I am very new to Matlab, and I feel completely overwhelmed by the use of arrays. What is the most efficient implementation of the following C++ code in Matlab?
A = std::vector<double>();
for (int i = 0; i < 100; i++) {
if (complicatedBoolFunction(i)) {
A.push_back(i);
}
}
Edit: By efficiency I mean to use as little resources as possible to grow the array A - that is, to avoid copy-pasting it into temporary memory
You can do this 2 ways
Pre-allocating for the maximum size, and removing unused elements. This has the advantage of pre-allocating memory in case the condition is often met...
A = NaN(100,1)
for ii = 0:99
if rand > 0.5 % some condition
A(ii+1) = ii; % some value
end
end
A(isnan(A)) = []; % remove unused elements
Appending to the array. This avoids making A way too large if appending is unlikely...
A = []; % empty array
for ii = 0:99
if rand > 0.5 % some condition
A(end+1, 1) = ii; % some value. Equivalent to 'A = [A; ii];'
end
end
A better, and more Matlab-esque way of doing this would be to vectorise your conditional function. This way you avoid looping and allocation issues...
ii = 0:99;
A = ii(rand(100, 1) > 0.5);
You can use any Boolean function you like as an indexing array, as long as it returns a logical array with the same number of elements as the array you're indexing (ii here) or integer indices of the elements to choose.
The most efficient implementation of such C++ code would be
i = 0:99;
A = i(complicatedBoolFunction(i));
Anyway you can grow an array with concatenation, which is (or was) usually not recommended, like the following
A = [];
for i = 0:99
if (complicatedBoolFunction(i))
A = [A i];
end
end
or much more efficiently like this:
A = [];
for i = 0:99
if (complicatedBoolFunction(i))
A(end + 1) = i;
end
end
