The expression A(A > 5) is equivalent to A(find(A > 5)). Logical indexing is closely related to the find function. Or you could replace all the spaces in a string matrix str with underscores. To replace all NaN elements of the matrix B with zero, use B(isnan(B)) = 0 For example, you could replace all the NaN elements in an array with another value by using a combination of isnan, logical indexing, and scalar expansion. Many MATLAB functions that start with is return logical arrays and are very useful for logical indexing. For example, A(A > 12) extracts all the elements of A that are greater than 12. The output is always in the form of a column vector. MATLAB extracts the matrix elements corresponding to the nonzero values of the logical array. In logical indexing, you use a single, logical array for the matrix subscript. This form of indexed assignment is called scalar expansion.Īnother indexing variation, logical indexing, has proven to be both useful and expressive. You can always, however, use a scalar on the right side: v() = 30 % Replace second and third elements by 30 Usually the number of elements on the right must be the same as the number of elements referred to by the indexing expression on the left. V(end:-1:1) % Reverse the order of elementsīy using an indexing expression on the left side of the equal sign, you can replace certain elements of the vector: v() = % Replace some elements of v You can even do arithmetic using end: v(2:end-1) % Extract the second through the next-to-last elementsĬombine the colon operator and end to achieve a variety of effects, such as extracting every k-th element or flipping the entire vector: v(1:2:end) % Extract all the odd elements The end operator can be used in a range: v(5:end) % Extract the fifth through the last elements The special end operator is an easy shorthand way to refer to the last element of v: v(end) % Extract the last element Swap the two halves of v to make a new vector: v2 = v() % Extract and swap the halves of v The colon notation in MATLAB provides an easy way to extract a range of elements from v: v(3:7) % Extract the third through the seventh elements Or the subscript can itself be another vector: v() % Extract the first, fifth, and sixth elements The subscript can be a single value: v(3) % Extract the third element Let's start with the simple case of a vector and a single subscript. One easy improvement is to broadcast the first line in your loop to avoid allocating a matrix for (sparseR + reshape(q' * sparseS, 199, 199)) and then another one for 0.5 * 0.05 * (sparseR + reshape(q' * sparseS, 199, 199)): tmp = 0.5. Modifying your code to pre-allocate those matrices may help a lot. In particular, you are constructing new matrices to hold a lot of intermediate quantities. You are seeing a lot of allocations because your code really does allocate a lot of memory. Running your code in a function, I see 3.699408 seconds (41.60 k allocations: 3.787 GiB, 5.39% gc time) which is already quite close to what you reported MATLAB as giving. Instead, put the code you’re timing in a function.
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