Implicit declaration of variables

F77 allows you to assign implicitly a particular data type to all variables whose symbolic names start with a common character. This is, however, a rather advanced approach and I personally do not recommend that you use these implicit declarations. If you are curious about learning details, please feel free to consult the F77 manuals on the web. In order to switch off the implicit declarations, we always insert after the identifier program, subroutine or function the following line:

      implicit none

This simple statement forces the compiler to strictly check whether you, as a programmer, assigned a type to each variable that you use in your code. If you omit to declare some variable(s), the compiler does not produce the executable code and exits with an error about an undeclared variable.