Page has been tested for compliance with INDUSTRY (not MS-only) standards, using the free, publicly accessible validator at validator.w3.org.In the previous tutorial we learned a way to draw a square....
MODE 12 PRINT "One plain" GCOL 14 MOVE 500,1000 DRAW 600,1000 DRAW 600,1100 DRAW 500,1100 DRAW 500,1000
That "works". but it is very pedestrian. We could do that as follows...
We can write code to draw the first square in a very different way... and yet it draws exactly the same squares! Here's the code. (Don't worry about all the spaces I put it to make things line up. They don't change what happens... but it is always good to make your code easy to read.)
MODE 12 PRINT "One fancy" x%=500 y%=1000 side%=100 GCOL 14 MOVE x%, y% DRAW x%+side%,y% DRAW x%+side%,y%+side% DRAW x%, y%+side% DRAW x%, y%
Save that. I suggest you call it bas4-one-fancy.bbc.
Study the code carefully until you think you really understand how it works.
When you think you understand how it works, change one of the numbers above one of the MOVE lines. Does your change have the effect you thought it would? Good! (If not, figure out why the change you made had the result it did! The computer doesn't make mistakes. It does what you told it to do. Not what your MEANT you wanted when you said what you did!)
Repeat that a few times, until you are sure, sure, sure you understand how this square drawer works.
Shut down you BASIC BBC IDE. Do some chores. Take a walk.
Then come back to the computer without looking at this, without looking at what you saved before your break... and try to write the version we finished with before the break from scratch!
You shouldn't try to "remember" it, exactly. You do need to remember The Ways To Do Things that you saw, and think about what you want... step by step... and then you can rebuild the code! (^_^)
IF you really understood it! Which will be true for very few people!
When you've given it your best shot, go back, re-read the stuff above here, and maybe try again, maybe press on.
What if you wanted code to draw TWO squares.
You could do it the boring and bad way...
MODE 12 PRINT "Two plain" GCOL 14 MOVE 500,1000 DRAW 600,1000 DRAW 600,1100 DRAW 500,1100 DRAW 500,1000 GCOL 12 MOVE 220,300 DRAW 1220,300 DRAW 1220,1300 DRAW 220,1300 DRAW 220,300
But that would be a BAD way to do it. The second square needs adjustments to 5 lines... and each time you make an adjustment, a chance to make a mistake is created.
Instead, add to "One Fancy", the code we did a moment ago. Turn that into what you see below...
MODE 12 PRINT "Two fancy" REM First square... x%=500 y%=1000 side%=100 GCOL 14 MOVE x%, y% DRAW x%+side%,y% DRAW x%+side%,y%+side% DRAW x%, y%+side% DRAW x%, y% REM Second square... x%=220 y%=300 side%=1000 GCOL 12 MOVE x%, y% DRAW x%+side%,y% DRAW x%+side%,y%+side% DRAW x%, y%+side% DRAW x%, y%
Notice that a big chunk of the new code is just a copy of the code we used to make the first square. If you've figured out doing "copy/paste" (see the first tutorial about doing squares), adding the "new" bits will be easy!
This really is a lot better!
Well... if that's all your going to do, I admit the difference isn't huge.
But! Now that you understand that much, I can show you something really cool... though you may not see that yet. It is an example of a very, very important skill in coding: Using subroutines.
Add the following to the end of the code, and run it again.
PRINT "Bit at End"
When you run it, you will get the two squares again, of course, and "Bit at End" up in the top right hand corner.
Now... even though it will make no sense to you for the moment, add...
END
... just before the PRINT "Bit at End"... and run the code again.
As you possibly guessed, even though it was still a mystery as to why you would want to do such a thing, this time you got the two squares still, but you did not get the PRINT "Bit at End". Don't you like it when things are simple? END measn... "end"! "Stop", "Go no further".
Pressing on...
Now replace the END with
DEF PROC_DoSquare MOVE x%, y% DRAW x%+side%,y% DRAW x%+side%,y%+side% DRAW x%, y%+side% DRAW x%, y% ENDPROC
... and change the heart of the progam, leaving you with...
MODE 12 PRINT "Two by subroutine" REM First square... x%=500 y%=1000 side%=100 GCOL 14 PROC_DoSquare REM Second square... x%=220 y%=300 side%=1000 GCOL 12 PROC_DoSquare END REM The rest of this is "details".... DEF PROC_DoSquare MOVE x%, y% DRAW x%+side%,y% DRAW x%+side%,y%+side% DRAW x%, y%+side% DRAW x%, y% ENDPROC
Do you see how much better that is that our early versions? I think the more you see subroutines (PROC_DoSquare is a subroutine), the more you will like them.
Among other things, they let you "make up new words" for the language! (like "PROC_DoSquare"!)
what happens in heart of the program...
REM First square... x%=500 y%=1000 side%=100 GCOL 14 PROC_DoSquare REM Second square... x%=220 y%=300 side%=1000 GCOL 12 PROC_DoSquare
... is now easier to see, isn't it? And of course, you don't have to worry about typos in endless MOVE/ DRAW statements.
One person's "amusing" is another person's "yawn", but I hope you will enjoy...
Save the final version of the two squares program, then start a new program, and make it...
SquaresToDoStill%=50 MODE 12 REPEAT x%=RND(1900) y%=RND(1400) side%=25*RND(18)+30 GCOL RND(14)+1 PROC_DoSquare WAIT(5) SquaresToDoStill%=SquaresToDoStill%-1 UNTIL SquaresToDoStill%=0 PRINT "Pretty? Goodbye." WAIT(200) QUIT END REM The rest of this is "details".... DEF PROC_DoSquare MOVE x%, y% DRAW x%+side%,y% DRAW x%+side%,y%+side% DRAW x%, y%+side% DRAW x%, y% ENDPROC
There's no need to try this, but it might amuse...
Revise the bit above and including the WAIT, so it looks like the following. You're only adding one line, but you're also putting some blank lines in to make things more readable.
GCOL RND(14)+1 PROC_DoSquare CIRCLE FILL x%,y%, 5*RND(12)+15 WAIT(5)
"CIRCLE FILL" is a built in word... just one of many you can use. For now, though, it is probably best to stick with building up your skill in using he core words.
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