# For loop vectorisation having structure arrays

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Melanie VT il 11 Mar 2023
Commentato: Melanie VT il 13 Mar 2023
Hi,
I'm still having trouble converting for loops into vectorised form. This time, I have a code snippet which generates two arrays (array2 & array3) using a for loop. Unfortunately, I couldn't manage to get a vectorised solution. I'd be more than happy to get your help.
Mel
array2 = zeros(height(array1),1);
array3 = zeros(height(array1),1);
for i = 1:length(array1)
temp_s1 = array1(i);
idx = [struct_data1(:).cid].' == temp_s1;
temp_s2 = struct_data1(idx);
array2(i) = temp_s2([struct_data1(idx).group2].'== 1).group1;
array3(i) = struct_data2([struct_data2(:).cid].' == temp_s1).gid;
end
##### 2 CommentiMostra NessunoNascondi Nessuno
Walter Roberson il 11 Mar 2023
array2(i) = temp_s2([struct_data1(idx).group2].'== 1).group1;
That assumes that you will get exactly one match from the == 1 in that line, and that the group1 returned is a scalar.
As outside observers, we have no particular reason to expect that you will ever sometimes get 0 matches or sometimes get more than one match.
We can guess that idx might have more than one true entry since you [struct_data1(idx).group2] which is code that is compatible with the possibility that more than one entry in struct_data1 is being referred to.
Melanie VT il 12 Mar 2023
I get what you mean @Walter Roberson! Thank you for drawing my attention to this matter🙏🏼

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### Risposta accettata

Jan il 12 Mar 2023
Modificato: Jan il 12 Mar 2023
What is the prupose of a vectorization here? Remember that vectorized code creates temporary arrays usually and this is expensive. Therefore loops are faster in many cases.
c1 = [struct_data1(:).cid].'; % Do this once only
c2 = [struct_data1(:).group2].';
c3 = [struct_data2(:).cid].';
for i = 1:length(array1)
temp_s1 = array1(i);
idx = (c1 == temp_s1);
temp_s2 = struct_data1(idx);
array2(i) = temp_s2(c2(idx) == 1).group1;
array3(i) = struct_data2(c3 == temp_s1).gid;
end
In my local Matlabr R2018b this reduces the runtime from 38 sec of the original to 0.4 sec.
Further 10% faster:
[~, index] = ismember(array1, c3);
array2 = zeros(size(array1, 1),1);
array3 = zeros(size(array1, 1),1);
for i = 1:length(array1)
temp_s1 = array1(i);
idx = (c1 == temp_s1);
temp_s2 = struct_data1(idx);
array2(i) = temp_s2(c2(idx) == 1).group1;
array3(i) = struct_data2(index(i)).gid;
end
##### 4 CommentiMostra 2 commenti meno recentiNascondi 2 commenti meno recenti
Jan il 13 Mar 2023
My first suggestion offered a speedup of a factor 100. The version 4 with ismembc2 is again 260 times faster. 26'000 is a satisfying acceleration and it is based on simplifications of the code, not a vectorization.
Most of the time in the fastest version is spent for combining the scalar fields of the struct to vectors. This shows, that the chosen representation of the data as structs might by clear and clean, but a bunch of vectors will be more efficient.
Melanie VT il 13 Mar 2023
That's a splendid job @Jan! My hat's off to you👒 I can't literally thank you enough. You're providing not only solutions but also capacity strengthening! This is why you're an L10-MVP, I guess. From now on, I will build my codes with this logic. Thanks a million; it's really exceptional🙏🏻

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