fortran visiblespaces=true

! This is free-form
temp = x; x = y; y = temp ! Swap x and y 
write(6,*) 'x and y are =',  & 
            x,y           ! Print x and y

visiblespaces=false

For back-compatibility with Fortran 77, Fortran 90 accepts a ``fixed source form''. In short, the fixed source form requirements are:

• all lines of code start in the (or higher) column; the first 6 columns are reserved for labels, continuation characters, etc.
• each statement is written on a separate line (i.e., statements are separated by newlines); a statement can be continued on the next line by writing (any) character in the column of the new line;
• comment lines start with a C in the first column.

Fixed format was born in 1950's, when each line of code was punched on a paper card (``punched card''). For debugging and maintaining the code it was clearly easier to have one statement per card. When running, a punched card had to be aligned, checked if it is a new statement or a continuation of the last, if it is a comment, etc. - lots of information stored in the first 6 columns. (A million-line code, quite common today, on punched cards, needed a truck to be carried from one computer to another).

For example, a fragment of a Fortran 90 program in fixed source form (in fact, Fortran 77 program) can be:

C---This-is-fixed-form 
C---Swap x and y 
      temp = x
      x = y
      y = temp
C---Print x and y 
      write(6,*) 'x and y are =',
    *             x,y

Our compiler (f90) will consider the program to be in fixed source format if the source file has the extension `` .f'' (for example, my_program.f). The compiler will consider the program to be in free source format if the source file has the extension `` .f90'' (for example, my_program.f90). Note that a free format source in a .f file may result in compilation errors.