Undeclared variables have an implicit type,
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!
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