PHP Program for Median of two sorted arrays of same size
Last Updated :
24 Oct, 2023
Write a PHP program for a given 2 sorted arrays A and B of size n each. the task is to find the median of the array obtained by merging the above 2 arrays(i.e. array of length 2n). The complexity should be O(log(n)).
Examples:
Input: ar1[] = {1, 12, 15, 26, 38}
ar2[] = {2, 13, 17, 30, 45}
Output: 16
Explanation:
After merging two arrays, we get {1, 2, 12, 13, 15, 17, 26, 30, 38, 45}
The middle two elements are 15 and 17
The average of middle elements is (15 + 17)/2 which is equal to 16
Note : Since size of the set for which we are looking for median is even (2n), we need take average of middle two numbers and return floor of the average.
PHP Program for Median of two sorted arrays of same size using Simply count while Merging:
Use merge procedure of merge sort. Keep track of count while comparing elements of two arrays. If count becomes n(For 2n elements), we have reached the median. Take the average of the elements at indexes n-1 and n in the merged array.
Below is the implementation of the above approach:
PHP
<?php
function getMedian( $ar1 , $ar2 , $n )
{
$i = 0;
$j = 0;
$count ;
$m1 = -1; $m2 = -1;
for ( $count = 0; $count <= $n ; $count ++)
{
if ( $i == $n )
{
$m1 = $m2 ;
$m2 = $ar2 [0];
break ;
}
else if ( $j == $n )
{
$m1 = $m2 ;
$m2 = $ar1 [0];
break ;
}
if ( $ar1 [ $i ] < $ar2 [ $j ])
{
$m1 = $m2 ;
$m2 = $ar1 [ $i ];
$i ++;
}
else
{
$m1 = $m2 ;
$m2 = $ar2 [ $j ];
$j ++;
}
}
return ( $m1 + $m2 ) / 2;
}
$ar1 = array (1, 12, 15, 26, 38);
$ar2 = array (2, 13, 17, 30, 45);
$n1 = sizeof( $ar1 );
$n2 = sizeof( $ar2 );
if ( $n1 == $n2 )
echo ( "Median is " .
getMedian( $ar1 , $ar2 , $n1 ));
else
echo ( "Doesn't work for arrays" .
"of unequal size" );
?>
|
Time Complexity: O(n)
Auxiliary Space: O(1)
PHP Program for Median of two sorted arrays of same size (By comparing the medians of two arrays):
Step-by-step approach:
- Merge the two input arrays ar1[] and ar2[].
- Sort ar1[] and ar2[] respectively.
- The median will be the last element of ar1[] + the first
- element of ar2[] divided by 2. [(ar1[n-1] + ar2[0])/2].
Below is the implementation of the above approach:
PHP
<?php
function getMedian( $ar1 , $ar2 , $n ) {
$j = 0;
$i = $n - 1;
while ( $ar1 [ $i ] > $ar2 [ $j ] && $j < $n && $i > -1) {
$temp = $ar1 [ $i ];
$ar1 [ $i ] = $ar2 [ $j ];
$ar2 [ $j ] = $temp ;
$j ++;
$i --;
}
sort( $ar1 );
sort( $ar2 );
return ( $ar1 [ $n - 1] + $ar2 [0]) / 2;
}
$ar1 = array (1, 12, 15, 26, 38);
$ar2 = array (2, 13, 17, 30, 45);
$n1 = count ( $ar1 );
$n2 = count ( $ar2 );
if ( $n1 == $n2 ) {
echo "Median is " . getMedian( $ar1 , $ar2 , $n1 );
} else {
echo "Doesn't work for arrays of unequal size" ;
}
?>
|
Time Complexity: O(nlogn)
Auxiliary Space: O(1)
PHP Program for Median of two sorted arrays of same size using Binary Search:
Step-by-step approach:
We can find the kth element by using binary search on whole range of constraints of elements.
- Initialize ans = 0.0
- Initialize low = -10^9, high = 10^9 and pos = n
- Run a loop while(low <= high):
- Calculate mid = (low + (high – low)>>1)
- Find total elements less or equal to mid in the given arrays
- If the count is less or equal to pos
- Update low = mid + 1
- Else high = mid – 1
- Store low in ans, i.e., ans = low.
- Again follow step3 with pos as n – 1
- Return (sum + low * 1.0)/2
- Median of two sorted arrays of same size
Below is the implementation of the above approach:
PHP
<?php
function getMedian( $arr1 , $arr2 , $n ) {
$low = (int)-1e9;
$high = (int)1e9;
$pos = $n ;
$ans = 0.0;
while ( $low <= $high ) {
$mid = $low + (( $high - $low ) >> 1);
$ub = count ( array_filter ( $arr1 , function ( $x ) use ( $mid ) { return $x <= $mid ; })) +
count ( array_filter ( $arr2 , function ( $x ) use ( $mid ) { return $x <= $mid ; }));
if ( $ub <= $pos ) {
$low = $mid + 1;
} else {
$high = $mid - 1;
}
}
$ans = $low ;
$pos --;
$low = (int)-1e9;
$high = (int)1e9;
while ( $low <= $high ) {
$mid = $low + (( $high - $low ) >> 1);
$ub = count ( array_filter ( $arr1 , function ( $x ) use ( $mid ) { return $x <= $mid ; })) +
count ( array_filter ( $arr2 , function ( $x ) use ( $mid ) { return $x <= $mid ; }));
if ( $ub <= $pos ) {
$low = $mid + 1;
} else {
$high = $mid - 1;
}
}
$ans = ( $ans + $low ) / 2;
return $ans ;
}
$arr1 = array (1, 4, 5, 6, 10);
$arr2 = array (2, 3, 4, 5, 7);
$n = count ( $arr1 );
$median = getMedian( $arr1 , $arr2 , $n );
echo "Median is " . $median . PHP_EOL;
?>
|
Time Complexity: O(log n)
Auxiliary Space: O(1)
Please refer complete article on Median of two sorted arrays of same size for more details!
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