Random Walk and Random Squawk - PROC, POINT(x,y)  Tutorial 008


Here we introduce use of a PROC (Procedure). This program is a development of one by Alan Thomas in his book "Further Programming for the BBC Micro" which is full of good ideas. Here I have removed his GOTOs and used instead a REPEAT ... UNTIL loop. (The use of GOTO in all but the smallest programs can cause programs to be almost unreadable as their structure soon becomes unclear. If we avoid GOTO then we can dispense with line numbers, which makes program development much easier and quicker).

Although the PROC in this program is not strictly necessary it does make for readability, which is important for the author as well as for readers. Its easy to forget how one programmed  something, so its good to choose "meaningful" variable names - thus in the following program "jump" is better than just "j".

Select/Copy/Paste the following program into BB4W, run and perhaps save it to disc (hard or floppy). Then read the annotated listing to see how it all works

      REM "Random Squawk" by RG Weston
      REM 19 Feb 2003
      REM After Alan Thomas p.11 but...
      REM structured, coloured, made audible etc.
      REM ** Models a gas molecule's motion **
      :
      MODE8:OFF
      PRINT'TAB(30)"Random Walk"
      ORIGIN 640,512
      MOVE 0,0
      x=0 : y=0
      jump=50
      REPEAT
        PROCwalk
      UNTIL POINT(x,y) = -1
      RUN
      END
      :
      DEF PROCwalk
      dx=RND(jump):dy=RND(jump)
      R=RND(2):S=RND(2)
      IF R=1 THEN x=x+dx ELSE x=x-dx
      IF S=1 THEN y=y+dy ELSE y=y-dy
      DRAW x,y
      GCOL0,RND(15)
      delay=INKEY(RND(10))
      SOUND 1,-(7+RND(8)),y,1
      REM PRINTTAB(30,30)POINT(x,y)
      ENDPROC

Annotated listing :

  100 REM "Random Squawk" by RG Weston
  110 REM 19 Feb 2003
  120 REM After Alan Thomas p.11 but...
  130 REM structured, coloured, made audible etc.
  140 REM ** Models a gas molecule's motion **
  150 :
  160 MODE8:OFF
  170 PRINT'TAB(30)"Random Walk"
  180 ORIGIN 640,512   *** sets graphics origin to centre of screen ***
  190 MOVE 0,0      *** Moves graphics cursor  to centre of screen ****
  200 x=0 : y=0      *** sets DRAW parameters for line 330 intitially to zero
  210 jump=50   *** max jump size for line 290 ***
  220 REPEAT *** main program loop ends at line 240 ***
  230   PROCwalk  *** This procedure is defined at line 280 ***
  240 UNTIL POINT(x,y)= -1 *** main loop repeats until the graphics cursor moves off screen.... ***
  250 RUN   *** When the program exits the main loop  RUN instructs the computer to start the program again ***
  260 END *** Shows end of program - put any Procedure definitions beyond here.
  270 : *** just a spacer ***
  280 DEF PROCwalk  *** Definition of PROCwalk ***
  290 dx=RND(jump):dy=RND(jump) *** dx and dy are the changes to the value of the graphics co-ordinates x and y ***
  300 R=RND(2):S=RND(2) ***decide which way to jump R=1 for right, R=2 for left, S=1 for up, S=2 for down... ***
  310 IF R=1 THEN x=x+dx ELSE x=x-dx
  320 IF S=1 THEN y=y+dy ELSE y=y-dy
  330 DRAW x,y *** Draws the line to the new position ***
  340 GCOL0,RND(15) *** changes graphics colour to make it easier to follow the jumps ***
  350 delay=INKEY(RND(10)) *** Random delay between jumps is more realistic for molecular collisions than a fixed one ***
  360 SOUND 1,-(7+RND(8)),y,1 *** hear the "y" co-ordinate as a sound! ***
  370 REM PRINTTAB(30,30)POINT(x,y) *** the REM here prevents the instruction working ***
  380 ENDPROC *** signals the end of the procedure.

Note :

At line 370, if you delete just the start of the line so as to remove only the three letters  REM, so that the line reads

PRINTTAB(30,30)POINT(x,y)

then the line will then be able to be active in your listing in the BB4W editing pane (window). If you run the program now, you will then see it printing the values returned by the POINT function which shows you the colour of the screen at the position (x,y).
Only when (x,y) wanders off the screen does it return a value -1 and the program restarts.

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