Creating, Accessing and understanding multidimensional arrays in php - php

Question

I have implemented the following small example:
$nodeList;
for($i = 0; $i < 10;$i++) {
$nodeList[$i] = $i;
for($j = 0; $j < 3;$j++) {
$nodeList[$i][$j] = $j;
}
}
foreach($nodeList[0] as $nodeEl) {
print "NodeEl: ".$nodeEl." | ";
}
print nl2br("\n\r");
$testList = array
(
array(1,2,3),
array(4,5,6),
array(7,8,9),
array(10,11,12),
);
foreach($testList[0] as $testEl) {
print "TestEl: ".$testEl." | ";
}
Where the output for $nodeList is null (var_dump / print_r too) and the output for $testList is TestEl: 1 | TestEl: 2 | TestEl: 3, as expected.
In my understanding those two solutions should create roughly the same output - but instead there is no output for the first one at all. Because the second dimension of the array is not even created.
Reading up on http://php.net/manual/de/language.types.array.php creates the strong feeling that the [] operator is only for dereferencing / accessing of the array, but then again the docs provide a sample where they assign a value to a certain key the same way I do $arr["x"] = 42.
What is the difference between these two ways of array access?
How can I achieve filling a n-dimensional array in a way similar to the way I try to fill $nodeList?

Answer 1

You should make sure to have error reporting turned on, because warnings are generated for your code:
E_WARNING : type 2 -- Cannot use a scalar value as an array -- at line 7
This concerns the following statement:
$nodeList[$i] = $i;
If you want to create a 2D array, there is no meaning in assigning a number on the first level. Instead you want $nodeList[$i] to be an array. PHP does that implicitely (creating the array) when you access it with brackets [...], so you can just leave out the offending statement, and do:
for($i = 0; $i < 10;$i++) {
for($j = 0; $j < 3;$j++) {
$nodeList[$i][$j] = $j;
}
}
You can even leave out the $j in the last bracket pair, which means PHP will just add to the array using the next available numerical index:
for($i = 0; $i < 10;$i++) {
for($j = 0; $j < 3;$j++) {
$nodeList[$i][] = $j;
}
}
Adding a value at every level
If you really need to store $i at the first level of the 2D array, then consider using a more complex structure where each element is an associative array with two keys: one for the value and another for the nested array:
for($i = 0; $i < 10; $i++) {
$nodeList[$i] = array(
"value" => $i,
"children" => array()
);
for($j = 0; $j < 3;$j++) {
$nodeList[$i]["children"][] = array(
"value" => "$i.$j" // just example of value, could be just $j
);
}
}
$nodeList will be like this then:
array (
array (
'value' => 0,
'children' => array (
array ('value' => '0.0'),
array ('value' => '0.1'),
array ('value' => '0.2'),
),
),
array (
'value' => 1,
'children' => array (
array ('value' => '1.0'),
array ('value' => '1.1'),
array ('value' => '1.2'),
),
),
//...etc
);

Answer 2

You should write
<?php
$nodeList;
for($i = 0; $i < 10;$i++) {
for($j = 0; $j < 3;$j++) {
$nodeList[$i][$j] = $j;
}
}
foreach($nodeList[0] as $nodeEl) {
print "NodeEl: ".$nodeEl." | ";
}

Answer 3

You need to declare $nodeList as array like
$nodeList=array();
and for 2D array
$nodeList= array(array());