Perl语言入门-第四章-子程序-习题
1. 题目2. 代码
1 #-----------------------------------------------------------#
2 # Source: Learning Perl, chapter4
3 # Date: 2012-01-14
4 # Author: xiaodongrush
5 #-----------------------------------------------------------#
6 # code-1 #
7 #-----------------------------------------------------------#
8 $split = "-------------------------------------------------";
9 $code1 = 'sub sum {' . "\n" .
10 'my $result = 0;' . "\n" .
11 'foreach(@_) {' . "\n" .
12 ' $result += $_;' . "\n" .
13 '}' . "\n" .
14 '$result;' . "\n" .
15 '}' . "\n" .
16 'print &sum(1,3,5,7,9). "\n";';
17 print "code-1\n" . $code1 . "\n" . $split;
18 #-----------------------------------------------------------#
19 # execute-1 #
20 #-----------------------------------------------------------#
21 print "\nexecute-1\n";
22 sub sum {
23 my $result = 0;
24 foreach(@_) {
25 $result += $_;
26 }
27 $result;
28 }
29 print &sum(1,3,5,7,9) . "\n";
30 #-----------------------------------------------------------#
31 # code-2 #
32 #-----------------------------------------------------------#
33 $code2 = 'print &sum(1..1000) + "\n";';
34 print $split . "\ncode-2\n" . $code2 . "\n" . $split;
35 #-----------------------------------------------------------#
36 # execute-2 #
37 #-----------------------------------------------------------#
38 print "\nexecute-2\n";
39 print &sum(1..1000) . "\n";
40 #-----------------------------------------------------------#
41 # code-3 #
42 #-----------------------------------------------------------#
43 $code3 = 'sub average {' . "\n" .
44 'my $result = 0;' . "\n" .
45 'if(@_ == 0) { return 0; }' . "\n" .
46 'foreach(@_) { $result += $_; }' . "\n" .
47 '$result /= @_;' . "\n" .
48 '}' . "\n" .
49 'sub above_average {' . "\n" .
50 'my @array;' . "\n" .
51 'my $aver = &average(@_);' . "\n" .
52 'foreach(@_) { if($_ > $aver) { push(@array, $_); } }'. "\n" .
53 '@array;' . "\n" .
54 "}" ;
55 print $split . "\ncode-3\n" . $code3 . "\n" . $split;
56 #-----------------------------------------------------------#
57 # execute-3 #
58 #-----------------------------------------------------------#
59 print "\nexecute-3\n";
60 sub average {
61 my $result = 0;
62 if(@_ == 0) { return 0; }
63 foreach(@_) { $result += $_; }
64 $result /= @_;
65 }
66 sub above_average {
67 my @array;
68 my $aver = &average(@_);
69 foreach(@_) { if($_ > $aver) { push(@array, $_); } }
70 @array;
71 }
72 print &average(1..10) . "\n";
73 foreach(&above_average(1..10)) { print $_ . "\n";}
74 #-----------------------------------------------------------#
75 # code-4 #
76 #-----------------------------------------------------------#
77 $code4 = 'use feature qw(state);' . "\n" .
78 'sub greet {' . "\n" .
79 'state $last_name = undef;' . "\n" .
80 'print "Hi " . @_ . "! ";' . "\n" .
81 'if($last_name eq undef) { print "You are the first one here!\n"; }' . "\n" .
82 'else { print "I\'ve seen: " . $last_name . "\n";}' . "\n" .
83 '$last_name = @_;' . "\n" .
84 '}' . "\n" .
85 '&greet("Fred");' . "\n" .
86 '&greet("Barney");' ;
87 print $split . "\ncode-4\n" . $code4 . "\n" . $split;
88 #-----------------------------------------------------------#
89 # execute-4 #
90 #-----------------------------------------------------------#
91 print "\nexecute-4\n";
92 use feature qw(state);
93 sub greet {
94 state $last_name;
95 print "Hi " . @_ . "! ";
96 if(!defined($last_name)) { print "You are the first one here!\n"; }
97 else { print "I've seen: " . $last_name . "\n";}
98 $last_name = @_;
99 }
100 &greet("Fred");
101 &greet("Barney");
102 #-----------------------------------------------------------#
103 # code-5 #
104 #-----------------------------------------------------------#
105 $code5 = 'use feature qw(state);' . "\n" .
106 'sub greet_version2 {' . "\n" .
107 'state @last_names;' . "\n" .
108 'print "Hi " . @_ . "! ";' . "\n" .
109 'if(@last_names == 0){ print "You are the first one here!\n"; }'
110 . "\n" .
111 'else {' . "\n" .
112 ' print "I\'ve seen: ";' . "\n" .
113 ' foreach(@last_names) { print " " . $_; }' . "\n" .
114 ' print "\n";' . "\n" .
115 '}' . "\n" .
116 'push(@last_names, @_);' . "\n" .
117 '}' . "\n" .
118 '&greet_version2("Fred");' . "\n" .
119 '&greet_version2("Barney");' . "\n" .
120 '&greet_version2("Wilma");' . "\n" .
121 '&greet_version2("Betty");' ;
122 print $split . "\ncode-5\n" . $code5 . "\n" . $split;
123 #-----------------------------------------------------------#
124 # execute-5 #
125 #-----------------------------------------------------------#
126 print "\nexecute-5\n";
127 use feature qw(state);
128 sub greet_version2 {
129 state @last_names;
130 print "Hi " . @_ . "! ";
131 if(@last_names == 0) { print "You are the first one here!\n"; }
132 else {
133 print "I've seen: ";
134 foreach(@last_names) { print " " . $_; }
135 print "\n";
136 }
137 push(@last_names, @_);
138 }
139 &greet_version2("Fred");
140 &greet_version2("Barney");
141 &greet_version2("Wilma");
142 &greet_version2("Betty");
143 #-----------------------------------------------------------#
144 print "END!!!";
145 <STDIN>
146 #-----------------------------------------------------------#
147 # 总结
148 # 1. 子程序定义用sub关键字，子程序调尽量用&（当与内置函数重名，
149 # 或者函数定义在函数调用之后时，必须用&）
150 # 2. 子程序参数列表：@_，子程序返回值：最后一行，或者return。
151 # 3. state这个是在perl 5.0.10里面提供的功能，要么加入use feature qw(state);
152 # 要么加入use 5.010;
153 # 3. reverse，sort，这些函数与chomp不同，更像返回值，而chomp直接修改了原数组，或者原变量
154 # 4. 字符串判断是否存在用：$last_name eq undef
155 # 5. 单引号字符串内的单引号需要转义
156 # 'else { print "I\'ve seen: " . $last_name . "\n";}' . "\n" .
157 # 6. 私有持久列表变量不能初始化，即state @array = undef; 这是错误的。
158 # 7. 标量在没有被赋值前，是undef状态，undef在数值上下文中，转化为0，
159 # 在字符串上下文，转化为空串，判断一个变量是否是undef状态，用defined($_var)
160 # 8. 数组在没有被赋值前，不是undef状态，仅仅是个空数组，判断其是否为空：@array == 0 3. 输出
4. 文件
/Files/pangxiaodong/LearningPerl/ch4-answer.rar
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