Short-circuit evaluation
by Richard Russell, December 2009
Consider a compound conditional test such as the following:
IF condition1 AND condition2 AND condition3 THEN
It can be seen that if condition1 is FALSE then the entire expression must be FALSE, irrespective of the results of the other conditions. Ideally, in such a case it would be better not to waste time evaluating condition2 and condition3 because they cannot affect the overall outcome. Similarly if condition2 is false, ideally you shouldn't evaluate condition3.
Skipping the subsequent conditions in this way, when they cannot affect the result, is called Short-Circuit Evaluation and some languages (e.g. C) provide special operators to support it. However, BASIC generally doesn't.
Because the performance benefits from Short-Circuit Evaluation may be substantial (especially if the expressions involve one or more function calls) it would be highly desirable to emulate this behaviour, and fortunately in BBC BASIC there is an easy way to do so:
IF condition1 IF condition2 IF condition3 THEN
Here the ANDs have simply been changed to IFs. If condition1 is FALSE neither condition2 nor condition3 is evaluated. If condition1 is TRUE but condition2 is FALSE, condition3 is not evaluated.
A similar situation arises when the conditions are combined using OR:
IF condition1 OR condition2 OR condition3 THEN
Here, if condition1 is TRUE (strictly, not FALSE) the entire expression must be TRUE, irrespective of the results of the other conditions, and it would be better not to waste time evaluating condition2 and condition3.
Unfortunately there isn't such a straightforward way of emulating this in BASIC as in the AND case. However it is possible to achieve the desired effect as follows:
IF condition1=FALSE IF condition2=FALSE IF condition3=FALSE THEN ELSE REM Carry out the required actions here ENDIF
Effectively what we have done here is to use De-Morgan's theorem to convert the OR expression into an AND expression, and to reverse the result by putting the required actions into the ELSE clause.
To gain the maximum benefit from Short-Circuit Evaluation you should make the first condition in the set the one least likely to be met, and you should make the slowest-to-evaluate condition the last in the set.
You must take into account any side effects from evaluating the conditions. If one or more of the conditional expressions calls a user-defined function, and if that function has a side-effect which can affect the operation of the program, then changing from conventional evaluation to Short-Circuit evaluation will cause that side-effect to occur conditionally upon the result(s) of earlier conditions rather than than always occurring.