Need help using the array_udiff()

[eldan88]

Hey Guys. I am trying to understand how the array_udiff works but  I am having a hard time understanding why the call back function returns a interger? How does that return value preform the camparison?

 

For example in the code below, what does the return 0 , or return 1 or -1 have anything to do with the comparison? Thank you in advance!

function myfunction($a,$b)
{
if ($a===$b)
  {
  return 0;
  }
  return ($a>$b)?1:-1;
}

$a1=array("a"=>"red","b"=>"green","c"=>"blue");
$a2=array("a"=>"blue","b"=>"black","e"=>"blue");

$result=array_udiff($a1,$a2,"myfunction");
print_r($result);

[Barand]

Each pair of values is compared during the sort and passed to your function as $a and $b.

 

Those return values indicate whether $a should sort above (-1) or below (+1) $b, or should be considered equal (0).

 

(It doesn't have to be 1 and -1, any negative and positive numbers will have the same effect.)

[eldan88]

Hey Barand, Thanks for the explination but I still can't seem to figure it out.

 

What do you mean exactly by "Sort Above" also why would it sore above if its a negative number???

 

Also how is it logically possible to check if $a is greater than $b???

[Barand]

"Sort above" means "comes before" in the sorted output list.

 

 

 

Also how is it logically possible to check if $a is greater than $b

 

you could try using "if ($a > $b)"

[Barand]

Perhaps an example will help. I have an array of products and prices and I want them in ascending order of price but those with zero price should be listed at the end.

$data = array (
            array ('prodcode' => 'A123', 'price' => 10),
            array ('prodcode' => 'E123', 'price' => 0),
            array ('prodcode' => 'D123', 'price' => 40),
            array ('prodcode' => 'C123', 'price' => 30),
            array ('prodcode' => 'E124', 'price' => 0),
            array ('prodcode' => 'B123', 'price' => 20)
        );
uasort ($data, 'mysort');

function mysort ($a, $b)
{
    if ($a['price']==0) 
        return 1;  // $a should sort below $b
    elseif ($b['price']==0)
        return -1; // $a should sort above $b
    else
        return ($a['price'] - $b['price']);  // $a sorts above $b if $a < $b
} 

foreach ($data as $p) {
    echo "{$p['prodcode']} {$p['price']}<br>";
}

/*
OUTPUT:

A123 10
B123 20
C123 30
D123 40
E124 0
E123 0
*/

[jazzman1]

 

but those with zero price should be listed at the end.

 

Yes, b/s every number is less than every capital and lower letters -  http://www.asciitable.com/

function myfunction($a,$b)
{
if ($a===$b) {
    
  return 0;
  
  }
  
  return ($a > $b) ? 1 : -1;
}

$a = 'a';

$b = 'B';

var_dump(myfunction($a, $b)); // int(1)

Capital "B" is less than lower 'a'.

 

Hm.......NO....I am wrong

<?php

function myfunction($a,$b)
{
if ($a===$b) {
    
  return 0;
  
  }
  
  return ($a > $b) ? 1 : -1;
}

$a = 1;

$b = 'a';

var_dump(myfunction($a, $b)); // int(1)

But not in javascript:

function MyFunction(a,b) {
  
  if(a === b) return 'equal values';
  
  return (a < b) ? 'a1 is less than a2' : 'a1 is greater than a2';
  
}

var a1 = 'a';

var a2 = 1;

alert(MyFunction(a1,a2))

I made the first test in JS Sorry for that.

[eldan88]

Hey Barand. Thanks for providing the example.  I understand it a little better, but I am stilll a little bit confused in some parts.

 

1) I don't understand how $a > $b if they are both strings and not numbers

2) Why would  a negative number be sorted above $b isn't supposed to be below be since it a negative number?

 

For example in the follwing example, how can $a be greater, less than or equal to $b?? .... That's what confuses me the most

$a = array("Alex","jon");
$b = array("Mike", "jon");
echo "<pre>";

print_r(array_udiff($a,$b,function($a,$b){
    if ($a < $b) {
        fb("I am coming out of the  if (a < b)");
        return -1;


    } elseif ($a > $b) {
        fb("I am coming out of the a > b");
        return 1;

    } else {
        fb("I am coming out of the else");
        return 0;

    };
}));

[Barand]

1. Strings have a sort order, just as numbers do. ABC is less than ACB

 

So if you sort the list

ACB

ABC

 

in your function if $a = 'ACB' and $b = 'ABC'

$a is greater than $b so return +1 ie $a sorts below $b in the output list

ABC ($b)

ACB ($a)

[eldan88]

ahhh okay. I got it now. Thank you  so much for making it more clear to me now