:set aaa {"jan";"feb";"apr"};

:global aaa {"jan";"feb";"mar";"apr"};

:global a {b=1}
/environment print
:set ($a->"b")
/environment print
:set $a
/environment print

:global a {"val1";"val2";"val3"} <- I realize there are no keys identified here, but RouterOS sets those values keys as numbered starting at 0.  Sometimes this is easier for certain things I do.
/environment print
:set ($a->0) <- This erases the keys value but not the key, whereas your method deletes it by the key, which is exactly what I needed last night, so again thank you ;)
/environment print
:set $a
/environment print

:local delete "mar"
:local array [:toarray ""]
:local aaa ([:toarray "jan,feb,mar,apr"])
:foreach i in=$aaa do={
	:if ( $i!=$delete ) do={ 
		:set array ( $array, $i )
		}
	}
:set aaa $array
:put $aaa

:local array1String (";".[:tostr $array1].";")
:foreach arrayElement in=[$array2] do={
:if ($arrayString~";+$arrayElement;+") do={ action to be performed on matchers here }\
else={ action to be performed on non-matchers here }}

{
:local test {"a"=1;"b"=2;"c"=3}
:put $test
:set ($test->"b")
:put $test
}

{
:local test   {"bb";"dddd";"a";"ccc"}
:put $test
:local arrlen [:len $test]
:local arrpos [:find $test "dddd" -1]
:set test ([:pick $test 0 $arrpos],[:pick $test ($arrpos + 1) $arrlen])
:put $test
}

{

:local test   {"bb";"dddd";"a";"ccc";"abcd";"befgh"}
:local search "c" ; # can be a simple string or a RegEx

:put $test
:local arrlen [:len $test]
:local arrpos

:foreach item in=$test do={
    :if ($item ~ $search) do={
        :set arrpos [:find $test $item -1]
        :set test ([:pick $test 0 $arrpos],[:pick $test ($arrpos + 1) $arrlen])
    }
}

:if ([:typeof $arrpos] = "nothing") do={ :put "Nothing replaced, $search not found" }

:put $test
}

:local intf [:toarray ""]
:foreach i in=[/intferface/find] do={
:set intf ($intf,[/intferface/get $i name])}

:local list [:toarray ""]
:foreach l in=[/intferface/list/member/find where list=list1] do={
:set list ($list,[/intferface/list/member/get $l intferface])}

:local listStr (";".[:tostr $list].";")

:foreach i in=[$intf] do={
:if ($listStr~";+$i;+") do={}\
else={/intferface/list/member/add list=list1 intferface=$i disabled=yes}}

:local array1String ";$[:tostr $array1];"

:foreach arrayElement in=$array2 do={
    :if ($arrayString~";+$arrayElement;+") do={
# action to be performed on matchers here
    } else={
# action to be performed on non-matchers here
    }
}

/interface
:foreach i in=[find] do={
    :local iname [get $i name]
    :if ([:len [list member find where list=list1 and interface=$iname]] = 0) do={
        list member add list=list1 interface=$iname disabled=yes
    }
}