How to replace specific values in a vector in C++?

Question

I have a vector with some values (3, 3, 6, 4, 9, 6, 1, 4, 6, 6, 7, 3), and I want to replace each 3 with a 54 or each 6 with a 1, for example and so on.

So I need to

Answer 1

The tool for the job is std::replace:

std::vector vec { 3, 3, 6, /* ... */ };
std::replace(vec.begin(), vec.end(), 3, 54); // replaces in-place

See it in action.