Tag: arrays

Compile-time array with runtime variable size

The code in this post is bad. Its purpose is only to show what I want to say.

An array has fixed size and compile-time allocation and it’s preferred in some real-time situations. If you want a variable number of elements, you could choose a vector. A vector has runtime allocation on the heap, which cannot always offer the predictability and performance required by some applications.

But what if you need both? Compile-time allocation and a different number of elements each time a function is called with some data from a source. You can use an array and a variable for the number of elements:

// Declare the maximum number of elements and the array
constexpr std::size_t max_size = 10;
std::array<int, max_size> elements{4, 5};

// Each call, you will get the elements array and
// the number of elements sent
std::size_t number_of_elements = 2;

// You should make sure the number of elements is not greater
// than the maximum
if (number_of_elements > max_size) {
    throw std::out_of_range{"too many elements"};
}

auto work = [](const std::array<int, max_size>& elements, const std::size_t number_of_elements) {
    // Then iterate through all the elements passed this time
    for (auto it = elements.begin(); it != elements.begin() + number_of_elements; ++it) {
        // ...
    }
};

work(elements, number_of_elements);

Some problems with this approach:

    • all functions that need this array must include the number of elements in their signature
    • the number of elements could be altered by mistake
    • each time you iterate the array you must specify which is its end

It would great to have a container similar to an array/vector that would prevent the issues above. Maybe there is already an implementation for this but I didn’t find it. So I wrote a few lines which partially implement this container. The idea is to have perfect integration with all STD algorithms, to be able to use it like it was an array.

My container is far from the idea of perfect, it just shows some features. I’m actually wrapping an STD array and using two iterators: one for writing data into the array and one for reading. The usage is pretty simple. Continue reading Compile-time array with runtime variable size

Merge JSON arrays by key in PostgreSQL

UPDATE:

While testing more cases, I’ve come up with a solution that covers my needs better. Instead of taking distinct values, I used a FULL JOIN between the two data sets:

SELECT json_agg(data) FROM (
	SELECT coalesce(new.id, old.id) AS id, coalesce(new.value, old.value) AS value
	FROM jsonb_to_recordset('[{"id": 1, "value": "A"}, {"id": 2, "value": "B"}, {"id": 3, "value": "C"}, {"id": 6, "value": "6"}]') AS old(id int, value text)
	FULL JOIN jsonb_to_recordset('[{"id": 1, "value": "NEW A"}, {"id": 2, "value": "NEW B"}, {"id": 5, "value": "5"}]') AS new(id int, value text)
	ON old.id = new.id
) data

Given a JSON column which holds an array of objects, each object having “id” and “value” properties, I wanted to update the array with a new one, by merging the values by the “id” property.
If same id is present in an object of both arrays, the new one will be used.

Old:

[
    {"id": 2, "value": "B"},
    {"id": 1, "value": "A"}
]

New:

[
    {"id": 1, "value": "X"},
    {"id": 3, "value": "C"},
    {"id": 4, "value": "C"}
]

Result:

[
    {"id":1, "value":"X"},
    {"id":2, "value":"B"},
    {"id":3, "value":"C"},
    {"id":4, "value":"C"}
]

And I wanted to do this in one database roundtrip, not take the data in back end, merge it, and send it back to the database.

First, I transformed the array objects into records and prioritized the new array by giving its values a position; and I put both sets together:

SELECT 1 AS position, * FROM json_to_recordset('[{"id": 2, "value": "B"}, {"id": 1, "value": "A"}]') AS src(id int, value text) -- old
UNION ALL
SELECT 0 AS position, * FROM json_to_recordset('[{"id": 1, "value": "X"}, {"id": 3, "value": "C"}, {"id": 4, "value": "C"}]') AS dst(id int, value text) -- new

Then I extracted only the id and the value, the position being used only to sort data:

SELECT id, value FROM (
	SELECT 1 AS position, * FROM json_to_recordset('[{"id": 2, "value": "B"}, {"id": 1, "value": "A"}]') AS src(id int, value text) -- old
	UNION ALL
	SELECT 0 AS position, * FROM json_to_recordset('[{"id": 1, "value": "X"}, {"id": 3, "value": "C"}, {"id": 4, "value": "C"}]') AS dst(id int, value text) -- new
) AS ordered ORDER BY position

Next step, I took the distinct rows by id:

SELECT DISTINCT ON (id) id, value FROM (
	SELECT id, value FROM (
		SELECT 1 AS position, * FROM json_to_recordset('[{"id": 2, "value": "B"}, {"id": 1, "value": "A"}]') AS src(id int, value text) -- old
		UNION ALL
		SELECT 0 AS position, * FROM json_to_recordset('[{"id": 1, "value": "X"}, {"id": 3, "value": "C"}, {"id": 4, "value": "C"}]') AS dst(id int, value text) -- new
	) AS ordered ORDER BY position
) data

And finally, the result needs to be converted to JSON:

SELECT json_agg(uniqueid) FROM (
	SELECT DISTINCT ON (id) id, value FROM (
		SELECT id, value FROM (
			SELECT 1 AS position, * FROM json_to_recordset('[{"id": 2, "value": "B"}, {"id": 1, "value": "A"}]') AS src(id int, value text) -- old
			UNION ALL
			SELECT 0 AS position, * FROM json_to_recordset('[{"id": 1, "value": "X"}, {"id": 3, "value": "C"}, {"id": 4, "value": "C"}]') AS dst(id int, value text) -- new
		) AS ordered ORDER BY position
	) data
) uniqueid