I had problem about the gcc version

[Chen tom <tomcp85 at hotmail dot com>]

hello ,

      20 days ago, I had tried to install the gcc-master.zip packet, but failed, so I deleted the file I have installed, carelessly . two days ago I have the gcc-7 installed, there is a problem as followed:

tom@tom-Inspiron-7447:~$ gcc -o test test.c
gcc: error trying to exec 'cc1': execvp: No such file or directory
tom@tom-Inspiron-7447:~$ gcc-7 -o test test.c
tom@tom-Inspiron-7447:~$ ./test
Hello world!
so I want to completely delete the documents installed, "version 8.0.0", and set gcc to version 7, I have done some orders as follow

tom@tom-Inspiron-7447:/usr/local/bin$ sudo update-alternatives --remove-all gcc
tom@tom-Inspiron-7447:/usr/local/bin$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 70
update-alternatives: using /usr/bin/gcc-7 to provide /usr/bin/gcc (gcc) in auto mode
tom@tom-Inspiron-7447:/usr/local/bin$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
Target: x86_64-pc-linux-gnu
Configured with: ./configure
Thread model: posix
gcc version 8.0.0 20170912 (experimental) (GCC)
tom@tom-Inspiron-7447:/usr/local/bin$ sudo update-alternatives --config gcc
There is only one alternative in link group gcc (providing /usr/bin/gcc): /usr/bin/gcc-7
Nothing to configure.
tom@tom-Inspiron-7447:/usr/local/bin$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
Target: x86_64-pc-linux-gnu
Configured with: ./configure
Thread model: posix
gcc version 8.0.0 20170912 (experimental) (GCC)
The orders did not work, I don't know how to do it ,would you give me any advice.