matlab高性能代码编写指南-重循环效率

循环！ 循环！
基本上matlab是不推荐大家使用大量循环的，但是如果万一真的遇到一些大规模的循环，代码的写法会有什么要求吗？
猜猜看，快的循环是慢的循环的几倍呢？
a) 10倍  b) 5倍  c) 2.5倍 d) 1.5倍

%%2重循环的效率
t1 = 0;
t2 = 0;
p = magic(1000);
v = zeros(1000);
for num = 1 : 100
tic;
for n = 1 : size(p,1) %低效率的循环
    for m = 1 : size(p,2)
        v(n,m) = p(n,m);
    end
end
t1 = toc + t1;
tic;
for m = 1 : size(p,2) %高效率的循环
    for n = 1 : size(p,1)
        v(n,m) = p(n,m);
    end
end
t2 = toc + t2;
end
ratio = t1/t2;
msg = sprintf('Loop 2 is %2.1fx faster than loop 1',ratio);
disp(msg);

%%3重循环的效率
t1 = 0;
t2 = 0;
p = randi(1000,[100,100,100]);
v = zeros(100,100,100);
for num = 1 : 100
tic;
for n = 1 : size(p,1) %低效率的循环
    for m = 1 : size(p,2)
        for l = 1 : size(p,3)
            v(n,m,l) = p(n,m,l);
        end
    end
end
t1 = toc + t1;
tic;
for m = 1 : size(p,3) %高效率的循环
    for n = 1 : size(p,2)
        for l = 1 : size(p,1)
            v(n,m) = p(n,m);
        end
    end
end
t2 = toc + t2;
end
ratio = t1/t2;
msg = sprintf('3DLoop 2 is %2.1fx faster than 3Dloop 1',ratio);
disp(msg);


答案：二重循环loop2 大约是loop1的2.5倍， 三重循环loop2大约是loop1的5倍。