Hello, and welcome to the sixth part of the Runtime Basic Tutorial
FIRST STEPS IN BASIC
By now you should have had plenty of practice typing in the short
listing given in the previous articles. If you have, and have managed to
follow the series so far, you've got what it takes to be a programmer!
On the other hand, it's quite possible that all this talk to variables,
real and integer numbers and rounding up and down is making your head
spin. Well, don;t worry! Fortunately for you, programming in BASIC
doesn't always involve complex calculations and maths.
In the last two articles, we looked at the two types of BASIC
variable that deal with numbers. In this article, we're going to introduce a
third type called a "STRING". A string variable is used to store a
sequence - or string - of characters, which can be letter, numbers or any
other character from the ST character set. A string can always be told
apart from another type of variable by the dollar sign ($) after the name.
Thus both A$ and name$ are acceptable as string variables. (Remember
that the rules we discussed for naming variables still apply. Basic only
looks at the first eight letters, and each variable must have a unique name
to avoid confusion).
Unlike the numeric variables we have already look at, a string
variable cannot have mathematical operations performed on it. The only
thing that can be done to two strings is to add them together. Joining tow
strings together in this way is known as concentration. Attempting to do
something like a division is meaningless, and causes a BASIC error. For
10 A$ = "APPLES"
20 B$ = "PEARS"
30 PRINT A$ / B$
To us, dividing APPLES by PEARS might seem to be perfectly
reasonable, but to BASIC it is an impossible operation. Remember that
BASIC can only work with numbers and doesn't understand APPLES and
PEARS as anything other than a string of letters.
With this limitation, it might seem at first that strings a practically
useless. However, strings are one of the most useful features of BASIC,
as we shall shortly see. Here's our first sample program for this month:
10 INPUT "What is your Christian name:";a$
20 INPUT "What is your surname:";b$
30 PRINT "Hello ";a$;"";b$
The program will respond:
What is your Christian name:?PETER
What is your surname:?PHILLIPS
Hello PETER PHILLIPS
Remember that the INPUT statement automatically adds a
questions mark to the end of the prompt, so you don't need to worry about
putting one in. It might also be useful to leave a couple of spaces after the
colons (:) to make the program look better.
The second point with this program is the variable names.
Remember that you should always to to give variables a meaningful
name that helps you keep track of them.
In the following program, we can see how string concentration
works. Also notice that the meanings of the variable names are now much
10 INPUT "What is your Christian name:";first$
20 INPUT "What is your surname:";last$
30 hello$ = "Hello " + first$ + "" + last$
40 PRINT hello$
Notice how the semi-colons (;) in the PRINT statement of
PROGRAM ONE have been replace by additions (+) in the string
concentration in line 30 of the new program.
Strings are always added from the left to right so if line 30 read:
30 hello$ = "Hello " + last$ + "" + first$
then the result would be printed as:
Hello PHILLIPS PETER
Program Two is also a longer way of achieving the same result.
However, if you want to use hello$ again to address the user by name
several times within the same program, then this is the best way to do it.
techniques like this are often used in games to help address the player in a
CUTS BOTH WAYS
Characters, no matter what sort they may be, words, sentences, or
whatever can be manipulated provided they are held as a string. If you
think of a text string like a real piece of string, you won't go far wrong.
We've already seen how to tie two pieces of string together, now how
about cutting them up?
The table below gives a list of all the operations that can be used to
cut up strings, as well as some others that can be useful. The three most
commonly used operations are LEFT$, RIGHT$ and MED$.
LEFT$ Get left part of string
MED$ Get middle part of string
RIGHT$ Get right part of string
LEN Find the length of the string
INSTR IN STRing. Search for one string inside another.
VAL Returns the numeric value of the string
STR$ Convert a numeric value to a string
To see how some of these operations work, type in the following
program and run it:
10 a$ = "Left" + "Middle" + "Right"
20 PRINT "Left word = ";LEFT$ (a$,4)
30 PRINT "Right word = ";RIGHT$ (A$,5)
40 PRINT "Middle word = ";MED$ (a$,5,6)
This program will return with these answers:
Left word = Left
Right word = Right
Middle word = Middle
If you try Program Three for yourself, you'll see what the answers
are, but not really understand why the program is returning these results.
It is often necessary to go through a program line by line to understand
what is happening. Lets try it with this one.
LINE 10 a$ = "Left" + "Middle" + "Right"
This is a string concentration operation. It creates a string called a$
that contains the letters "LeftMiddleRight". Notice that there are no
spaces between the three words.
LINE 20 PRINT "Left word = ";LEFT$ (a$,4)
We already know what a PRINT statement does and why, so its
what's after the semi-colon that matters. This part of the statement
introduce the first of the new string functions, LEFT$.
Notice that LEFT$ has been given two parameters inside the
brackets, separated by a comma (,). The syntax for the command is:
x$ = LEFT$ (y$,X)
where x$ is the character to the leftmost of y$ as denoted by the
value of X. So in our example, what left string does is this:
Take the four characters (4 = value of X) to the leftmost of string
a$ (a$ is "LeftMiddleRight") and print them to the screen.
This is translated as the word "Left" as the letters L, E, F, T are the
leftmost four characters of "LeftMiddleRight".
LINE 30 PRINT "Right word = ";RIGHT$ (A$,5)
RIGHT string has exactly the same syntax as LEFT$, except that it
counts from the right hand end of the string. In our example it takes the
five letters to the farthest right of the string which are R, I, G, H, and T
and prints them.
LINE 40 PRINT "Middle word = ";MED$ (a$,5,6)
MED$ has a slightly different syntax from the other two. It
introduces a third parameter. The complete syntax is:
x$ = MED$ (y$, start, extent)
The x$ and y$ parts are exactly the same as for LEFT$ and
RIGHT$ but the START and EXTENT parts are new. They are used like
START is the number of character into the string to count
EXTENT is the number of characters to be included
So our example translates as:
Count five (start) characters into the string LeftMiddleRight (a$)
and then take the next six characters (extent).
This then finds the letters M, I, D, D, L, and E because M is the
fifth character and MIDDLE is six letters long. (Note that the START
character is included in the number counted by EXTENT).
BEFORE NEXT TIME
Try and write a program to input two strings, concentrate them and
then output the result. This should be fairly easy if you follow the
Try to write a program to find letters of the alphabet, both forwards
and backwards. What is the ninth letter of the alphabet? What is the sixth
letter from the end? HINT: Set up your a$ to contain the letters of the
alphabet, and an INPUT statement to get the number of the letter
required. Then use LEFT$ and RIGHT$ to find the correct letters. I'll
include a sample of the sort of program you should have written next
Marko, Suomen Atari-sivut / ArkiSTo 2003