A numerical relation is a statement that compares two numerical expressions. The relation is either true or false (when the comparison is made), and the Sorcerer can always tell which, just by evaluating the expressions and comparing the resulting numbers.
1=1 This is true
1=0 This is false
X=6 This is true if X has the value of 6 at the time
the comparison is made. Otherwise it is false.
A+B=C This is true is the value of A plus the value of B
equals the value of C at the time of the evaluation.
Otherwise it is false.
The equal sign as used in these examples is not the assignment operator (the one that goes with a LET statement).
Numerical relations have this form:
<first expression> <relation operator> <second expression>
The relation operators are:
= equal 1=1 is true, 1=0 is false
<> or >< not equal 2<>3 is true, 2<>2 is false
< less than 2<4 is true, 0<-2 is false
> greater than 1>-1 is true, 2>3 is false
<= or =< less than or equal 1<=1 and 6<=7 are true, 1<=0 is false
>= or => greater than or equal -1>=-1 and -1>=-2 are true, 0>=1 is false
A numerical expression is a mathematical entity that always has a number value. Similarly, a logical expression is an entity that always has a logical value (sometimes called a truth value). Numerical expressions can have any number for their values, but logical expressions can take on only two different logical values: true and false. (The Sorcerer gives expresslon that are true the value of -1, and those that are false it gives the value of 0. The Sorcerer always knows when to treat 0 and -1 as numbers and when to treat them as truth values.)
Numerical relations are simple examples of logical expressions. Logical operators let you build more complicated expressions out of simple ones. There are three of these operators:
<first expression> AND <second expression>
<first expression> OR <second expression>
The NOT expression is true when the expression is false, and vice versa.
NOT X = 1 is true if X does not have value l,
and is false if X does have value 1.
This is the same as the relation X <> 1.
The AND expression is true in the case of both of the simpler expressions being true. If either or both of them are false, then the AND expression is false.
X = 1 AND A = B is true just in the case that X has value 1 and
A has the same value as B, otherwise it is false.
The OR expression is true if either of the expressions is true or if they are both true. It is false only if both of the expressions are false.
X < 1 OR X = 1 is true if X is less than 1 or if X is equal to 1.
It is false if X is greater than 1. This is just
the same as the relation X <= 1.
X = 1 OR Y = 2 is true if X has the value 1, or if Y has the value 2,
or both conditions hold. Otherwise it is false.
The OR operator is sometimes called the inclusive OR, because it includes as true the case where both simpler expressions are true.
There is another logical operator called the exclusive OR (XOR), which Standard BASIC does not recognize. An exclusive OR expression is true if either of the simpler expressions are true, but not if both are.
Most people use the English word "or" as an exclusive OR operator. Don't let this confuse you; an OR expression in Standard BASIC is true if both of its parts are true (and, of course, if either is true). Think of the Standard BASIC OR as the legalistic term "and/or."
Before the Sorcerer can decide whether a logical expression is true or false, it must decide whether the numerical relations in the expression are true or false. And before it can make that decision, it must compute the values of the individual numerical expressions in the relation.
In Chapter 5 you learned the order of precedence for numerical operations. The numerical relations and logical operators are part of this order of precedence. This is the complete sequence:
Logical decisions may be made dependent on the values of counters, accumulators and flags.
You may repeat a program (or a part of a program) an indefinite number of times with GOTO statements. You can keep track of the number of repetitions with a counter. You can instruct the Sorcerer to make a decision when the counter reaches a certain value.
You may wish to keep running totals of certain values; this you do with accumulators.
You may want something to happen at a certain point in a program. You tell the Sorcerer that when a particular variable called a flag, has a certain value, it is time for that something to happen. You usually set a value that the variable would not normally reach. (For example, the program may instruct the operator to enter a number of ages, and to enter 0 when he wishes to end the program, that being an age that would not normally be input.) Each time through a loop, the program examines the variable to see if it is equal to (or greater than, or less than) the variable.
In Chapter 4 you saw how the Sorcerer makes decisions with IF...THEN statements. Here are some more examples, using numerical relations and logical expressions with variables, among them counters, accumulators and flags.
100 IF X=Y THEN 100
70 IF (A=B OR C<>D+2) THEN PRINT "HELP!"
200 IF (OSCAR(X)=SAM(2)) THEN STOP
10 IF NOT (A*2=6 OR X<Y) AND A(Q)=X^2 THEN 300
Here is a program that asks you to choose three numbers and then tells you something about those numbers. This program has a counter (the variable named CR) to keep track of how many times the operation is performed. The counter, together with a relational operator, also ensures that the message about how to terminate the program is only printed once, at the first run-through of the program.
10 LET CR=0
20 PRINT:PRINT "PICK THREE NUMBERS";
30 IF CR>0 THEN 60
50 PRINT "(YOU MAY END THIS PROGRAM BY ENTERING THREE ZEROS.)"
60 LET CR=CR+1
70 INPUT A,B,C
80 IF A=B OR B=C OR A=C THEN 110
90 PRINT "YOUR NUMBERS ARE ALL DIFFERENT."
100 GOTO 20
110 IF A=B AND B=C THEN 140
120 PRINT "TWO OF YOUR NUMBERS ARE THE SAME."
130 GOTO 20
140 IF A<>0 THEN 180
150 PRINT "YOU TRIED";CR-1;"SETS OF NUMBERS ";
160 PRINT "(EXCLUDING '0,0,0')."
180 PRINT "ALL OF YOUR NUMBERS ARE THE SAME."
190 GOTO 20
Notice that in 150 1 is subtracted from the CR total to avoid counting the entry of three zeros as a try. These three zeros are flags.
The city council wishes to know the average ages of children in various municipally funded projects. They have a program that runs on their new Sorcerer. This program instructs the operator to enter the sex and age of each child in turn. If he accidentally enters the wrong number (for instance, no one over the age of thirteen is enrolled in these projects), the program tells him to make the entry again.
10 C1=0:A1=0:A2=0:S1=0:S2=0:T=0:REM INITIALIZATION
30 PRINT "ENTER CHILD'S SEX FIRST, ACCORDING TO THIS CODE:"
40 PRINT "1=BOY, 2=GIRL. THEN ENTER CHILD'S AGE."
50 PRINT "EXAMPLE, FOR BILLY SMITH, AGE 10, ENTER: 1,10"
60 PRINT "(TO FIND THE AVERAGES, ENTER 0,0)"
90 PRINT "CHILD #";C1;
100 INPUT SX,AJ
110 IF SX=0 AND AJ=0 THEN 230
120 IF SX=1 OR SX=2 THEN IF AJ>0 AND AJ<14 THEN 150
130 PRINT "BAD INPUT. PLEASE TRY AGAIN."
140 GOTO 90
150 IF SX=2 THEN 190
160 LET S1=S1+1
170 LET A1=A1+AJ
180 GOTO 210
190 LET S2=S2+1
200 LET A2=A2+AJ
210 LET T=T+AJ
220 GOTO 80
240 IF S1>0 THEN PRINT "THE AVERAGE AGE OF THE BOYS IS";A1/S1
250 IF S2>0 THEN PRINT "THE AVERAGE AGE OF THE GIRLS IS";A2/S2
260 IF S1=0 AND S2=0 THEN PRINT "NO ENTRIES":STOP
270 PRINT "THE AVERAGE AGE OF ALL THE CHILDREN IS";T/(S1+S2)
ENTER CHILD'S SEX FIRST, ACCORDING
TO THIS CODE: 1=BOY, 2=GIRL. THEN
ENTER CHILD'S AGE. FOR EXAMPLE, FOR
BILLY SMITH, AGE 10, ENTER: 1,10
(TO FIND THE AVERAGES, ENTER 0,0.)
CHILD #1? 1,3
CHILD #2? 2,4
CHILD #3? 3,2
BAD INPUT. PLEASE TRY AGAIN.
CHILD #3? 2,0
BAD INPUT. PLEASE TRY AGAIN.
CHILD #3? 2,2
CHILD #4? 1,6
CHILD #5? 2,3
CHILD #6? 0,0
THE AVERAGE AGE OF THE BOYS IS 4.5
THE AVERAGE AGE OF THE GIRLS IS 3
THE AVERAGE AGE OF ALL THE
CHILDREN IS 3.6
Notice that in line 120 an IF...THEN statement is followed by another IF ... THEN. If both conditions are not fullfilled (i.e., if an improper entry is made), then the program does not continue to line 150. The "error message" prints instead, and the program requests a repeat of the input by returning to line 90 (By not going to line 80, the counter is not incremented until the program receives a proper input.)
Line 120 could be replaced by an equivalent statement:
120 IF (SX=1 OR SX=2) AND (AJ>0 AND AJ<14) THEN 150
These are the counters: C1 counts the number of inputs, S1 counts the boys and S2 counts the girls. The age accumulators are A1, for the boys, A2, for the girls and T, for all the children. SX=0 and AJ=0 are the flag conditions the program tests for.
Remember from Chapter 5 that, because the Sorcerer uses floating point numbers, it does not always store numbers the way you expect. You may tell the Sorcerer that when X=2 something must happen. That 2 may be the result of a computation that the Sorcerer has rounded off for screen display. The number may in actuality be 1.999994 which the Sorcerer does not think is the same as 2. Who knows what numbers lurk out there somewhere beyond the six digits that the Sorcerer prints on the screen?
To prove this point, try this program:
20 INPUT "CHOOSE TWO NUMBERS";X,Y
30 PRINT "YOUR FIRST NUMBER IS";X
40 PRINT "YOUR SECOND NUMBER IS";Y
50 IF X=Y THEN PRINT "X=Y":GOTO 10
60 PRINT "X<>Y":GOTO 10
CHOOSE TWO NUMBERS? 1,1
YOUR FIRST NUMBER IS 1
YOUR SECOND NUMBER IS 1
CHOOSE TWO NUMBERS? .9999994,1
YOUR FIRST NUMBER IS 1
YOUR SECOND NUMBER IS 1
You can avoid this problem by not using the "=" and "<>" operators. Replace them with the "<" and ">" operators.
A store owner came to the software department at Exidy to complain about an account--balancing program that refused to work. One step in the program required that a variable be equal zero. The statement was of the order IF X=0 THEN do such-and-such. His program never did what it was supposed to because X never did exactly equal 0; it was always a few fractions of a cent larger or smaller. The software people got the "bug" out of his program by substituting the statement:
IF X>-.001 AND X<.001 THEN
(This could be expressed more simply as
IF ABS(X)<.001 THEN ...
See Chapter 8.)
In other words, if X was within 1 /10 of a cent of the $100, then the decision was made. (This is the close-enough-for government-work principle.)
The <= operator is often used with integers. If you know the comparison will be made with an integer, remember that <=1 and <2 mean the same. If not, decide how accurate you must be, and then replace <=2 with, for instance, <2.001.
Another solution is to round or truncate numbers before making comparisons. The problem is not caused by BASIC but, rather, one intrinsic to computers.
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The Trailing Edge