How to apply a function to each element of an array column in Postgres?

Question

A Pg query returns an array. I would like to retrieve that with each element formatted to 3 decimal places. How can I apply a function to each element of an array? Something

Answer 1

You may need to create a stored function. Here is the one that does what you need:

CREATE OR REPLACE FUNCTION array_round(float[], int)
RETURNS float[]
AS
$$
DECLARE
   arrFloats ALIAS FOR $1;
   roundParam ALIAS FOR $2;
   retVal float[];
BEGIN
   FOR I IN array_lower(arrFloats, 1)..array_upper(arrFloats, 1) LOOP
    retVal[I] := round(CAST(arrFloats[I] as numeric), roundParam);
   END LOOP;
RETURN retVal;
END;
$$
LANGUAGE plpgsql 
   STABLE 
RETURNS NULL ON NULL INPUT;

Then call something like this:

# SELECT array_round(ARRAY[1.53224,0.23411234], 2);
 array_round 
-------------
 {1.53,0.23}
(1 row)

Answer 2

You need to turn the array into a row set. For example, using generate_series:

SELECT ARRAY(SELECT ROUND(ARRAY[1.53224,0.23411234])[i], 2) FROM generate_series(1,2) AS s(i));    

I know that's pretty ugly. There should be a helper function to make such mappings easier.

Perhaps something like (yes it's horrible, slow, and brittle dynamic code):

CREATE OR REPLACE FUNCTION map_with_arg(TEXT, ANYARRAY, TEXT)
RETURNS ANYARRAY
IMMUTABLE STRICT
LANGUAGE 'plpgsql' AS
$$
DECLARE
    i INTEGER;
    t TEXT;
    cmd TEXT;
BEGIN
    FOR i IN array_lower($2, 1) .. array_upper($2, 1) LOOP
        cmd := 'SELECT ('||quote_ident($1)||'('||quote_nullable($2[i])||', '||quote_nullable($3)||'))::TEXT';
        EXECUTE cmd INTO t;
        $2[i] := t;
    END LOOP;
    RETURN $2;
END;
$$;

select map_with_arg('repeat', array['can','to']::TEXT[], '2');
 map_with_arg
---------------
 {cancan,toto}

Update It occurs to me that we could use a single dynamic statement for the whole loop. This could mitigate some of the performance concerns.

CREATE OR REPLACE FUNCTION map_with_arg(TEXT, ANYARRAY, TEXT)
RETURNS ANYARRAY
IMMUTABLE STRICT
LANGUAGE 'plpgsql' AS
$$
DECLARE
    cmd TEXT;
    rv TEXT;
BEGIN
    cmd := 'SELECT ARRAY(SELECT (' || quote_ident($1)||'($1[i], '||quote_nullable($3)||'))::TEXT FROM generate_subscripts($1, 1) AS gs(i))';
    EXECUTE cmd USING $2 INTO rv;
    RETURN rv;
END;
$$;

Answer 3

First, turn the array into a set using unnest:

> SELECT n FROM unnest(ARRAY[1.53224,0.23411234]) AS n;
     n      
------------
    1.53224
 0.23411234
(2 rows)

Then, apply an expression to the column:

> SELECT ROUND(n, 2) FROM unnest(ARRAY[1.53224,0.23411234]) AS n;
 round 
-------
  1.53
  0.23
(2 rows)

Finally, use array_agg to turn the set back into an array:

> SELECT array_agg(ROUND(n, 2)) FROM unnest(ARRAY[1.53224,0.23411234]) AS n;
  array_agg  
-------------
 {1.53,0.23}
(1 row)

Answer 4

postgres=# select array(select round(unnest(array[1.2,2.4,3,4])));
   array   
-----------
{1,2,3,4}
(1 row)