Apple II User Manual

REMARKS

The REM (short for "remark") statement is used to insert comments or notes into a program.  When

BASIC encounters a REM statement, the rest of the line is ignored.

This serves mainly as an aid for the programmer and serves no useful function as far as the operation

of the program in solving a particular problem.

207  RELATIONAL TESTS

Suppose we wanted to write a program to check whether a number is zero.  With the statements

we've gone over so far, this could not be done.  What is needed is a statement which can be used

to conditionally branch to another statement.  The "IF-THEN" statement does just that.

Type in the following program:  (remember, type NEW first)

    10 INPUT B

    20 IF B=0 THEN 55

    30 PRINT "NON-ZERO"

    40 GOTO 10

    50 PRINT "ZERO"

    60 GOTO 10

When this program is typed and run, it will ask for a value for B.  Type in any value you wish.

The AIM 65 will then come to the "IF" statement.  Between the "IF" and the "THEN" portion

of the statement there are two expressions separated by a "relation."

A relation is one of the following six symbols:

    RELATION            MEANING

    --------            ------------------------

    =                   EQUAL TO

    >                   GREATER THAN

    <                   LESS THAN

    <>                  NOT EQUAL TO

    <= or =<            LESS THAN OR EQUAL TO

    => or >=            GREATER THAN OR EQUAL TO

The IF statement is either true or false, depending upon whether the two expressions satisfy the

relation.  For example, in the program we just did, if 0 was typed in for B the IF statement would

be true because 0=0.  In this case, since the number after the THEN is 50, execution of the program

would continue at line 50.  Therefore, "ZERO" would be printed and then the program would

jump back to line 10 (because of the GOTO statement in line 60).

Suppose a 1 was typed in for B.  Since 1=0 is false, the IF statement would be false and the program

would continue execution with the next line.  Therefore, "NON-ZERO" would be printed and the

GOTO in line 40 would send the program back to line 10.

A PROGRAM USING RELATIONS

Now try the following program for comparing two numbers:

    10 INPUT A,B

    20 IF A<=B THEN 50

    30 PRINT "A IS BIGGER"

    40 GOTO 10

    50 IF A<B THEN 80

    60 PRINT "THEY ARE THE SAME"

    70 GOTO 10

    80 PRINT "B IS BIGGER"

    90 GOTO 10

When this program is run, line 10 will input two numbers from the keyboard.  At line 20, if A is

greater than B, A<=B will be false.  This will cause the next statement to be executed, printing

"A IS BIGGER" and then line 40 sends the computer back to line 10 to begin again.

At line 20, if A has the same value as B, A<=B is true so we go to line 50.  At line 50, since A has

the same value as B, A<B is false; therefore, we go to the following statement and print "THEY

ARE THE SAME." Then line 70 sends us back to the beginning again.