PHP：从数组中获取值的算法[关闭]

I have a array with 2500 elements..I want simple algorithm to get the following.

If the input is 1 , then it should return 1st 50 elements. If the input is 2 it should return 51 to 100th element. if input is 3 it should return 101 to 150th element.

I am too much confused with this algorithm and my mind is not giving any idea,Some one please help me.

Note: i am coding my own pagination for 2500 url..And i need just algorithm idea..not coding

<?php
    $ar = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30];

    function return_frame($arr, $multiplier, $frame_size=50, $preserve_keys = false) {
        return array_slice($arr, ($multiplier - 1)*$frame_size, $frame_size, $preserve_keys);

    }

    print_r(return_frame($ar, 2, 15, false));
?>

Array
(
    [0] => 16
    [1] => 17
    [2] => 18
    [3] => 19
    [4] => 20
    [5] => 21
    [6] => 22
    [7] => 23
    [8] => 24
    [9] => 25
    [10] => 26
    [11] => 27
    [12] => 28
    [13] => 29
    [14] => 30
)

Take a look at: http://php.net/manual/en/function.array-slice.php

i think you just can add position and range on your sql code like this :

"select * from blabla order by post desc limit $position,$range"

You have indexes from 0 to 2499 right.

you want a $count of 50. Let me show you the thinking process.

so if you get 1 as a $paramter you want $start to be 0 and $end to be 49.

lets see:

1-$parameter is $start right. and 50-$paramter is $end . ok

for 2 you want $start to be 50 and $end to be 99;

the above does not work here. lets play with the $count

$parameter*count; is almost $end.. we have to play for the 0 indexing. so:

$end will be $parameter*$count-1. we can see that this is true for 1,2 and also 3 as $parameters. $start will be ($parameter-1)*$count . just quickly in the head that gives 0,50,100. just what we want.

as @cars10 suggested we do not need the $end. with array_slice($arr,($parameter-1)*$count,$count);