Name
LOOP-02: Make the termination conditions of a loop obvious
Synopsis
Loop logic is easier to determine if all the control logic
is in one place, either in the WHILE
or UNTIL
clauses or in a LEAVE
statement within the loop. It’s
particularly confusing to include a RETURN
statement within a loop.
To that end, we suggest that you avoid LEAVE
or RETURN
statements within WHILE
or REPEAT
UNTIL
loops.
Example
In the following example, borrowed from the prime number
routine in the preceding section, a WHILE
loop contains a LEAVE
clause—there are two ways for the
loop to terminate, and this makes the code harder to analyze and
trace:
SET j=2; SET is_prime=1; divisors: WHILE(j< i) DO IF MOD(i,j)=0 THEN SET is_prime=0; LEAVE divisors; END IF; SET j=j+1; END WHILE;
One way to improve the readability of the loop would be to
move all of the termination logic into the WHILE
clause:
SET j=2; SET is_prime=1; divisors: WHILE(j< i AND is_prime=1) DO IF MOD(i,j)=0 THEN SET is_prime=0; END IF; SET j=j+1; END WHILE;
Alternatively, we could employ a simple loop and place all termination logic within the loop.
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