example in the book does not work

[AlexanderKoshkarov]

Hello,
I started to read your book - it is nice! Thanks for it.
I stuck with gdb, where your examples on page 45 (GDB Variables) do not work for me. maybe I have too new version? (I downgraded gcc, but gdb my distro does not have packaged gdb lower then 7)
In short I cannot use "*x@3" for an array, here is a self explanatory log

[nix-shell:~/Downloads/test]$ gcc --version
gcc (GCC) 5.4.0
[nix-shell:~/Downloads/test]$ gdb --version
GNU gdb (GDB) 7.12.1
[nix-shell:~/Downloads/test]$ cat test.c
int main() {
    int x[20] = {};
    x[0] = 3;
}
[nix-shell:~/Downloads/test]$ gcc -O0 -g -Wall -std=gnu11 test.c
warning about not using x
[nix-shell:~/Downloads/test]$ gdb a.out 
(gdb) b 4
Breakpoint 1 at 0x400530: file test.c, line 4.
(gdb) r
Starting program: /home/kosh/Downloads/test/a.out 

Breakpoint 1, main () at test.c:4
4	}
(gdb) set $ptr=&x[3]
Attempt to take address of value not located in memory.
(gdb) p x
$1 = {3, <optimized out> <repeats 19 times>}
(gdb) p *x
Attempt to take address of value not located in memory.
(gdb) p *x@1
Attempt to take address of value not located in memory.
(gdb) p x[1]
$2 = <optimized out>
(gdb) p x[0]
$3 = 3

Is the problem with new gdb?