Getting a common value from 3 loops - help on understanding solution

[coding_n00b]

I need some help understanding the solution on how to get common values from 3 arrays.

 

Questions on the bits of the code I don't understand is in red.

 

Here is the code and solution:

 

$array1 = [1, 5, 10, 20, 40, 80];
$array2 = [6, 7, 20, 80, 100];
$array3 = [3, 4, 15, 20, 30, 70, 80, 120];

$values = [];

foreach ([$array1, $array2, $array3] as $ar) { <-- take the 3 arrays and assign it to $ar - I get this
    foreach ($ar as $value) { <-- How do you get from 3 arrays in $ar to just the values from the arrays?
    if (!isset($values[$value])) { <-- If the values array passing in the array values variable $value is not set then it equals 0 / why is this?
        $values[$value] = 0;
    }
    $a = $values[$value]++; <-- Why do you increment this here for what purpose?
    }
}

$commonValues = [];

foreach ($values as $value => $count) { <-- Can you explain what this foreach is doing? Stepping through it?
    if ($count > 2) {
        $commonValues[] = $value;
    }
}

 

print_r($common_values);

[Barand]

Use array_intersect

$array1 = [1, 5, 10, 20, 40, 80];
$array2 = [6, 7, 20, 80, 100];
$array3 = [3, 4, 15, 20, 30, 70, 80, 120];

$common = array_intersect($array1, array_intersect($array2,$array3));

echo "Common values are " . join(', ', $common);    //--> 20, 80

[Barand]

Definition

Test data : that data for which the program works.

 

Your solution loops through the values in the three arrays and counts the occurrence of each value. If the occurrence is >2 it assumes it is a value common common to the three arrays. If however an array contained repeating values that would also appear as "common" in the solution EG in

$array1 = [1, 5, 5, 10, 20, 40, 80];
$array2 = [5, 6, 6, 6, 7, 20, 80, 100];
$array3 = [3, 4, 15, 20, 30, 70, 80, 120];

5 and 6 would also be wrongly included in the $commonValues array

[coding_n00b]

 

Use array_intersect

$array1 = [1, 5, 10, 20, 40, 80];
$array2 = [6, 7, 20, 80, 100];
$array3 = [3, 4, 15, 20, 30, 70, 80, 120];

$common = array_intersect($array1, array_intersect($array2,$array3));

echo "Common values are " . join(', ', $common);    //--> 20, 80

Yes, thanks I am aware you can use that but for learning purposes I am doing it without built-in PHP functions.

 

My main concern was not understanding the inner foreach loop and how it related to the if not isset portion of the code.

[jcbones]

Hope this answers the latest question:

 

<?php
//data used for testing:
$array1 = [1, 5, 10, 20, 40, 80];
$array2 = [6, 7, 20, 80, 100];
$array3 = [3, 4, 15, 20, 30, 70, 80, 120];
 
//declare a variable called values, empty array:
$values = [];
 
//combine arrays, assign them as $ar.
foreach ([$array1, $array2, $array3] as $ar) { <-- take the 3 arrays and assign it to $ar - I get this
//loop through each $ar index, pulling just the $value;
    foreach ($ar as $value) { <-- How do you get from 3 arrays in $ar to just the values from the arrays?
    //if there is not an index in the $values array, that is the same as the $value of the current $ar index.
    if (!isset($values[$value])) { <-- If the values array passing in the array values variable $value is not set then it equals 0 / why is this?
    //then set the index, and the counter to 0, this just declares the index so that it doesn't throw a notice.
        $values[$value] = 0;
    }
    //then we increment that value, so we have a count of the occurences.  If this is the first occ, then we increment from 0 to 1.
    //*Note* no need to assign a variable name to this, it serves no purpose.
    $a = $values[$value]++; <-- Why do you increment this here for what purpose?
    }
}
 
//declare an empty array in variable $commonValues;
$commonValues = [];
 
//loop through the values array we created in the loop above. Remember that we are holding the numbers in index, and the count in value.
foreach ($values as $value => $count) { <-- Can you explain what this foreach is doing? Stepping through it?
//if the count is greater than 2,
    if ($count > 2) {
    //add it to the commonValues array, auto incrementing the index.
        $commonValues[] = $value;
    }
}
 
 //print the array $commonValues to the screen, Note, there is a mis-spelling here.
print_r($common_values);