Deleting matrix elements by = [] vs reassigning matrix - matlab

Question

Is there any difference between these two methods for deleting elements in Matlab:
ElementsToDelete = [0 0 1 0 1 0 0 1 1 0]
A = 1:10
A(ElementsToDelete) = []
%Versus
A = 1:10
A = A(~ElementsToDelete)
Are there times when one method is more appropriate than the other? Is there a difference in efficiency? Or are they completely interchangeable?

Answer 1

Try this:
A = rand(1e3, 1);
b = A<0.5;
tic;
for ii = 1:1e5
a = A;
a(b) = [];
end
toc
tic;
for ii = 1:1e5
a = A;
a = a(~b);
end
toc
Results:
Elapsed time is 1.654146 seconds
Elapsed time is 1.126325 seconds
So the difference is a speed factor of 1.5 in favour of re-assigning. This however, is worse:
A = rand(1e4, 1);
stop = 0;
for jj = 1:10
a = A;
start = tic;
for ii = 1:1e5
a(a < rand) = [];
end
stop = stop + toc(start);
end
avg1 = stop/10
stop = 0;
for jj = 1:10
a = A;
start = tic;
for ii = 1:1e5
a = a(a > rand);
end
stop = stop + toc(start);
end
avg2 = stop/10
avg1/avg2
Results:
avg1 = 1.1740235 seconds
avg2 = 0.1850463 seconds
avg1/avg2 = 6.344485136963019
So, the factor's increased to well over 6.
My guess is that deletion (i.e., assigning with []) re-writes the entire array on each and every occurrence of a true in the internal loop through the logical indices. This is hopelessly inefficient, as becomes apparent when testing it like this. Re-assigning on the other hand can determine the size of the new array beforehand and initialize it accordingly; no re-writes needed.
Why the JIT does not compile the one into the other is a mystery to me, because deletion is a far more intuitive notation IMHO. But, as you see, it is inefficient compared to alternatives, and should thus be used sparingly. Never use it inside loops!