INTRODUCTION TO BASIC

by David Ross  ( NY Computer Interest Group / KCN News ) 

Using Commodore ' s Somewhat Random Numbers :  Part 1

In games, simulations, and scientific programs, a source of random numbers may be needed .   A number is random if its selection from a set of numbers is as probable as the selection of any other number from that set .   The RaNDom function in Commodore BASIC  --  RND ()   --  provides a source of somewhat random numbers .   The qualification  " somewhat "  is necessary since computers are determinant systems in which every process depends on what occurred before .   The random function starts with a given number, or seed, and alters it according to a fixed rule .   Since it follows such a rule, the result is not random, but because the rule is unknown to the user of the program, it appears to be random .   And, in fact, these pseudo random numbers are unpredictable enough to be useful in games and most simulations, but not in rigorous scientific applications . 

The random function produces a decimal fraction between 0 and 1, excluding both . 

Enter and run the following program : 

10 PRINT RND ( 1 )  20 GOTO 10

Line 10 sends a random number between 0 and 1, exclusive, to the screen .   The number 1 is used within the parentheses in this and the following programs without comment .   Its effect will be discussed fully in a later article .   The GOTO keyword in line 20 returns the execution of the program to line 10 .   To slow the output, hold down the control key .   To stop the program, press the RUN / STOP key .  

Since programs are likely to require random integers, not decimal fractions, it is necessary to manipulate the result of the random function to produce the desired values .   The program below, RANDOM . DEMO . 1, constructs step by step the expression which produces random numbers between 1 and 10, including both 1 and 10 .   Next, each line of the program is described in detail . 

  100 REM  *******  RANDOM . DEMO . 1  ******* 
  110  ::::::::::::::::::::::::::::::::: 
  120 LET RN  =  RND ( 1 ) 
  130 PRINT  " RN  =",, RN
  140 PRINT  " RN * 10  =",, RN * 10
  150 PRINT  " INT ( RN * 10 )   =", INT ( RN * 10 ) 
  160 PRINT  " INT ( RN * 10 )+ 1  =", INT ( RN * 10 )+ 1
  170 END

LINE 100 :  The REM keyword allows for documentation  --  REMarks  --  to be included in the program .   BASIC ignores everything after this keyword while the program is running . 

LINE 110 :  Colons are mainly used to separate statements in a BASIC line .   However, here a line of them is used to mark off the title from the main program . 

LINE 120 :  LET is the keyword used to introduce the assignment statement where in the value of the expression RND ( 1 )  becomes the value of the variable RN .   LET can be omitted and the statement rewritten as 120 RN  =  RND ( 1 ). 

LINE 130 :  The keyword PRINT sends the expressions that follow it to the screen .   The expression within quotation marks is a string, or literal .   It is sent to the screen just as it is .   The expression RN is a variable .   Its value is copied from memory and then sent to the screen .   The commas between these two expressions format the output .   After the first expression is displayed, the first comma tabulates to the eleventh column, and the second comma to the twenty - first column .   Then the value of the second expression is displayed on the screen starting in that twenty - first column .   This aligns the values of the variable in this line with corresponding values in lines 150 and 160 . 

LINE 140 :  This line is similar to line 130 except that the value of RN is multiplied by 10 .   Recall that the object of this program is to produce random numbers between 1 and 10, inclusive .   Since the range of these numbers is 10, the decimal fraction represented by RN is multiplied by 10 .   The product is a number between 0 and 10, including 0 but excluding 10 .   The PRINT keyword causes this product to be sent to the screen . 

LINE 150 :  It is now necessary to remove the trailing decimal fractions from the selected number .   INT ()  is a numeric function which returns an INTeger less than or equal to the expression within the parentheses .   The expression now produces an integer ranging from 0 to 9, inclusive, which is displayed on the screen . 

LINE 160 :  The addition of 1 to the random integer shifts its range to 1 through 10, inclusive .   The goal of the program has been reached and been displayed on the screen .  

LINE 170 :  The END keyword marks the end of the program and stops execution .   It is optional in this program since BASIC will also stop when the program runs out of lines . 

The above program, can now be rewritten in one statement : 

200 PRINT INT ( RND ( 1 )* 10 )+ 1

As an exercise, rewrite the above programs to produce random numbers from 1 to 100, 10 to 20, and  - 1 to  + 1, all inclusive .   Run the programs as many times as needed to check that the full range of values is produced .   A GOTO statement may be useful . 

In the next article, there will be a demonstration of a programing solution to the above exercise .   In the following articles, there will be programs and discussions showing how expressions within the parentheses of the random function affect the output and the use of the random function in a simple game .