QBASIC Lesson 2 - String handling

Okay, you've had an easy introduction, now we're going to progress onto
something useful. Strings. You've already heard about variables, right? A
variable is the computer's way of storing a number. But wat about storing
a word, a group of words or even a novel? This is where strings get involved.
The best way to show you is probably by example, so here is an example:

a$ = "Hello World!"
PRINT a$

Another completely pointless program, I know, but it does illustrate a point.
To use a string simply put the dollar sign after a variable name ($) and the
computer will know that it is supposed to be a string. Don't ask me why it's
a dollar sign; it's been like that for ages. By the way, a$ is pronounced
"a string".

So, a string works pretty much like a variable. Strings can be added together
as follows:

a$ = "Hello "
b$ = "World!"
c$ = a$ + b$
PRINT c$

However they cannot be multiplied, subtracted or divided. There are other
functions which provide virtually anything you could ask for. Experiment
with the following until you've worked out what they do (don't expect me
to do all the work!):

a$ = UCASE$("hello world!")
PRINT a$

a$ = LCASE$("HELLO WORLD!")
PRINT a$

a$ = RIGHT$("Hello World!", 6)
PRINT a$

a$ = LEFT$("Hello World!", 5)
PRINT a$

a$ = MID$("Hello World!", 3, 6)
PRINT a$

a = LEN("Hello World!")
PRINT a

Experiment with different numbers where applicable until you've fathomed what
the different functions do. These form the basics of string handling in
QBASIC. Using these commands along with the "+" command you should be able
to create pretty much any string you want. If you get stuck then have a look
in the QBASIC help file; however it is often much easier to discover these
things by yourself.

There is one possible hurdle, however. What if you have a variable, but need
to treat it as a string? Or a string, but need to treat it as a variable?
No problem. These two commands do exactly that:

n$ = "100"
a = VAL(n$)
PRINT a

n = "100"
a$ = STR$(n)
PRINT a$

In this way, you can take a string and load it's value into a variable, or
take a variable and load it's value into a string. This is immensly useful,
although at the moment I can't actually think of a useful application.

Okay, one final pair of commands before we finish talking and start
programming:

a$ = CHR$(98)
PRINT a$

a = ASC("b")
PRINT a

You may of heard of the ASCII character codes. ASCII is basically a labelling
system where every letter, symbol, in fact every symbol that you can see
in text on a computer, is given a number. For example, lower case a is 97.
There is a full list of the ASCII characters in the QBASIC help file.

Anyway, the command CHR$ produces the character for a particular ASCII code;
for example, give it the number 97 and it will give you an a. The command
ASC does exactly the opposite. Give it a character and it will give you the
ASCII code. Easy.

Okay, now for today's program:

CLS
INPUT "Please enter your surname", n$
INPUT "Please enter your first name", f$
INPUT "Please enter Mr/Mrs/Miss/Ms", t$
n = INT(RND * 10) + 1

p$ = t$ + f$ + n$ + " your number is:" + STR$(n)

PRINT p$

All this program does is ask your name and whether you wish to be addressed
as Mr, Mrs, Miss or Ms. It then picks a random number and gives it to you,
calling you by the name you entered. I think a line-by-line breakdown is
probably a good idea at this stage:

CLS
Clear the screen. No problem.

INPUT "Please enter your surname", n$
This asks for your surname and puts it in n$.

INPUT "Please enter your first name", f$
This asks for your first name and puts it in f$.

INPUT "Please enter Mr/Mrs/Miss/Ms", t$
This asks how to address you and puts it in t$.

n = INT(RND * 10) + 1
This puts a random number in n - I'll explain this more thoroughly at a later
date. For now, feel free to have a go at messing around with the INT and RND
commands to see what they do.

p$ = t$ + f$ + n$ + " your number is:" + STR$(n)
This is the bit that does the work. It states that p$ should be made to equal
t$ plus f$ plus n$ plus the text " your number is:" plus the number in n
converted to a string.

PRINT p$
This command merely prints out p$ onto the screen.

Okay, that wasn't too hard. Have a go at using the other commands to do things
like displaying the middle part of a name, displaying a name backwards or
any other thing you feel like having a got at. The next lesson will cover
IF statements and FOR...NEXT loops. Both of these are essential to programming
in basic, and when you've learned them you will be able to create many more
useful programs. Until then...