1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>10.3. Lists, Lists And More Lists</title>
<link rel="stylesheet" href="gimp-help-plain.css" type="text/css" />
<link rel="stylesheet" href="gimp-help-screen.css" type="text/css" />
<meta name="generator" content="DocBook XSL Stylesheets V1.66.1" />
<link rel="start" href="index.html" title=" " />
<link rel="up" href="ch02s10.html" title="10. A Script-Fu Tutorial" />
<link rel="prev" href="ch02s10s02.html" title="10.2. Variables And Functions" />
<link rel="next" href="ch02s10s04.html" title="10.4. Your First Script-Fu Script" />
</head>
<body>
<div xmlns="" class="navheader">
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center" id="chaptername">10.3. Lists, Lists And More Lists</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="ch02s10s02.html">Prev</a> </td>
<th width="60%" align="center" id="sectionname">10.3. Lists, Lists And More Lists</th>
<td width="20%" align="right"> <a accesskey="n" href="ch02s10s04.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="id3308199"></a>10.3. Lists, Lists And More Lists</h3>
</div>
</div>
</div>
<p>
We've trained you in variables and functions, and now enter the
murky swamps of Scheme's lists.
</p>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3316793"></a>Defining A List</h4>
</div>
</div>
</div>
<p>
Before we talk more about lists, it is necessary that you know
the difference between atomic values and lists.
</p>
<p>
You've already seen atomic values when we initialized
variables in the previous lesson. An atomic value is a single
value. So, for example, we can assign the variable "x" the
single value of 8 in the following statement:
</p>
<pre class="programlisting">
(let* ( (x 8) ) x)
</pre>
<p>
(We added the expression <tt class="varname">x</tt> at the end to print out the value
assigned to <tt class="varname">x</tt>-- normally you won't need to do this. Notice how
<tt class="code">let*</tt> operates just like a function: The value of
the last statement is the value returned.)
</p>
<p>
A variable may also refer to a list of values, rather than a
single value. To assign the variable <tt class="varname">x</tt> the
list of values 1, 3, 5, we'd type:
</p>
<pre class="programlisting">
(let* ( (x '(1 3 5))) x)
</pre>
<p>
Try typing both statements into the Script-Fu Console and
notice how it replies. When you type the first statement in,
it simply replies with the result:
</p>
<pre class="programlisting">
8
</pre>
<p>
However, when you type in the other statement, it replies with
the following result:
</p>
<pre class="programlisting">
(1 3 5)
</pre>
<p>
When it replies with the value 8 it is informing you that
<tt class="varname">x</tt> contains the atomic value 8. However,
when it replies with (1 3 5), it is then informing you that
<tt class="varname">x</tt> contains not a single value, but a list
of values. Notice that there are no commas in our declaration
or assignment of the list, nor in the printed result.
</p>
<p>
The syntax to define a list is:
</p>
<pre class="programlisting">
'(a b c)
</pre>
<p>
where <tt class="varname">a</tt>, <tt class="varname">b</tt>, and
<tt class="varname">c</tt> are literals. We use the apostrophe (')
to indicate that what follows in the parentheses is a list of
literal values, rather than a function or expression.
</p>
<p>
An empty list can be defined as such:
</p>
<pre class="programlisting">
'()
</pre>
<p>
or simply:
</p>
<pre class="programlisting">
()
</pre>
<p>
Lists can contain atomic values, as well as other lists:
</p>
<pre class="programlisting">
(let*
(
(x
'("The Gimp" (1 2 3) ("is" ("great" () ) ) )
)
)
x
)
</pre>
<p>
Notice that after the first apostrophe, you no longer need to
use an apostrophe when defining the inner lists. Go ahead and
copy the statement into the Script-Fu Console and see what it
returns.
</p>
<p>
You should notice that the result returned is not a list of
single, atomic values; rather, it is a list of a literal <tt class="code">("The
Gimp")</tt>, the list <tt class="code">(1 2 3)</tt>, etc.
</p>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3316666"></a>How To Think Of Lists</h4>
</div>
</div>
</div>
<p>
It's useful to think of lists as composed of a "head" and a
"tail." The head is the first element of the list, the tail
the rest of the list. You'll see why this is important when we
discuss how to add to lists and how to access elements in the
list.
</p>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3317060"></a>Creating Lists Through Concatenation (The Cons Function)</h4>
</div>
</div>
</div>
<p>
One of the more common functions you'll encounter is the cons
function. It takes a value and prepends it to its second
argument, a list. From the previous section, I suggested that
you think of a list as being composed of an element (the head)
and the remainder of the list (the tail). This is exactly how
cons functions -- it adds an element to the head of a
list. Thus, you could create a list as follows:
</p>
<pre class="programlisting">
(cons 1 '(2 3 4) )
</pre>
<p>
The result is the list <tt class="code">(1 2 3 4)</tt>.
</p>
<p>
You could also create a list with one element:
</p>
<pre class="programlisting">
(cons 1 () )
</pre>
<p>
You can use previously declared variables in place of any
literals, as you would expect.
</p>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3316697"></a>Defining A List Using The list Function</h4>
</div>
</div>
</div>
<p>
To define a list composed of literals or previously declared
variables, use the list function:
</p>
<pre class="programlisting">
(list 5 4 3 a b c)
</pre>
<p>
This will compose and return a list containing the values held
by the variables <tt class="varname">a</tt>, <tt class="varname">b</tt>
and <tt class="varname">c</tt>. For example:
</p>
<pre class="programlisting">
(let* (
(a 1)
(b 2)
(c 3)
)
(list 5 4 3 a b c)
)
</pre>
<p>
This code creates the list <tt class="code">(5 4 3 1 2 3)</tt>.
</p>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3317463"></a>Accessing Values In A List</h4>
</div>
</div>
</div>
<p>
To access the values in a list, use the functions <tt class="code">car</tt> and <tt class="code">cdr</tt>,
which return the first element of the list and the rest of the
list, respectively. These functions break the list down into
the head::tail construct I mentioned earlier.
</p>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3317501"></a>The <tt class="code">car</tt> Function</h4>
</div>
</div>
</div>
<p>
<tt class="code">car</tt> returns the first element of the list (the
head of the list). The list needs to be non-null. Thus, the
following returns the first element of the list:
</p>
<pre class="programlisting">
(car '("first" 2 "third"))
</pre>
<p>
which is:
</p>
<pre class="programlisting">
"first"
</pre>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3317352"></a>The <tt class="code">cdr</tt> function</h4>
</div>
</div>
</div>
<p>
<tt class="code">cdr</tt> returns the rest of the list after the first
element (the tail of the list). If there is only one element
in the list, it returns an empty list.
</p>
<pre class="programlisting">
(cdr '("first" 2 "third"))
</pre>
<p>
returns:
</p>
<pre class="programlisting">
(2 "third")
</pre>
<p>
whereas the following:
</p>
<pre class="programlisting">
(cdr '("one and only"))
</pre>
<p>
returns:
</p>
<pre class="programlisting">
()
</pre>
</div>
<div class="simplesect" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="id3317554"></a>Accessing Other Elements In A List</h4>
</div>
</div>
</div>
<p>
OK, great, we can get the first element in a list, as well as
the rest of the list, but how do we access the second, third
or other elements of a list? There exist several "convenience"
functions to access, for example, the head of the head of the
tail of a list (<tt class="code">caadr</tt>), the tail of the tail of a
list (<tt class="code">cddr</tt>), etc.
</p>
<p>
The basic naming convention is easy: The a's and d's represent
the heads and tails of lists, so
</p>
<pre class="programlisting">
(car (cdr (car x) ) )
</pre>
<p>
could be written as:
</p>
<pre class="programlisting">
(cadar x)
</pre>
<p>
To view a full list of the list functions, refer to the
Appendix, which lists the available functions for the version
of Scheme used by Script-Fu.
</p>
<p>
To get some practice with list-accessing functions, try typing
in the following (except all on one line if you're using the
console); use different variations of car and cdr to access
the different elements of the list:
</p>
<pre class="programlisting">
(let* (
(x '( (1 2 (3 4 5) 6) 7 8 (9 10) )
)
)
; place your car/cdr code here
)
</pre>
<p>
Try accessing the number 3 in the list using only two function
calls. If you can do that, you're on your way to becoming a
Script-Fu Master!
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25">
<img alt="[Note]" src="../images/note.png" />
</td>
<th align="left">Note</th>
</tr>
<tr>
<td colspan="2" align="left" valign="top">
<p>
In Scheme, a semicolon (";") marks a comment. It, and
anything that follows it on the same line, are ignored by the
script interpreter, so you can use this to add comments to jog
your memory when you look at the script later.
</p>
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="ch02s10s02.html">Prev</a> </td>
<td width="20%" align="center">
<a accesskey="u" href="ch02s10.html">Up</a>
</td>
<td width="40%" align="right"> <a accesskey="n" href="ch02s10s04.html">Next</a></td>
</tr>
<tr>
<td width="40%" align="left" valign="top">10.2. Variables And Functions </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> 10.4. Your First Script-Fu Script</td>
</tr>
</table>
</div>
</body>
</html>
|
