1.5    Fortran 90 v/s FORTRAN 77

    Although you probably have never seen FORTRAN 77, I will give you a brief summary of some of the changes and obsolete features.  Keep in mind that Fortran 90 is upwardly compatible with FORTRAN 77, meaning all features present in FORTRAN 77 are present in Fortran 90.  Some FORTRAN 77 features have been designated obsolete and should never be used because of better alternatives.  The features deemed obsolete in Fortran 90 are not available in Fortran 95 (they are actually obsolete).

    The first, and most obvious, change from FORTRAN 77 to Fortran 90 is the conversion from fixed source code to free source.  In fixed source form (which dates back to punch cards) statement labels had to appear in the first five columns, column six was used for a continuation indicator, and statements appeared on lines 7-72.   In Fortran 90 free source form means you are free to type comments and commands wherever you want.  An exclamation point, ! (also known as a "bang"), is used to indicate a comment statement in Fortran 90.

    Obsolete features include the arithmetic IF statement, ASSIGN and GO TO statements. Avoid use of these features in your code.  In fact, don't learn them.

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