Implicit Typing

Undeclared variables have an implicit type,

• if first letter is I, J, K, L, M or N then type is INTEGER;
• any other letter then type is REAL s.

Implicit typing is potentially very dangerous and should always be turned off by adding:

    IMPLICIT NONE

as the first line after any USE statements.

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!

For more information, click here