Flow-control structures: Difference between revisions
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Similarly to escaping from a subroutine, within a DO-loop, a GO TO might jump out of the loop(s) - perhaps for good reason. More interesting is the possibility of jumping ''into'' a DO-loop's scope, possibly after jumping out - who knows what its index variable might have been changed to. This is considered poor form by others not writing such code and some compilers will reject any attempts. With the F77 introduction of IF ... THEN ... ELSE ... END IF constructions, jumping out of a block is still acceptable but jumping in is frowned on (even if only from the THEN clause to some part of its ELSE clause) and may be prevented.
F90 offers a more decorous means for exiting DO-loops, including the additional DO WHILE loop, via the statements CYCLE and EXIT - the text "GO TO" does not appear as such, but the effect is the same. The CYCLE option means abandoning further statements within the block to test afresh the iteration condition, while EXIT means ending the iteration as if it had completed. Further syntax allows some compiler checking, as follows: <lang Fortran> XX:DO WHILE(condition)
statements...
NN:DO I = 1,N
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