There are two diametrically opposed schools of thought on type
specification. The first holds that all names should have their
types specified explicitly. This certainly helps programmers to
avoid mistakes, because they have to think more carefully about
each item. It also helps the compiler to diagnose errors more
easily, especially if the it knows that all names are going to be
declared in advance. Some Fortran compilers allow a statement of
the form ``IMPLICIT NONE" which makes all names typeless by
default and so requiring every name to be explicitly typed. Others
have a compile-time switch with the same effect. If yours does not
you may be able to produce a similar effect by using something
like:
IMPLICIT CHARACTER*1000000 (A-Z)
near the beginning of each program unit which is likely to cause an
error for anything not explicitly typed. One disadvantage of the
practice of declaring all names in advance is that the program may
become so cluttered with specification statements that it may
obscure its structure and algorithm.
The alternative way of working is to make maximum use of implicit types to reduce the number of statements. This means, of course, that the first letter of each name has to be chosen to suit the type, leaving no more than five to be chosen freely: this makes it harder than ever to devise meaningful symbolic names. As a result, Fortran programs often include names like RIMAGE or ISIZE or KOUNT. Clearly type statements are still needed for character type because it is usually necessary to use items of a number of different lengths.
Experience suggests that either system can be satisfactory provided it is used consistently. However the wholesale reassignment of initial letters with IMPLICIT statements usually increases the chance of making a mistake. IMPLICIT, if used at all, should only reassign one or two rarely-used letters to the less common data types, for example:
IMPLICIT DOUBLE PRECISION (Z), LOGICAL (Q),
COMPLEX (X)
It is also prudent to use an identical IMPLICIT statement in each
program unit, otherwise type mismatches are more likely to be
made in procedure calls.