PSET7: Extracting strings from queries with PHP

Question

Not a bug, but wondering if someone can explain why this works... I'm attempting to display someone's name and cash position.

In index, I query the tables for the relevant values. This returns an array of rows, as I've found out.

I'm then passing an element of the array to another view (portfolio). For example:

render("portfolio.php", ["title" => "Portfolio", "positions" => $positions, "cash" => $cash[0], "name" => $name[0]]);

However, when I go to portfolio.php and attempt to do something like this:

<h2>You have $<?=$cash ?> to spend</h2>

It throws up an invalid 'array to string conversion'.

OK so maybe it's put the cash in an array. Let's try:

<h2>You have $<?=$cash[0] ?> to spend</h2>

Nope. There you get an 'undefined offset' error.

OK, so maybe we need to call the the 'cash' value in the 'cash' array?

<h2>You have $<?=$cash['cash'] ?> to spend</h2>

It works!

The question is why? In C, if we index an array, it returns a value of type [int, string, whatever], not another array. Why isn't that the case in PHP?

Is it because the rows themselves are associative arrays, and so I can never actually extract a single value? If that's the case, how would I just pass one variable out of an associative array?

Answer 1

You have answered yourself:

Is it because the rows themselves are associative arrays, and so I can never actually extract a single value? If that's the case, how would I just pass one variable out of an associative array?

CS50::query() returns a normal integer indexed array of rows and each row is an associative array. So, all things considered, the return value is an integer indexed array of associative arrays.

If you had tried to query for more than one user in the same query, let's say users with IDs >= 1 && <= 2, CS50::query would return something like (simplifying the fields a little):

$results = [["id" => 1, "cash" => 343, "username" => John], ["id" => 2, "cash" => 232, "username" => Mary]] 

So the first user, John, is $results[0], the second user, Mary, is $results[1]

If you query just for the ID 1, the return value would still be an array of rows, although with just one row in it, like this:

$results = [["id" => 1, "cash" => 343, "username" => John]]

So you'd still need to use $results[0] to get the associative array that represents the user John.

When you passed $cash[0] to your view, you thought you were passing something related to cash, but that was just the result of giving a misleading name to the array of associative arrays that I labeled as $results.

If you had used another name for the result value of the query (like $users for example), you'd be less confused.

You've done something like:

$cash = [["id" => 1, "cash" => 343, "username" => John]]

And when you do $cash[0] you're left with:

$cash[0] = ["id" => 1, "cash" => 343, "username" => John]

If you had given a better name like $users to your array, it would be clearer:

$users = [["id" => 1, "cash" => 343, "username" => John]]
$users[0] = ["id" => 1, "cash" => 343, "username" => John]
$users[0]["cash"] = 343

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So to finally answer your question, how do you make sure you pass just one variable to the view? Like this:

$rows = CS50::query(.....)
$user = $rows[0]
$cash = $user["cash"]

or more concisely:

$rows = CS50::query(.....)
$cash = $rows[0]["cash"]

Done. This way $cash would represent the cash value, not an associative array.