Initialize double array with nonzero values (blas)

Question

I have allocated a big double vector, lets say with 100000 element. At some point in my code, I want to set all elements to a constant, nonzero value. How can I do this without using a for loop over all elements? I am also using the blas package, if it helps.

Answer 1

You could use std::fill (#include <algorithm>):

std::fill(v.begin(), v.end(), 1);

This is essentially also only a loop of course..

Answer 2

'fill' is right from what you've said.

Be aware that it's also possible to construct a vector full of a specified value:

std::vector<double> vec(100000, 3.14);

So if "at some point" means "immediately after construction", do this instead. Also, it means you can do this:

std::vector<double>(100000, 3.14).swap(vec);

which might be useful if "at some point" means "immediately after changing the size", and you expect/want the vector to be reallocated ("expect" if you're making it bigger than its prior capacity, "want" if you're making it much smaller and want it trimmed to save memory).