update-alternatives not working

[Windywind]

`$ sudo update-alternatives --install /usr/bin/gcc-4.7 50
update-alternatives: --install needs

Use 'update-alternatives --help' for program usage information.
`

Followed the instruction, and tried to update my gcc on my Ubuntu 16.04, but get this error. I know something is missing, but I don;t know how to do this.