for循环——有趣例子 - 数学建模社区-数学中国

% Script file: timings.m
%
% Purpose:
% This program calculates the time required to
% calculate the squares of all integers from 1 to
% 10,000 in three different ways:
% 1. Using a for loop with an uninitialized output
% array.
% 2. Using a for loop with an preallocated output
% array.
% 3. Using vectors.
%
% Record of revisions:
% Date Programmer Description of change
% ==== ========== =====================
% 12/08/97 S. J. Chapman Original code
%
% Define variables:
% ii, jj Loop index
% average1 Average time for calculation 1
% average2 Average time for calculation 2
% average3 Average time for calculation 3
% maxcount Number of times to loop calculation
% square Array of squares
% leap_day Extra day for leap year
% month Month(mm)
% year Year(yyyy)
% Perform calculation with an uninitialized array
% "square". This calculation is done only once
% because it is so slow.
maxcount = 1; % One repetition
tic; % Start timer
for jj = 1:maxcount
clear square % Clear output array
for ii = 1:10000
square(ii) = ii^2; % Calculate square
end
end
average1 = (toc)/maxcount; % Calculate average time
% Perform calculation with a preallocated array
% "square". This calculation is averaged over 10
% loops.
maxcount = 10; % One repetition
tic; % Start timer
for jj = 1:maxcount
clear square % Clear output array
square = zeros(1,10000); % Preinitialize array
for ii = 1:10000
square(ii) = ii^2; % Calculate square
end
end
average2 = (toc)/maxcount; % Calculate average time
% Perform calculation with vectors. This calculation
% averaged over 100 executions.
maxcount = 100; % One repetition
tic; % Start timer
for jj = 1:maxcount
clear square % Clear output array
ii = 1:10000; % Set up vector
square = ii.^2; % Calculate square
end
average3 = (toc)/maxcount; % Calculate average time
% Display results
fprintf('Loop / uninitialized array = %8.4f\n', average1);
fprintf('Loop / initialized array = %8.4f\n', average2);
fprintf('Vectorized = %8.4f\n', average3);

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