for循环——有趣例子

森之张卫东

例4.5

比较向量算法和循环为了比较循环和向量算法执行若无事所用的时间，用两种方法编程并测试三个运算所花的时间。

1.用for循环计算1到10000的之间每一个整数的平方，而事先不初始化平方数组。

2.用for循环计算1到10000的之间每一个整数的平方，而事先初始化平方数组。

3.用向量算法计算计算1到10000的之间每一个整数的平方。

答案:

这个程序必须用上面提供的三种方示计算出1到10000之间的每一个整数的平方，并测试每一个种算法的时间。测试时间要用到matlab函数tic和toc。tic函数复位内建计时器，而toc函数则从最后一次调用tic以秒开始计时。

因为在许多的计算机中它的时间钟是相当粗略的，所以有必要多运行几次以获得相应的平均数。

下面就是用三种方法编出的matlab

程序。

1. % Script file: timings.m
   
2. %
   
3. % Purpose:
   
4. % This program calculates the time required to
   
5. % calculate the squares of all integers from 1 to
   
6. % 10,000 in three different ways:
   
7. % 1. Using a for loop with an uninitialized output
   
8. % array.
   
9. % 2. Using a for loop with an preallocated output
   
10. % array.
    
11. % 3. Using vectors.
    
12. %
    
13. % Record of revisions:
    
14. % Date Programmer Description of change
    
15. % ==== ========== =====================
    
16. % 12/08/97 S. J. Chapman Original code
    
17. %
    
18. % Define variables:
    
19. % ii, jj Loop index
    
20. % average1 Average time for calculation 1
    
21. % average2 Average time for calculation 2
    
22. % average3 Average time for calculation 3
    
23. % maxcount Number of times to loop calculation
    
24. % square Array of squares
    
25. % leap_day Extra day for leap year
    
26. % month Month(mm)
    
27. % year Year(yyyy)
    
28. % Perform calculation with an uninitialized array
    
29. % "square". This calculation is done only once
    
30. % because it is so slow.
    
31. maxcount = 1; % One repetition
    
32. tic; % Start timer
    
33. for jj = 1:maxcount
    
34.     clear square % Clear output array
    
35.     for ii = 1:10000
    
36.         square(ii) = ii^2; % Calculate square
    
37.     end
    
38. end
    
39. average1 = (toc)/maxcount; % Calculate average time
    
40. % Perform calculation with a preallocated array
    
41. % "square". This calculation is averaged over 10
    
42. % loops.
    
43. maxcount = 10; % One repetition
    
44. tic; % Start timer
    
45. for jj = 1:maxcount
    
46.     clear square % Clear output array
    
47.     square = zeros(1,10000); % Preinitialize array
    
48.     for ii = 1:10000
    
49.         square(ii) = ii^2; % Calculate square
    
50.     end
    
51. end
    
52. average2 = (toc)/maxcount; % Calculate average time
    
53. % Perform calculation with vectors. This calculation
    
54. % averaged over 100 executions.
    
55. maxcount = 100; % One repetition
    
56. tic; % Start timer
    
57. for jj = 1:maxcount
    
58.     clear square % Clear output array
    
59.     ii = 1:10000; % Set up vector
    
60.     square = ii.^2; % Calculate square
    
61. end
    
62. average3 = (toc)/maxcount; % Calculate average time
    
63. % Display results
    
64. fprintf('Loop / uninitialized array = %8.4f\n', average1);
    
65. fprintf('Loop / initialized array = %8.4f\n', average2);
    
66. fprintf('Vectorized = %8.4f\n', average3);

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森之张卫东

代码格式优美，大家共同学习吧！！！

peter凯

不错有哦有哟与哦有哦

peter凯

这个真不错得得