Functions and Methods 函数和方法 agentzh ( 章亦春 )

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1 Functions and Methods

2 Functions and Methods 函数和方法 agentzh ( 章亦春 )

3 Consistent interface, Simple interface, rich interface 一致的接口, 简单的接口, 丰富的接口

4 C# seems to have hundreds of different collection classes, used inconsistently in the.net libraries. -- Ned Batchelder C# 似乎拥有成百上千个不同的集合类, 它们在.NET 类库中的使用是如此的不一致

5 Perl 6's array Simple interface with rich functionalities Perl 6 数组 : 简单的接口, 丰富的功能

6 # use it as an ordinary = 'dog', 'cat', 'mouse', # prints 'dog' say ~@item[1..-1]; # prints 'cat mouse = 'moose'); # append = (@item, 'camel', 'moose'); # just the same

7 # use it as a stack 'elk'; # just another way $top $top $top # ditto

8 # use it as a queue 128, 'hello'; $elem $elem $elem = say "length of the queue:

9 Make simple things easy and hard things possible. -- Larry Wall 让简单的事情很容易办到, 让困难的事情有可能办到

10 Writing a string to a file should be a simple task, however... 向一个文件写入一个字符串本该是一件很简单的任务, 可是

11 # The Java way: import java.io.*; class WriteFile { public static void main(string args[]) { FileOutputStream fostream; PrintStream pstream; try { fostream = new FileOutputStream("somefile.txt"); pstream = new PrintStream( fostream ); pstream.println ("This is written to a file"); pstream.close(); } catch (Exception e) { System.err.println ("Error writing to file " + e); } } }

12 /* The C way: */ #include <stdio.h> #include <stdlib.h> int main() { FILE* fh; if ((fh = fopen("somefile.txt", "w") == NULL) { fprintf(stderr, "Can't open file: %s", strerror(errno)); return 1; } fprintf(fh, "This is written to a file\n"); fclose(fh); return 0; }

13 # The Perl 5 way: open my $fh, "> somefile.txt" or die "Can't open file: $!"; print $fh "This is written to a file\n" or die "Can't print to file: $!"; close $fh;

14 output arguments <=> return values 输出参数 <=> 返回值

15 /* pass results via return values */ int add(int a, int b) { return a + b; }... if (add(1, 2) == 3) { printf("ok"); } printf("%d", add(5, 6));

16 /* pass results via output arguments */ void add(int a, int b, int& c) { c = a + b; }... int temp; add(1, 2, temp); if (temp == 3) { printf("ok"); } add(5, 6, temp); printf("%d", temp);

17 Multiple return values 多重返回值

18 # The Perl 6 way: sub div ($a, $b) { return ($a/$b, $a%$b); }... my ($quo, $rem) = div(5, 2);

19 /* The C way: */ void div (int a, int b, int* ptr_quo, int* ptr_rem) { *ptr_quo = a / b, *ptr_rem = a % b; }... int quo, rem; div(5, 2, &quo, &rem);

20 // The C++ way: void div (int a, int b, int& quo, int& rem) { quo = a / b; rem = a % b; }... int quo, rem; div(5, 2, quo, rem);

21 The power of notation 记法的威力

22 Inventing a language doesn't necessarily mean building the successor to Java; often a thorny problem can be cleared up by a change of notation "The Practice of Programming" 发明一种语言并不一定意味着创建 Java 的继承者 ; 通常一个很棘手的问题可以通过记法上的改变而获得澄清 -- 程序设计实践

23 Case #1: Formated output 案例 #1: 格式化输出

24 // The C++ style: cout << "{ list[" << i << "][" << j << "] = " << elems[i][j] << " }\n";

25 // The C++ style: cout << "{ list[" << i << "][" << j << "] = " << elems[i][j] << " }\n"; This is ugly.

26 // The C style: printf("{ list[%d][%d] = %f }\n", i, j, elems[i][j]);

27 // The C style: printf("{ list[%d][%d] = %f }\n", i, j, elems[i][j]); This is much better.

28 The problem is that many developers choose the solution before defining the problem. It's not the case that any programming language is "one size fits all". -- Ovid 问题就在于许多开发人员在定义问题之前就选定了解决方案并不存在一种编程语言能做到一劳永逸

29 Case #2: A simple-minded CSV file parser in Perl 6 案例 #2: 一个简单的逗号分隔 (CSV) 文件的解析器 ( 使用 Perl 6)

30 my $csv_src = slurp $csv_file; = split /[\n\s]+/, $csv_src; -> $line { = split /\s*,\s*/, $line; # process the fields here... }

31 Job done!

32 In large applications covering problem domains suitable for both Perl and Java, the Java programmer can't hold a candle to me. -- Ovid 对于 Perl 和 Java 都适合的大型应用, Java 程序员根本无法赶上我的编程效率

33 Higher order functions (λ calculus) 高阶函数 (λ 演算 )

34 C# 3.0 ( C# Orcas ) introduces several language extensions that build on C# 2.0 to support the creation and use of higher order, functional style class libraries. The extensions enable construction of compositional APIs that have equal expressive power of query languages in domains such as relational databases and XML. -- C# Version 3.0 Specification

35 Closures A piece of code manipulable in arbitrary contexts

36 my $foo = { say }; my $bar = $foo; $bar.(); # prints 3 $foo.(); # ditto

37 Closures can remember the context in which it was created.

38 sub factory($seed) { return { say $seed }; } my $foo = factory(7); $foo.(); # prints 7 my $bar = factory(100); $bar.(); # prints 100 $foo.(); # still prints 7

39 Higher order functions 0 loops 高阶函数 0 循环

40 Case #1: We want to print out all the elements contained in an array of arrays. 案例 #1: 我们想打印出一个数组的数组里的所有元素

41 // The C# way (using loops): int[][] elems = new int[][] { new int[] {1,2}, new int[] {3,4,5} }; for (int i = 0; i < elems.length; i++) for (int j = 0; j < elems[i].length; j++) Console.WriteLine(elems[i][j]);

42 # The Perl 6 way: = [1,2], [3,4,5]; map { map {.say }, $\_

43 # Anyway, we can still use loops in Perl 6: = [1,2], [3,4,5]; { for \@\$\_ {.say } }

44 Case #2: We want to filter out customers with income higher than $ 案例 #2: 我们想到筛选出所有收入在 5000 美元以上的客户

45 # Traditional way in Perl 6 (using loops): -> $customer { if $customer.income() > 5000 { $customer; } }

46 # As before, but more concise: { if.income > 5000 { $\_; } }

47 # A functional-style solution: = grep {.income > 5000

48 Higher order functions As expressive as SQL 高阶函数拥有和 SQL 一样的表达力

49 Case #1: Customer filtering and sorting 案例 #1: 客户筛选与排序

50 select * from customers where income > 5000 and gender = 'female' order by name

51 = sort { $^a.name cmp $^b.name }, grep {.income > 5000 and.gender eq 'female'

52 Case #2: Boy student statistics 案例 #2: 男生统计信息

53 select class_id, count(*) as count from students where gender = 'male' group by class_id order by count desc

54 my %class; map { %class{.class_id}++ }, grep {.gender eq 'male' = reverse sort { $^a.value <=> $^b.value }, %class.pairs;

55 JIT slide making... 即时幻灯片制做

57 Thank you!

Microsoft Word - 把时间当作朋友（2011第3版）3.0.b.06.doc

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