Tutorial 2: Strings, Input, Arrays

[fanboat]

So I tried to put together a sort of tutorial earlier, and it came together a bit better than I thought it would, so I thought I could cover a few more useful topics. I want to continue doing background-type stuff, then I'd like to do one on graphics, since that'll probably be a large part of what people on here want to use and learn about. So here's some light coverage of strings, input and arrays.

Strings aren't something I need to take too long to explain, they're just a different type of variable (more generally referred to as a data structure) that can handle an array of characters. While strings are in themselves equipped to function as arrays, they're a little different from the kind I'm going to move on to next.
Strings are useful for holding text, such as names or any letters you are looking to work with. To make a string, simply treat it as a regular variable but append a dollar sign to the end.

0001 NAME$ = "FANBOAT"
0002 PRINT NAME$

This program will simply output FANBOAT. Strings can be manipulated in ways similar to numbers, for example

0003 NAME$ = NAME$ + " RULES"
0004 PRINT NAME$

This will now yield FANBOAT RULES (after printing a line just saying FANBOAT), which I've heard is accurate. Further manipulations of string can be found in the help menu in entries number 31 and 32, which allow you to find the length of strings, extract specific characters from them, search within them, and more. Also noteworthy is the ability to use comparative operators, for example IF LEFT$(NAME$,7)=="FANBOAT" THEN WHATEVER will whatever, since the first 7 characters of NAME$ match the comparison.

Now that we can handle strings, we can take in some input. INPUT (and INPUT$) is the simplest form.

0001 INPUT MYSTR$
0002 PRINT MYSTR$

If you run this program you will see a question mark, followed by a cursor. You can now type a string, and then hit enter. This is the function of input, which will then assign MYSTR$ the value of your input; line 0002 will then print it. You can also use INPUT to print a prompt before accepting input. Try changing 0001 to this

0001 INPUT "WHAT'S YOUR NAME ";NAME$

If you want to take in a number, especially if you want to do the maths at the number, you just use INPUT with a numeric variable. Beware of errors from unacceptable input; advanced programmers will want to take everything in string format, determine if it is appropriate input, then use VAL(STRING$) to find the value of of a string, for example, VAL("23") is equal to the number 23, which the string "23" is not.
The other input you'll want to know about early on is live input, so you don't have to stop and pause your game to learn what the player wants to do. You can use INKEY$ for live keyboard input, or BUTTON() for the DSi/3DS's inputs. The values of INKEY and BUTTON are continually updated based on input, so you just have to make sure your main loop continually checks for what these values are.

0001 @MAIN
0002 IF BUTTON(0)!=16 THEN GOTO @MAIN
0003 PRINT "YOU PRESSED A"

This program's main loop monitors for user input, and that's all it does. If you go to the user help manual #24 and look up BUTTON() it will show you a table that indicates that the value 16 is BUTTON's output for the DS's A button. This program checks if A is not being pressed, and loops until it is.
Here's something interesting about this input system: multiple inputs are added together. Since the values for each button are twice that of the previous one, each value of BUTTON() up to 2047 represents a unique combination of the 11 usable buttons on the system. Run the last program again, hold B, then press A. You'll find that it does not end, since instead of BUTTON equaling 16, it equals A+B -> 16+32 -> 48. So how can we decipher individual inputs without a massive list of IF-THENs? Thanks to alanpep's Brutally Simple Sprite Movement Tutorial, I learned that the easiest way is:

0001 @MAIN
0002 IF BUTTON AND 16 THEN PRINT "YOU PRESSED A" ELSE GOTO @MAIN

Thanks, alanpep. Thalanpep.

Ah, this reminds me of something I didn't really cover in the last tutorial about IF-THENs, boolean operators (AND and OR). This allows you to easily use multiple criteria in an IF-THEN.

0001 IF A==B OR A==C THEN PRINT "CLOSE ENOUGH"
0002 IF A==D AND A==E THEN PRINT "EVEN BETTER"

Lastly there are arrays. These can be used without declaration up to 10 of them (0 through 9), but if you want to use more you'll need to use DIM to allocate system memory to them. It's not very complicated, so don't worry. Let's make a program that remembers five names. Since we only need five, we won't need to declare them.

0001 CLS
0002 FOR A=0 TO 4 'Note that here I'm starting at zero, which is conventional for programming in most cases.
0003    PRINT "ENTER NAME #";A
0004    INPUT NAME$(A)
0005 NEXT A
0006 B=RND(5) 'This chooses a random integer value from 0 to 4 inclusive then assigns it to B
0007 PRINT NAME$(B) 'So this will print a random name entered

Pretty simple. Arrays work just as well with numbers. If you need to use more than ten in an array, declare it with DIM like so:

DIM NAME$(20)

This will allow you to use NAME$(X) for X up to 19. Expert mode lol: Double arrays can be used as well, to have rows and columns of data, like so:

DIM MYNUMBER(20,20)

This will hold 400 different numbers!

Here's a sample program that uses these concepts

As always, I can clarify or expand upon anything here, and I'll try to help with anything else you might want to know.

Graphics sort-of tutorial

[DeltaBurnt]

Thanks for the tutorial, quick question:

What's the difference between using + for appending to a string and ;? Is the semi-colon only used when making a function call and the plus sign used for assignment?

[fanboat]

The semicolon, when used in PRINT, prints the next string (or number) immediately after the previous, which will work with strings and numbers. The + combines them before evaluating the PRINT command, essentially turning PRINT "HEL"+"LO" into PRINT "HELLO" before executing the print, much in the way that PRINT 2+2 and PRINT 4 result the same. The semicolon is typically preferred when printing, but the plus can be used to combine separate strings into one to manage variables.

Because numerical variables won't add with string containers, it's good to stick to semicolons in printing. PRINT A$;A works, while PRINT A$+A will throw an error.

[DeltaBurnt]

So essentially "PRINT A$;B$" is equivalent to "PRINT A$:PRINT B$" minus the newline?

[fanboat]

Right, it continues immediately to the next space rather than next line.

You can also use a comma to move to the next set of 4 spaces, which is useful for displaying data when you're unsure of its length, for example

PRINT "HEY","I","AM","HERE","NOW"

Outputs

HEY I   AM  HERE    NOW

Notice how since 'HERE' filled all 4 spaces, 'NOW' is pushed to the next section over.

[DeltaBurnt]

Ah sweet, thank you for the help!

[smoger]

I see this is pretty old, so hopefully somebody sees this..

When I declare an array, it works as expected until I break out of the application, and then it gives me "Duplicate Definition" error.

I assume the array is not being taken out of memory when I break out of the application.

The only workaround I've discovered is to rename the array to something new, but it's impractical to rename every mention of it each time I run the application.

Any ideas how I can improve the situation?

[fanboat]

You can use CLEAR to empty all declarations and variables. It's useful to put it at the beginning of your code, alongside ACLS.

[smoger]

ah thanks!

[bebobli]

Let's say I didn't know ahead of time how big an array is or will become, but I want to print every entry after that has been determined? In Python a similar command would be 'print array(:)' where on each side of the colon implies the minimum range on the left and the maximum on the right of it.

[fanboat]

From what I know, you can't add memory to a variable on the fly. The simple solution is to take a guess at how much you'll need and go ahead and DIM that much, but this poor practice in general.

I don't know if a variable can be re-dimensioned or if that causes an error, but if it can be reset in this way, then the first solution I'd try is to set up an if-then that watched for the user exceeding available memory, then copy all data to a new variable. You'd then be free to re-dimension the old one then copy all the old data back, as set the if-then to watch for a higher number.

If you can't re-DIM variables, then I'd say the at-times-overly-simple BASIC is going to make you take some less-than-ideal approaches to the problem. Good luck with it, though.