How to pass and array and modify it in a function in postgres - postgresql

Question

I am trying to write a function that does this: basically create an array with same data elements as the array that is passed to the function but with some change sometimes - like if some element is even then the flag should change from N to Y. The procedure takes as input an array that has two elements - a number and a flag. eg. (1,'N'), (2,'N'). Now if the passed number is even then the proc should modify that value and change to (2,'Y') whereas the other one remains as (1,'N').
Basically my array basics are not clear and reading through details has not helped so this question.
I tried the following but it is not working...can you please suggest:
CREATE TYPE test_n_t_num AS (
v_n double precision,
is_even character varying(1));
create function temp_n_proc_2(p_nums IN OUT test_n_t_num[])
as
$$
declare
v_nums test_n_t_num[];
v_cnt double precision;
BEGIN
v_cnt := cardinality(p_nums);
v_nums := ARRAY[]::test_n_t_num[];
for i in 1..v_cnt LOOP
if p_nums[i].v_n_double % 2 = 0 then
v_nums [i].is_even := 'Y';
p_nums [i].is_even := 'Y'
else
v_nums [i].is_even := p_nums [i].is_even;
end if;
v_nums[i] := {p_nums[i].v_n,v_nums [i].is_even};
END LOOP;
END;
$$
language plpgsql;
Also later I need to loop through and print out the values in the array v_nums - one that is defined in the function.
Thank you,
Nirav

Answer 1

I had a similar issue when I was trying to do this sort of thing back in the day. Basically you can't assign to composite objects using array notation, i.e. your_array[1].field := value doesn't work. Here's something that does (I don't use cardinality since I'm still on 9.3 and that was added in 9.4):
CREATE TYPE public.test1 AS (a INTEGER, is_even BOOLEAN);
CREATE OR REPLACE FUNCTION f1(ar INOUT public.test1[]) AS $$
DECLARE
t public.test1;
BEGIN
RAISE NOTICE '%', ar;
FOR i IN 1..ARRAY_LENGTH(ar, 1) LOOP
t := ar[i];
t.is_even := t.a % 2 = 0;
ar[i] := t;
END LOOP;
RAISE NOTICE '%', ar;
END
$$ LANGUAGE plpgsql;
So basically create a variable of that type, read the indexed item into the variable, modify the fields, then copy the variable's content back to the array.
SELECT * FROM f1('{"(1,)","(4,)","(6,)"}'::public.test1[]) returns {"(1,f)","(4,t)","(6,t)"}
The messages printed (this is using pgadmin 3) are:
NOTICE: {"(1,)","(4,)","(6,)"}
NOTICE: {"(1,f)","(4,t)","(6,t)"}