You didn't, as your code does not even compile.
if(g_mapname) is wrong because g_mapname is an array, you probably meant g_mapname[0](to check if the string is not empty).
switch(g_mapname) switch cannot be used on strings.
case Alien: Alien is not a string, "Alien" is(unless you defined somewhere new Allien[] = "Alien", which you probably did not, and if you did you should have posted this in your answer). Alien, as you used it in your code, is an unknown symbol and will produce a compilation error.
Regardless, the code you posted will not compile because of issues 1 and 2 anyway.
A simple test plugin:
PHP Code:
#include <amxmodx>
public plugin_init()
{
new array[32]
if(array)
{
switch(array)
{
case "aaa":
{
}
case "bbb":
{
}
}
}
}
Can you compile it? You can't because it is not valid.
Please test your code before posting and trying to help others so you don't end up confusing them instead.