Implicit declarations

In Fortran implicit declarations are allowed. Suppose we did not declare the variables I, J, X, Y but used them somewhere in the program. The Fortran compiler will not complain; rather, it will automatically declare I, J as integers and X, Y as reals. The rule is that undeclared variables which have the first letter I, J, K, L, M or N are considered INTEGER-s, and undeclared variables which start in A through H and O through Z are considered REAL-s. The automatic declarations based on implicit types are called implicit declarations. Some fourty years ago programmers found it cumbersome to explicitly declare all the variables all the time !

In F90 implicit declarations are permitted, but undesirable. In general, their use is a very bad programming habit, as it can mask programming errors, and can negatively impact future software development and maintainance. For example, a misspelling of a variable name will result in a new variable declaration, which can be further assigned etc, with the user being totally unaware.

An example (from A.C. Marshall) is

   do30i = 1.100
     <statements>
30 continue

Instead of a DO loop, because of the misprints, we will end up with a new real variable, do30i.

In consequence, we will always disable the implicit declarations by placing the command

implicit none

as the first line after any USE statements (i.e. before the declarations sequence). With this command in place, the existence of variables that are not explicitly declared will lead to a copilation error.