Ergebnisse für

0 - 2 von 2

cui,xingxing
Letzte Aktivitätam 2 Okt. 2024

An efficient way to create struct arrays

Create a struct arrays where each struct has field names "a," "b," and "c," which store different types of data. What efficient methods do you have to assign values from individual variables "a," "b," and "c" to each struct element? Here are five methods I've provided, listed in order of decreasing efficiency. What do you think?

Create an array of 10,000 structures, each containing each of the elements corresponding to the a,b,c variables.

num = 10000;
a = (1:num)';
b = string(a);
c = rand(3,3,num);

Here are the methods;

%% method1

t1 =tic;

s = struct("a",[], ...

"b",[], ...

"c",[]);

s1 = repmat(s,num,1);

for i = 1:num

s1(i).a = a(i);

s1(i).b = b(i);

s1(i).c = c(:,:,i);

end

t1 = toc(t1);

%% method2

t2 =tic;

for i = num:-1:1

s2(i).a = a(i);

s2(i).b = b(i);

s2(i).c = c(:,:,i);

end

t2 = toc(t2);

%% method3

t3 =tic;

for i = 1:num

s3(i).a = a(i);

s3(i).b = b(i);

s3(i).c = c(:,:,i);

end

t3 = toc(t3);

%% method4

t4 =tic;

ct = permute(c,[3,2,1]);

t = table(a,b,ct);

s4 = table2struct(t);

t4 = toc(t4);

%% method5

t5 =tic;

s5 = struct("a",num2cell(a),...

"b",num2cell(b),...

"c",squeeze(mat2cell(c,3,3,ones(num,1))));

t5 = toc(t5);

%% plot

bar([t1,t2,t3,t4,t5])

xtickformat('method %g')

ylabel("time(second)")

yline(mean([t1,t2,t3,t4,t5]))

Vollständigen Beitrag anzeigen

Tips & Tricks

goc3
Letzte Aktivitätam 8 Sep. 2024

+1 for Backwards Loops!

Base case:

Suppose you need to do a computation many times. We are going to assume that this computation cannot be vectorized. The simplest case is to use a for loop:

number_of_elements = 1e6;
test_fcn = @(x) sqrt(x) / x;
tic
for i = 1:number_of_elements
    x(i) = test_fcn(i);
end
t_forward = toc;
disp(t_forward + " seconds")
0.10925 seconds

Preallocation:

This can easily be sped up by preallocating the variable that houses results:

tic
x = zeros(number_of_elements, 1);
for i = 1:number_of_elements
    x(i) = test_fcn(i);
end
t_forward_prealloc = toc;
disp(t_forward_prealloc + " seconds")
0.035106 seconds

In this example, preallocation speeds up the loop by a factor of about three to four (running in R2024a). Comment below if you get dramatically different results.

disp(sprintf("%.1f", t_forward / t_forward_prealloc))
3.1

Run it in reverse:

Is there a way to skip the explicit preallocation and still be fast? Indeed, there is.

clear x
tic
for i = number_of_elements:-1:1
    x(i) = test_fcn(i);
end
t_backward = toc;
disp(t_backward + " seconds")
0.032392 seconds

By running the loop backwards, the preallocation is implicitly performed during the first iteration and the loop runs in about the same time (within statistical noise):

disp(sprintf("%.2f", t_forward_prealloc / t_backward))
1.08

Do you get similar results when running this code? Let us know your thoughts in the comments below.

Beneficial side effect:

Have you ever had to use a for loop to delete elements from a vector? If so, keeping track of index offsets can be tricky, as deleting any element shifts all those that come after. By running the for loop in reverse, you don't need to worry about index offsets while deleting elements.

Vollständigen Beitrag anzeigen

0 - 2 von 2