Question

I would like to know what the most efficient c++ implementation of the following matlab idiom is.

Suppose I have 3 vectors in matlab, x, y and idx.

x = [13,2,5.5,22,107]
y = [-3,100,200]
idx = [1,2,5]

I want to replace positions 1,2 and 5 of x with the contents of y. In matlab I do

x[idx] = y

What is the best way to do this in c++?

Answer 1

The Armadillo library probably comes closest as one of its goals is to make things easy for folks used to Matlab.

Here is a short example (and uvec is a typdef for vector of unsigned ints)

// set four specific elements of X to 1
uvec indices;
indices << 2 << 3 << 6 << 8;

X.elem(indices) = ones<vec>(4);

Obviously, the right-hand side could be any other vector of the same dimension as the index.

But there are few language-imposed constraints you cannot overcome:

• zero-based indexing at the C++ level (which you could alter, but few C / C++ programmers will consider it a good idea)
• certain operators, including [