Replacing values of specific entries in an Array In Julia

Question

I am wondering about how to implement efficiently a code that replaces some entries of an array with specific values. In other words, I have a matrix and a vector with some indices of the matrix where a logical condition holds. I want to change these entries for some values that I have in a vector.

So far, I've doing it with a loop, but perhaps there is a better strategy that uses filtering or something like that.

A small example would be:

A = collect(1:8); println(A)
   B = [10,20]
   C = A.<=2
   k = 1
   for t=1:8
                if C[t] ==  1
                    A[t] = B[k]
                    k = k+1
            else
       end
    end

However, I need to do this in inside a quite intensive loop, with a bigger matrix. The indices I have to change are always the same, but the vector of values (the counterpart of B) changes in each iteration.

Thanks so much!

Answer 1

You can simply write:

A = collect(1:8)
B = [10,20]
C = findall(A.<=2)
A[C] = B

The use of findall function could be also rewritten as C = findall(x -> x <= 2, A) to reduce the number of allocations. The key is that it is better that C holds actual indices than logical indices. Technically A[C] = B would also accept logical indices in C but then it is slower. And assuming that C is computed once while A[C] = B is computed many times it is better to precompute the indices you want to use.

If C changes with every iteration then it might be better to write C .= A .<= 2 and then A[C] = B (note that I use in-pace update of C here as when it is updated many times it is faster). You would have to benchmark what is better in your specific case.