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compound if

summary

    This subchapter looks at compound if.

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compound if

    This subchapter looks at compound if.

    A compound if is testing more than one condition at once. A compound if can be formed with nested ifs, but sometimes it is more clear to use a compound if (which makes your program easier to maintain for the next few decades). Also, the compound if structure will come up again when creating loops.

    The compound if makes use of Boolean logic, in particular the AND and the OR. The following example is not intended to be in any particular programming language, but is valid for Pascal.

    IF ( (item1 = TRUE) AND (item2 = TRUE) )
        THEN DoSomething();
    IF ( (item3 = TRUE) OR (item4 = TRUE) )
        THEN DoSomethingElse();
    IF ( NOT (item5 = TRUE) )
        THEN DoNegateThing();

    IF ( NOT (item6 = TRUE) AND (item7 = false) )
        THEN DoAnotherThing();

    In the example, there is a test to see if both item1 AND item2 are true. The procedure (action) DoSomething is only performed if both item1 and item2 are true. If either is false or if both are false, then the procedure (function/action) DoSomething is skipped.

    In the example, there is a test to see if either item3 OR item4 is true. The procedure (action) DoSomethingElse is performed if either item3 and item3 is true. If both are false, then the procedure (function/action) DoSomethingElse is skipped.

    The first NOT example simply tests to see if something is NOT true. In this case, we test to see if item5 is true and then take the opposite as our answer. If item5 was true, then we are testing for item5 to not be true and if item5 was false then we are testing for item5 not being false.

    The second NOT example is a bit more complicated. There are four parts to this particular condition test. First, we determine if item6 is true. Second, we determine if item7 is false (note that the order of these two tests is not guaranteed in all programming languages). The third step is to do the AND on the two first halves of our test. The final step is to take the opposite of the third step as our final answer. See the chart below:

initial starting position
step		item6	item7		item6	item7		item6	item7		item6	item7
0		TRUE	TRUE		TRUE	FALSE		FALSE	TRUE		FALSE	FALSE
first test (item6 = TRUE)
1		item6 test			item6 test			item6 test			item6 test
1		TRUE			TRUE			FALSE			FALSE
second test (item7 = FALSE)
2		item7 test			item7 test			item7 test			item7 test
2		FALSE			TRUE			FALSE			TRUE
third test AND
3		item6 test	item7 test		item6 test	item7 test		item6 test	item7 test		item6 test	item7 test
3		TRUE	FALSE		TRUE	TRUE		FALSE	FALSE		FALSE	TRUE
3		AND test			AND test			AND test			AND test
3		FALSE			TRUE			FALSE			FALSE
final test NOT
3		NOT test			NOT test			NOT test			NOT test
3		TRUE			FALSE			TRUE			TRUE

LISP    AND(x) — LISP function that takes zero or more arguments of type NIL or nonNIL and gives the logical AND.

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