Implicit Typing

If a variable is referenced but not declared (in its scoping unit) then, by default, it is implicitly declared. The type that it assumes depends on the first letter,

• I, J, K, L, M and N represent integers;
• all other letters define real variables.

This short cut is potentially very dangerous as a misspelling of a variable name can declare and assign to a brand new variable with the user being totally unaware.

Implicit typing should always be turned off by adding:

    IMPLICIT NONE

as the first line after any USE statements. In this way the user will be informed of any undeclared variables used in the program.

Consider,

    DO 30 I = 1.1000
     ...
 30 CONTINUE

in fixed format with implicit typing this declares a REAL variable DO30I and sets it to 1.1000 instead of performing a loop 1000 times! Legend has it that the example sighted caused the crash of the American Space Shuttle. An expensive missing comma!

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