这种观察并不重要,因为浪费在循环语句上的时间性能可能远高于循环本身.但无论如何,我会分享它,因为我搜索并找不到关于此的话题.我总是有这样的印象：预先分配我将循环的数组,然后循环它,将比直接循环它更好,并决定检查它.代码将比较这两个fors之间的效率：

disp('Pure for with column on statement:')

tic

for k=1:N

end

toc

disp('Pure for with column declared before statement:')

tic

m=1:N;

for k=m

end

toc

但我得到的结果是：

Pure for with column on statement:

Elapsed time is 0.003309 seconds.

Pure for with column declared before statement:

Elapsed time is 0.208744 seconds.

为什么会这样？不应该预先分配更快？

事实上,matlab帮助说：

Long loops are more memory efficient when the colon expression appears

in the FOR statement since the index vector is never created.

因此,与我的期望相反,for语句中的列表达式更好,因为它不分配向量,因此更快.

我做了以下脚本来测试其他场合我也认为会更快：

% For comparison:

N=1000000;

disp('Pure for loop on cell declared on statement:')

tic

for k=repmat({1},1,N)

end

toc

disp('Pure for loop on cell declared before statement:')

tic

mcell=repmat({1},1,N);

for k=mcell

end

toc

disp('Pure for loop calculating length on statement:')

tic

for k=1:length(mcell)

end

toc

disp('Pure for loop calculating length before statement:')

tic

lMcell = length(mcell);

for k=1:lMcell

end

toc

disp('Pure while loop using le:')

% While comparison:

tic

k=1;

while (k<=N)

k=k+1;

end

toc

disp('Pure while loop using lt+1:')

% While comparison:

tic

k=1;

while (k

k=k+1;

end

toc

disp('Pure while loop using lt+1 pre allocated:')

tic

k=1;

myComp = N+1;

while (k

k=k+1;

end

toc

时间是：

Pure for loop on cell declared on statement:

Elapsed time is 0.259250 seconds.

Pure for loop on cell declared before statement:

Elapsed time is 0.260368 seconds.

Pure for loop calculating length on statement:

Elapsed time is 0.012132 seconds.

Pure for loop calculating length before statement:

Elapsed time is 0.003027 seconds.

Pure while loop using le:

Elapsed time is 0.005679 seconds.

Pure while loop using lt+1:

Elapsed time is 0.006433 seconds.

Pure while loop using lt+1 pre allocated:

Elapsed time is 0.005664 seconds.

结论：

>只需通过逗号语句循环就可以获得一些性能,但与for循环所花费的时间相比,这可以忽略不计.

>对于细胞,差异似乎可以忽略不计.

>最好在进行循环之前预先分配长度.

>虽然没有预先分配向量,但是while具有与for相同的效率,这如前所述是有意义的

>正如预期的那样,最好在while语句之前计算固定表达式.

但我无法回答的问题是,细胞怎么样,为什么没有时间差异？开销可能远低于观察到的开销？或者它必须分配单元格,因为它不是一个基本类型的双重？

如果您知道有关此主题的其他技巧,请填写.

只需添加时间来显示转动功能的结果(‘accel’,’off’),如@Magla的回答所述.

Pure for with column on statement:

Elapsed time is 0.181592 seconds.

Pure for with column declared before statement:

Elapsed time is 0.180011 seconds.

Pure for loop on cell declared on statement:

Elapsed time is 0.242995 seconds.

Pure for loop on cell declared before statement:

Elapsed time is 0.228705 seconds.

Pure for loop calculating length on statement:

Elapsed time is 0.178931 seconds.

Pure for loop calculating length before statement:

Elapsed time is 0.178486 seconds.

Pure while loop using le:

Elapsed time is 1.138081 seconds.

Pure while loop using lt+1:

Elapsed time is 1.241420 seconds.

Pure while loop using lt+1 pre allocated:

Elapsed time is 1.162546 seconds.

结果现在按预期区域……