Commit | Line | Data |
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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perltrap - Perl traps for the unwary | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
9fda99eb DC |
7 | The biggest trap of all is forgetting to C<use warnings> or use the B<-w> |
8 | switch; see L<perllexwarn> and L<perlrun>. The second biggest trap is not | |
9 | making your entire program runnable under C<use strict>. The third biggest | |
10 | trap is not reading the list of changes in this version of Perl; see | |
11 | L<perldelta>. | |
a0d0e21e LW |
12 | |
13 | =head2 Awk Traps | |
14 | ||
15 | Accustomed B<awk> users should take special note of the following: | |
16 | ||
17 | =over 4 | |
18 | ||
19 | =item * | |
20 | ||
6014d0cb MS |
21 | A Perl program executes only once, not once for each input line. You can |
22 | do an implicit loop with C<-n> or C<-p>. | |
23 | ||
24 | =item * | |
25 | ||
a0d0e21e LW |
26 | The English module, loaded via |
27 | ||
28 | use English; | |
29 | ||
54310121 | 30 | allows you to refer to special variables (like C<$/>) with names (like |
19799a22 | 31 | $RS), as though they were in B<awk>; see L<perlvar> for details. |
a0d0e21e LW |
32 | |
33 | =item * | |
34 | ||
35 | Semicolons are required after all simple statements in Perl (except | |
36 | at the end of a block). Newline is not a statement delimiter. | |
37 | ||
38 | =item * | |
39 | ||
40 | Curly brackets are required on C<if>s and C<while>s. | |
41 | ||
42 | =item * | |
43 | ||
5db417f7 | 44 | Variables begin with "$", "@" or "%" in Perl. |
a0d0e21e LW |
45 | |
46 | =item * | |
47 | ||
48 | Arrays index from 0. Likewise string positions in substr() and | |
49 | index(). | |
50 | ||
51 | =item * | |
52 | ||
53 | You have to decide whether your array has numeric or string indices. | |
54 | ||
55 | =item * | |
56 | ||
aa689395 | 57 | Hash values do not spring into existence upon mere reference. |
a0d0e21e LW |
58 | |
59 | =item * | |
60 | ||
61 | You have to decide whether you want to use string or numeric | |
62 | comparisons. | |
63 | ||
64 | =item * | |
65 | ||
66 | Reading an input line does not split it for you. You get to split it | |
54310121 | 67 | to an array yourself. And the split() operator has different |
68 | arguments than B<awk>'s. | |
a0d0e21e LW |
69 | |
70 | =item * | |
71 | ||
72 | The current input line is normally in $_, not $0. It generally does | |
73 | not have the newline stripped. ($0 is the name of the program | |
74 | executed.) See L<perlvar>. | |
75 | ||
76 | =item * | |
77 | ||
c47ff5f1 | 78 | $<I<digit>> does not refer to fields--it refers to substrings matched |
8b0a4b75 | 79 | by the last match pattern. |
a0d0e21e LW |
80 | |
81 | =item * | |
82 | ||
83 | The print() statement does not add field and record separators unless | |
8b0a4b75 | 84 | you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using |
a0d0e21e LW |
85 | the English module. |
86 | ||
87 | =item * | |
88 | ||
89 | You must open your files before you print to them. | |
90 | ||
91 | =item * | |
92 | ||
93 | The range operator is "..", not comma. The comma operator works as in | |
94 | C. | |
95 | ||
96 | =item * | |
97 | ||
98 | The match operator is "=~", not "~". ("~" is the one's complement | |
99 | operator, as in C.) | |
100 | ||
101 | =item * | |
102 | ||
103 | The exponentiation operator is "**", not "^". "^" is the XOR | |
104 | operator, as in C. (You know, one could get the feeling that B<awk> is | |
105 | basically incompatible with C.) | |
106 | ||
107 | =item * | |
108 | ||
109 | The concatenation operator is ".", not the null string. (Using the | |
5f05dabc | 110 | null string would render C</pat/ /pat/> unparsable, because the third slash |
111 | would be interpreted as a division operator--the tokenizer is in fact | |
c47ff5f1 | 112 | slightly context sensitive for operators like "/", "?", and ">". |
a0d0e21e LW |
113 | And in fact, "." itself can be the beginning of a number.) |
114 | ||
115 | =item * | |
116 | ||
117 | The C<next>, C<exit>, and C<continue> keywords work differently. | |
118 | ||
119 | =item * | |
120 | ||
121 | ||
122 | The following variables work differently: | |
123 | ||
124 | Awk Perl | |
9fda99eb | 125 | ARGC scalar @ARGV (compare with $#ARGV) |
a0d0e21e LW |
126 | ARGV[0] $0 |
127 | FILENAME $ARGV | |
128 | FNR $. - something | |
129 | FS (whatever you like) | |
130 | NF $#Fld, or some such | |
131 | NR $. | |
132 | OFMT $# | |
133 | OFS $, | |
134 | ORS $\ | |
135 | RLENGTH length($&) | |
136 | RS $/ | |
137 | RSTART length($`) | |
138 | SUBSEP $; | |
139 | ||
140 | =item * | |
141 | ||
142 | You cannot set $RS to a pattern, only a string. | |
143 | ||
144 | =item * | |
145 | ||
146 | When in doubt, run the B<awk> construct through B<a2p> and see what it | |
147 | gives you. | |
148 | ||
149 | =back | |
150 | ||
6ec4bd10 | 151 | =head2 C/C++ Traps |
a0d0e21e | 152 | |
6ec4bd10 | 153 | Cerebral C and C++ programmers should take note of the following: |
a0d0e21e LW |
154 | |
155 | =over 4 | |
156 | ||
157 | =item * | |
158 | ||
159 | Curly brackets are required on C<if>'s and C<while>'s. | |
160 | ||
161 | =item * | |
162 | ||
163 | You must use C<elsif> rather than C<else if>. | |
164 | ||
165 | =item * | |
166 | ||
6ec4bd10 MS |
167 | The C<break> and C<continue> keywords from C become in Perl C<last> |
168 | and C<next>, respectively. Unlike in C, these do I<not> work within a | |
169 | C<do { } while> construct. See L<perlsyn/"Loop Control">. | |
a0d0e21e LW |
170 | |
171 | =item * | |
172 | ||
cabc01fc | 173 | The switch statement is called C<given/when> and only available in |
48238296 | 174 | perl 5.10 or newer. See L<perlsyn/"Switch Statements">. |
a0d0e21e LW |
175 | |
176 | =item * | |
177 | ||
5db417f7 | 178 | Variables begin with "$", "@" or "%" in Perl. |
a0d0e21e LW |
179 | |
180 | =item * | |
181 | ||
6014d0cb MS |
182 | Comments begin with "#", not "/*" or "//". Perl may interpret C/C++ |
183 | comments as division operators, unterminated regular expressions or | |
184 | the defined-or operator. | |
a0d0e21e LW |
185 | |
186 | =item * | |
187 | ||
188 | You can't take the address of anything, although a similar operator | |
5f05dabc | 189 | in Perl is the backslash, which creates a reference. |
a0d0e21e LW |
190 | |
191 | =item * | |
192 | ||
4633a7c4 LW |
193 | C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]> |
194 | ends up in C<$0>. | |
a0d0e21e LW |
195 | |
196 | =item * | |
197 | ||
198 | System calls such as link(), unlink(), rename(), etc. return nonzero for | |
9fda99eb | 199 | success, not 0. (system(), however, returns zero for success.) |
a0d0e21e LW |
200 | |
201 | =item * | |
202 | ||
203 | Signal handlers deal with signal names, not numbers. Use C<kill -l> | |
204 | to find their names on your system. | |
205 | ||
206 | =back | |
207 | ||
208 | =head2 Sed Traps | |
209 | ||
210 | Seasoned B<sed> programmers should take note of the following: | |
211 | ||
212 | =over 4 | |
213 | ||
214 | =item * | |
215 | ||
6014d0cb MS |
216 | A Perl program executes only once, not once for each input line. You can |
217 | do an implicit loop with C<-n> or C<-p>. | |
218 | ||
219 | =item * | |
220 | ||
a0d0e21e LW |
221 | Backreferences in substitutions use "$" rather than "\". |
222 | ||
223 | =item * | |
224 | ||
225 | The pattern matching metacharacters "(", ")", and "|" do not have backslashes | |
226 | in front. | |
227 | ||
228 | =item * | |
229 | ||
230 | The range operator is C<...>, rather than comma. | |
231 | ||
232 | =back | |
233 | ||
234 | =head2 Shell Traps | |
235 | ||
236 | Sharp shell programmers should take note of the following: | |
237 | ||
238 | =over 4 | |
239 | ||
240 | =item * | |
241 | ||
54310121 | 242 | The backtick operator does variable interpolation without regard to |
a0d0e21e LW |
243 | the presence of single quotes in the command. |
244 | ||
245 | =item * | |
246 | ||
54310121 | 247 | The backtick operator does no translation of the return value, unlike B<csh>. |
a0d0e21e LW |
248 | |
249 | =item * | |
250 | ||
251 | Shells (especially B<csh>) do several levels of substitution on each | |
5f05dabc | 252 | command line. Perl does substitution in only certain constructs |
54310121 | 253 | such as double quotes, backticks, angle brackets, and search patterns. |
a0d0e21e LW |
254 | |
255 | =item * | |
256 | ||
257 | Shells interpret scripts a little bit at a time. Perl compiles the | |
258 | entire program before executing it (except for C<BEGIN> blocks, which | |
259 | execute at compile time). | |
260 | ||
261 | =item * | |
262 | ||
263 | The arguments are available via @ARGV, not $1, $2, etc. | |
264 | ||
265 | =item * | |
266 | ||
267 | The environment is not automatically made available as separate scalar | |
268 | variables. | |
269 | ||
8886331d NC |
270 | =item * |
271 | ||
272 | The shell's C<test> uses "=", "!=", "<" etc for string comparisons and "-eq", | |
273 | "-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which | |
274 | uses C<eq>, C<ne>, C<lt> for string comparisons, and C<==>, C<!=> C<< < >> etc | |
275 | for numeric comparisons. | |
276 | ||
a0d0e21e LW |
277 | =back |
278 | ||
279 | =head2 Perl Traps | |
280 | ||
281 | Practicing Perl Programmers should take note of the following: | |
282 | ||
283 | =over 4 | |
284 | ||
285 | =item * | |
286 | ||
287 | Remember that many operations behave differently in a list | |
288 | context than they do in a scalar one. See L<perldata> for details. | |
289 | ||
290 | =item * | |
291 | ||
68dc0745 | 292 | Avoid barewords if you can, especially all lowercase ones. |
54310121 | 293 | You can't tell by just looking at it whether a bareword is |
294 | a function or a string. By using quotes on strings and | |
5f05dabc | 295 | parentheses on function calls, you won't ever get them confused. |
a0d0e21e LW |
296 | |
297 | =item * | |
298 | ||
54310121 | 299 | You cannot discern from mere inspection which builtins |
300 | are unary operators (like chop() and chdir()) | |
a0d0e21e | 301 | and which are list operators (like print() and unlink()). |
9fda99eb DC |
302 | (Unless prototyped, user-defined subroutines can B<only> be list |
303 | operators, never unary ones.) See L<perlop> and L<perlsub>. | |
a0d0e21e LW |
304 | |
305 | =item * | |
306 | ||
748a9306 | 307 | People have a hard time remembering that some functions |
a0d0e21e | 308 | default to $_, or @ARGV, or whatever, but that others which |
54310121 | 309 | you might expect to do not. |
a0d0e21e | 310 | |
6dbacca0 | 311 | =item * |
a0d0e21e | 312 | |
c47ff5f1 | 313 | The <FH> construct is not the name of the filehandle, it is a readline |
5f05dabc | 314 | operation on that handle. The data read is assigned to $_ only if the |
748a9306 LW |
315 | file read is the sole condition in a while loop: |
316 | ||
317 | while (<FH>) { } | |
54310121 | 318 | while (defined($_ = <FH>)) { }.. |
748a9306 LW |
319 | <FH>; # data discarded! |
320 | ||
6dbacca0 | 321 | =item * |
748a9306 | 322 | |
19799a22 | 323 | Remember not to use C<=> when you need C<=~>; |
a0d0e21e LW |
324 | these two constructs are quite different: |
325 | ||
326 | $x = /foo/; | |
327 | $x =~ /foo/; | |
328 | ||
329 | =item * | |
330 | ||
54310121 | 331 | The C<do {}> construct isn't a real loop that you can use |
a0d0e21e LW |
332 | loop control on. |
333 | ||
334 | =item * | |
335 | ||
54310121 | 336 | Use C<my()> for local variables whenever you can get away with |
337 | it (but see L<perlform> for where you can't). | |
338 | Using C<local()> actually gives a local value to a global | |
a0d0e21e LW |
339 | variable, which leaves you open to unforeseen side-effects |
340 | of dynamic scoping. | |
341 | ||
c07a80fd | 342 | =item * |
343 | ||
344 | If you localize an exported variable in a module, its exported value will | |
345 | not change. The local name becomes an alias to a new value but the | |
346 | external name is still an alias for the original. | |
347 | ||
a0d0e21e LW |
348 | =back |
349 | ||
54310121 | 350 | As always, if any of these are ever officially declared as bugs, |
6dbacca0 | 351 | they'll be fixed and removed. |
352 |