Simply because it is slow. In the GP's example if a was a much bigger array, copying it over to b would be expensive. On a related note, in C++ for many contain

Simply because it is slow. In the GP's example if a was a much bigger array, copying it over to b would be expensive. On a related note, in C++ for many containers (if not all) in the standard library, copying is the norm; in "The C++ Programming Language" Bjarne Stroustrup warns that it may be slow.