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An efficient way to create struct arrays

cui,xingxing on 15 Sep 2024 at 0:59 (Edited on 15 Sep 2024 at 2:23)
Latest activity Reply by Matt J on 2 Oct 2024 at 21:18

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]))

7 Comments

Matt J
Matt J on 2 Oct 2024 at 21:18
Note that it's usually a bad idea to do this. If your data start off as separate arrays, accessing data is usually more efficient if the data remains in this form, though they could be bundled for convenience into a scalar struct,
num = 10000;
a = (1:num)';
b = string(a);
c = rand(3,3,num);
s.a=a;
s.b=b;
s.c=c;
Jan Studnicka
Jan Studnicka on 23 Sep 2024 at 11:34
num = 10000;
a = (1:num)';
b = string(a);
c = rand(3,3,num);
%% method0 (avoiding REPMAT)
t0 = tic;
d = cell(1,num);
s = struct("a",d, "b",d, "c",d);
for ii = 1:num
s(ii).a = a(ii);
s(ii).b = b(ii);
s(ii).c = c(:,:,ii);
end
t0 = toc(t0)
t0 = 0.0484
%% method6 (avoiding REPMAT and CELL)
t6 = tic;
s6 = createArray(num,1,'struct');
for i = 1:num
s6(i).a = a(i);
s6(i).b = b(i);
s6(i).c = c(:,:,i);
end
t6 = toc(t6)
t6 = 0.0446
Stephen23
Stephen23 on 23 Sep 2024 at 12:50
+1 neat approach. Note that it requires >=R2024a.
Stephen23
Stephen23 on 19 Sep 2024 at 14:52
num = 10000;
a = (1:num).';
b = string(a);
c = rand(3,3,num);
%% method0 (avoiding REPMAT)
tic
d = cell(1,num);
s = struct("a",d, "b",d, "c",d);
for ii = 1:num
s(ii).a = a(ii);
s(ii).b = b(ii);
s(ii).c = c(:,:,ii);
end
toc
Elapsed time is 0.049320 seconds.
%% method1
tic
s = struct("a",[], "b",[], "c",[]);
s = repmat(s,num,1);
for ii = 1:num
s(ii).a = a(ii);
s(ii).b = b(ii);
s(ii).c = c(:,:,ii);
end
toc
Elapsed time is 0.058009 seconds.
cui,xingxing
cui,xingxing on 20 Sep 2024 at 1:06
Thanks for the additions.
Christian Schröder
Christian Schröder on 15 Sep 2024 at 9:27 (Edited on 15 Sep 2024 at 9:27)
That's really interesting, thank you - curious that loops should be faster than methods avoiding loops here. You can get more robust timings with the timeit() builtin, BTW.
cui,xingxing
cui,xingxing on 15 Sep 2024 at 1:54
I tested it on my local computer with slightly different results, except for stability, but that's generally the trend.
env: MATLAB-Desktop R2024a, win10
Output of time-consuming scenarios for Method 1 to Method 5:
历时 0.056723 秒。
历时 0.044524 秒。
历时 0.056564 秒。
历时 0.119374 秒。
历时 0.171208 秒。

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