plpgsql function to generate random readable strings

I have written the following function but it's isn't returning anything when I run it. Can somebody help identify the issue?

CREATE OR REPLACE FUNCTION GenerateReadableRandomString (
len INT
) RETURNS varchar AS
$$
DECLARE
validchars VARCHAR;
randomstr VARCHAR;
randint INT;
i INT;

BEGIN开发者_开发技巧

validchars := 'ABCEFHJKLMNPRTWXY3478';
i := 0;

LOOP
    randint := ceil(random() * char_length(validchars));
    randomstr := randomstr || substring(validchars from randint for 1);
    i := i + 1;
    EXIT WHEN i = len;
END LOOP;

RETURN randomstr;
END;
$$
LANGUAGE plpgsql;

faster code can be

CREATE OR REPLACE FUNCTION rstr(int)
RETURNS text AS $$
SELECT array_to_string(ARRAY(SELECT substring('ABCEFHJKLMNPRTWXY3478' FROM (random()*21)::int + 1 FOR 1) 
                                FROM generate_series(1,$1)),
                       '') 
$$ LANGUAGE sql;

Yeah the problem is that you haven't initialized your variable randomstr. And when you concat something with null you get null