A = 5 (or just A-5) assigns the value 5 to the vari able A. In PRINT A = 5. the " = " is a relational operator. (There is a lot of fancy nomenclature for these fairly simple concepts!) The term on the left is merely compared to the term on the right. No assignment or exchange of values is implied. In fact, some computer languages use different sym bols for the assignment '" = " and the relational '*=" so the computer and the programmer alike can more easily distinguish between them. You have probably used the other relational oper ators: >.<.>=.<—.<>. Once again, these operators separate two terms. The computer inter prets the value of the entire expression as being ei ther true or false, for example, PRINT 2*3 > is stored in the computer as -1 instead of +1? A rhetorical question, of course. The number zero is stored as sixteen zeroes in integer format. The op posite (or complement) of sixteen zeroes is sixteen ones, since the computer stores only zeroes and ones. The number stored as sixteen ones is the in teger value -1. We won't go into why that is. but it is clear that the opposite of false is true and the opposite of (0000 0000 0000 0000) is (1111 1111 1111 1111). The computer treats these numbers as the in tegers 0 and -1 respectively. The integer +1 is stored as (0000 0000 0000 0001). which obviously isn't the opposite of false. Can you figure out the purpose of this program: 2 + 3 would output -1. Why? Because the state •10 INPUT ment "six is greater than five" is a true statement. •20 C$ = •30 IF A$ •40 PRINT No doubt all of this seems a bit obscure if it is new to you. Let's see how well you are doing. "ENTER TWO WORDS"; A$,B$ B$ < B$ THEN C$ = A$ CS Predict the output of each of these lines. Predict what would be printed in line 40 if you en 10 PRINT 2 ;> 3 20 PRINT 8 -- 3 30 PRINT A <; a* 2 40 50 PRINT PRINT 5 -h (B 5 -- B tered CAT, DOG in response to line 10. This is a Line 40 requires some careful thinking. The ex simple alphabetize!-. Hopefully you are now able to recognize the logical expression in this program. It is the L"A$ < BS in line 30. The IF statement is always followed by a logical expression. If the first word you type comes before the second word al phabetically, then the expression AS < BS is true pression in parentheses is evaluated to be — 1 and the statement following THEN is executed. (true). The rest is math: 5 + (-1) is 4. Do you Otherwise CS retains the value of B$, and line 40 prints whichever of the two words is first in the = 3 = = -i 0) 0 see why line 50 would output 0? In that case, 5 + B would be evaluated to be 5 (since B is 0). The expression 5 = 0 is obviously false, so the printed dictionary. Line 20 is a handy way to eliminate a cumber used? Suppose you arc writing a game in which a bonus is to be added to the player's score after some GOTO statement in line 30. In line 20. we assume BS will be printed. If AS should be printed instead, we straighten it out in line 30. If we did not give C$ a value in line 30, our program would rounds 2 and 10. One way to do this very simply look like this: value is 0. Why would a statement such as line 40 ever be is to use logical expressions: •10 SC = SC *(RD«10) + (-20)*(RD=2) + (-30) Here SC is the score and RD is the current round. If (RD=2) is true, then -20 is multiplied by -1 giving 29 which is added to the score. If (RD=10) is true, its value of -1 is multiplied by -30 to give 30 which is added to the score. In any other rounds, both logical terms are false, and a value of 0 is added to the score. Of course, there are other ways of giving the same results, but once you un derstand logical expressions, this is a very handy method. Is there some reason that the logical value True 18 AHOY! -10 •20 INPUT IF A$ A$,B$ < B$ THEN CS = A$ : GOTO 40 .30 C$ •40 PRINT = B$ C$ The program flow in the first example is smoother than it is in the second example, don't you agree? Type this short program and run it: ■5 REM >> •10 REM TRUTH TABLES << TRUE = -1 : FALSE = •20 PRINT " B)" •30 READ A,B A"," B", "(A OR 0