share/docs/tutorial/ch05.html

<!doctype html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8">
<style type="text/css">
td, th { border: 1px solid #c3c3c3; padding: 0 3px 0 3px; }
table { border-collapse: collapse; }
img { max-width: 100%; }
</style>
<meta name="generator" content="ReText 7.2.3">
<title>ch05</title>
<style type="text/css">
</style>
</head>
<body>
<p><a href="ch04.html">Previous</a> <a href="index.html">Index</a> <a href="ch06.html">Next</a></p>
<hr>
<h1>5 - Variables and Expressions</h1>
<h4>Table of Contents</h4>
<ul>
<li><a href="#5.1">5.1 Introducing variables</a></li>
<li><a href="#5.2">5.2 Using LET</a></li>
<li><a href="#5.3">5.3 Variable names</a></li>
<li><a href="#5.4">5.4 String variables</a></li>
<li><a href="#5.5">5.5 Displaying variables</a></li>
<li><a href="#5.6">5.6 String operators</a></li>
<li><a href="#5.7">5.7 Numeric operators</a></li>
<li><a href="#5.8">5.8 Expressions</a></li>
<li><a href="#5.9">5.9 Operator precedence</a></li>
<li><a href="#5.10">5.10 Using brackets</a></li>
<li><a href="#5.11">5.11 Getting input</a></li>
<li><a href="#5.12">5.12 Avoiding LET</a></li>
<li><a href="#5.13">5.13 Illegal expressions</a></li>
</ul>
<hr>
<h2><a name="5.1">5.1 Introducing variables</a></h2>
<p>When we want to store something in memory - a number, a word, a sentence, perhaps an entire whole room description - we give it a symbolic name.</p>
<p>For example, if you want to store your character's health points, you might use the name <strong>health</strong>.</p>
<pre><code>    health = 100
</code></pre>
<p>To store your character's energy or movement points, you would use a different name.</p>
<pre><code>    energy = 250
</code></pre>
<p>The name you choose doesn't matter; the important thing is to choose a name that can be easily understood. The following line is perfectly good Axbasic, but it's still a really bad idea:</p>
<pre><code>    fpwpxzkiu = 99
</code></pre>
<p>In Axbasic, a <em>variable</em> is a value that's been stored in memory and given a symbolic name. It's called a variable because we can change the value any time we want.</p>
<pre><code>    health = 90
    health = 80
    health = 70
    ...
</code></pre>
<h2><a name="5.2">5.2 Using LET</a></h2>
<p>If you try running the following Axbasic script, you'll see an error message.</p>
<pre><code>    health = 100
    END
</code></pre>
<p>We've already mentioned a general rule in BASIC, namely that every statement must begin with a keyword. The first line starts with a variable, not a keyword, and that's not allowed.</p>
<p>When we want to create or modify a variable, we use the keyword LET.</p>
<pre><code>    LET health = 100
    END
</code></pre>
<p>Every statement now starts with a keyword, so the script will now run. (There is a way to avoid LET altogether; we'll discuss it later in this Section.)</p>
<h2><a name="5.3">5.3 Variable names</a></h2>
<p>There are some rules about variable names.</p>
<ul>
<li>The first character must be a letter</li>
<li>Other characters can be letters, numbers or underline (underscore) characters</li>
<li>The name can be as long as you want</li>
<li>Case doesn't matter</li>
<li>You can't use an Axbasic keyword like PRINT or END</li>
</ul>
<p>All of the following variable names are allowed:</p>
<pre><code>    hp
    health
    health_points
    the_health_points_for_my_character_are_stored_here
</code></pre>
<p>But this line would cause an error:</p>
<pre><code>    LET print = 10
</code></pre>
<p>Traditonally, keywords like PRINT and END are typed in capital letters but variable names like <strong>health</strong> and <strong>energy</strong> are typed in lower-case letters. Observing this tradition will make your scripts much easier to understand.</p>
<p>If your variable name contains several words, you should use underline characters. Don't simply use a much longer word - it's difficult to read, <em>especially</em> for someone whose first language is not English</p>
<pre><code>    thisisaverybadchoiceforavariablename
    do_this_instead
    orDoThisIfYouPrefer
</code></pre>
<p>Strictly speaking, it doesn't matter if you type in capitals, lower-case letters or a mixture of the two. Axbasic thinks that all of these are the same variable:</p>
<pre><code>    health
    HEALTH
    hEaLtH
</code></pre>
<h2><a name="5.4">5.4 String variables</a></h2>
<p>A string is any value that's not a number - a letter, a word, a sentence, or something even longer.</p>
<p>In Axbasic, strings are always written between double quotes. We've already seen a few examples.</p>
<pre><code>    PRINT "Hello world"
    MOVE "north"
    SEND "inventory"
</code></pre>
<p>For strings, the variable name always ends with a dollar character.</p>
<pre><code>    LET name$ = "bilbo"
    LET address$ = "the shire"
    LET favourite_colour$ = "gold"
</code></pre>
<p>If you forget to type either the dollar or the double quotes, you'll see an error message. For example, this script won't work because the variable <strong>number</strong> is expecting a numeric value, not a string.</p>
<pre><code>    LET number = "bilbo"
</code></pre>
<p>However, the converse isn't true. If you use a string variable and a numeric value, Axbasic will convert the value into a string. Both of the following lines behave in exactly the same way:</p>
<pre><code>    LET value$ = "100"
    LET value$ = 100
</code></pre>
<p>String variables can have the same name as an Axbasic keyword - you can use variables called <strong>print$</strong> and <strong>end$</strong>, if you really want to. (We've already mentioned that numeric variables can't be called <strong>print</strong> or <strong>end</strong>.)</p>
<h2><a name="5.5">5.5 Displaying variables</a></h2>
<p>The following script creates a variable, displays its value, then modifies the variable and displays the new value.</p>
<pre><code>    LET number = 1
    PRINT number
    LET number = 2
    PRINT number
    END
</code></pre>
<p>Of course, if we wanted to store words rather than numbers, we'd have to use a string variable.</p>
<pre><code>    LET name$ = "bilbo"
    PRINT name$
    LET name$ = "gandalf"
    PRINT name$
    END
</code></pre>
<h2><a name="5.6">5.6 String operators</a></h2>
<p>You can join two strings together by using the ampersand character.</p>
<pre><code>    LET name$ = "bilbo"
    LET surname$ = "baggins"
    PRINT name$ &amp; surname$
    END
</code></pre>
<p>This script joins the two strings together, and PRINTs them as a single line. The output looks like this:</p>
<pre><code>    bilbobaggins
</code></pre>
<p>That's not very pleasing on the eye, but we can join as many strings as we want.</p>
<pre><code>    LET name$ = "bilbo"
    LET surname$ = "baggins"
    PRINT name$ &amp; " " &amp; surname$
    END
</code></pre>
<p>The output now looks like this:</p>
<pre><code>    bilbo baggins
</code></pre>
<h2><a name="5.7">5.7 Numeric operators</a></h2>
<p>We can use the plus character to add two numbers.</p>
<pre><code>    LET first = 10
    LET second = 20
    PRINT first + second
    END
</code></pre>
<p>Hopefully, this script will display a single number:</p>
<pre><code>    30
</code></pre>
<p>You can also use the minus character for subtractions. Multiplication is done with the asterisk character and division is done with the forward slash character.</p>
<pre><code>    ! Let's do some sums
    LET first = 10
    LET second = 2
    ! Add
    PRINT first + second
    ! Minus
    PRINT first - second
    ! Multiply
    PRINT first * second
    ! Divide
    PRINT first / second
    END
</code></pre>
<p>The output looks like this:</p>
<pre><code>    12
    8
    20
    5
</code></pre>
<p>You can use the circumflex accent character to raise a number to a power.</p>
<pre><code>    ! Find the square of 2
    PRINT 2 ^ 2
    ! Find 2 to the power of 3
    PRINT 2 ^ 3
    ! Find 2 to the power of 10
    PRINT 2 ^ 10
    END
</code></pre>
<p>The output looks like this:</p>
<pre><code>    4
    8
    1024
</code></pre>
<h2><a name="5.8">5.8 Expressions</a></h2>
<p>The characters we've just used ( &amp; + - * / and ^ ) are called <em>operators</em>.</p>
<p>When we put a series of values and operators together, we get an <em>expression</em>. All of the following lines are expressions.</p>
<pre><code>    42
    1 + 2
    10 - 5 + 1
    3 * 6 * 2
    "bilbo" &amp; "baggins"
</code></pre>
<p>The important thing to remember about expressions is that they can be reduced to a single value. In other words, we can work out the "answer".</p>
<pre><code>    42
    3
    6
    36
    bilbobaggins
</code></pre>
<p>Before using an expression, Axbasic always works out the answer. We call this <em>evaluating the expression</em>. In this example, Axbasic evaluates the expression first, <em>before</em> PRINTing anything. The script PRINTs the number 15; it doesn't PRINT the numbers 1 to 5.</p>
<pre><code>    PRINT 1 + 2 + 3 + 4 + 5
    END
</code></pre>
<h2><a name="5.9">5.9 Operator precedence</a></h2>
<p>Take a look at the following script, and try to work out which number it PRINTs.</p>
<pre><code>    PRINT 1 + 2 * 4
    END
</code></pre>
<p>If you think the value is 9, you're right. But you might have thought the script would display 12; after all, 1 plus 2 is 3, and 3 multiplied by 4 is 12.</p>
<p>All programming languages - including Axbasic - have some notion of <em>operator precedence</em>, which is a fancy way of saying that multiplication should be done before addition.</p>
<p>In other words, for the expression:</p>
<pre><code>    1 + 2 * 4
</code></pre>
<p>We do the multiplication <em>first</em>, which gives us:</p>
<pre><code>    1 + 8
</code></pre>
<p><em>Then</em> we do the addition, giving us the final value of 9. Axbasic <em>always</em> gives this same value, no matter how many times you run the script.</p>
<p>(For anyone who is wondering, operators have <em>left-to-right associativity</em>. Because there are comparitively few operators compared to more modern languages, associativity is irrelevant in the behaviour of Axbasic scripts.)</p>
<h2><a name="5.10">5.10 Using brackets</a></h2>
<p>At this point, all programming tutorials will tell you that writing an expression like <strong>1 + 2 * 4</strong> is a <em>really bad idea</em>, even though it's perfectly legal Axbasic.</p>
<p>A much better way is to use brackets. In an expression, everything inside a pair of brackets is worked out first.</p>
<pre><code>    (1 + 2) * 4
</code></pre>
<p>In this example, 1 plus 2 is 3. We work that part out first because it's inside a pair of brackets. Then we work out that 3 multiplied by 4 is 12.</p>
<p>It's possible to use brackets within brackets, if you need to.</p>
<pre><code>    ( (1 + 2) * 4 ) / 2
</code></pre>
<p>In this example, we work out the innermost sum first. 1 plus 2 is 3, and 3 multipled by 4 is 12. Then we divide 12 by 2, to get the final answer of 6.</p>
<p>You can use as many brackets as you want, but the expression must contain exactly the same number of open brackets and close brackets.</p>
<h2><a name="5.11">5.11 Getting input</a></h2>
<p>One way to set a variable's value is to use LET. Another way is to ask the user to type the value themselves.</p>
<p>An INPUT statement waits for the user to type something, and then stores it in a variable</p>
<pre><code>    PRINT "What is your name?"
    INPUT name$
    PRINT "Your name is"
    PRINT name$
    END
</code></pre>
<p>Axbasic creates a dialogue (popup) window. After the user types their name and presses ENTER, the window disappears. The user's response is stored in the variable <strong>name$</strong>.</p>
<h2><a name="5.12">5.12 Avoiding LET</a></h2>
<p>After a while, BASIC programmers grow weary of typing LET every time they want to set a variable, and begin wondering why they can't just type something like this:</p>
<pre><code>    number = 10
</code></pre>
<p>Well, in fact it's possible to do tell Axbasic that you don't want to use LET at all. Just put this line somewhere in your script:</p>
<pre><code>    OPTION NOLET
</code></pre>
<p>Now you don't need to use LET at all.</p>
<pre><code>    OPTION NOLET
    first = 10
    second = 5
    third = 2
    PRINT first * second * third
    END
</code></pre>
<p>By the way, before Axbasic starts executing a script, it checks every line, looking for OPTION statements. If it finds any, it applies them immediately.</p>
<p>In other words, your OPTION NOLET can appear anywhere in the script. In practice, though, it's a very bad idea to put it anywhere but at the beginning. (Your computer doesn't mind checking every line in the script, but humans won't thank you for it.)</p>
<h2><a name="5.13">5.13 Illegal expressions</a></h2>
<p>In Axbasic, as in all other programming languages, you can't divide any number by zero. This simply isn't allowed. (A mathematician would say the operation is <em>undefined</em>.)</p>
<p>If you try running this script, you'll get a <em>Division by zero</em> error.</p>
<pre><code>    PRINT 10 / 0
    END
</code></pre>
<p>However, multiplying by zero or raising a number to the power of zero <em>is</em> allowed. (The results are always zero and one, respectively.)</p>
<hr>
<p><a href="ch04.html">Previous</a> <a href="index.html">Index</a> <a href="ch06.html">Next</a></p>

</body>
</html>