Php , add previous sub-array value to to next sub-array value

Programming languages PHP
#1 

<?php


$arr = [ [1,2,4], [4,2,6], [2,1,8] ];

// in each sub-array's end value

//add previous sub-array's end-value



$arr_count = count($arr);

$sub_arr_count = count($arr[0]);

for($i = 0; $i < $arr_count; $i++){
    //( $i < 1 ) ? continue; : 'no iteration';
    //if($i<1){continue;}
    for($j = 0; $j < $sub_arr_count; $j++){
        $arr[$i][2] = $arr[$i-1][2] + $arr[$i][2] ;
    }
}


?>

fails and gives errors ,

why this error is ?
plz help

PHP Notice: Undefined offset: -1 in HelloWorld.php on line 20
PHP Notice: Trying to access array offset on value of type null in HelloWorld.php on line 20
PHP Notice: Undefined offset: -1 in /runtime/php/3ydr8gcjt/HelloWorld.php on line 20
PHP Notice: Trying to access array offset on value of type null in /runtime/php/3ydr8gcjt/HelloWorld.php on line 20
PHP Notice: Undefined offset: -1 in /runtime/php/3ydr8gcjt/HelloWorld.php on line 20
PHP Notice: Trying to access array offset on value of type null in /runtime/php/3ydr8gcjt/HelloWorld.php on line 20
#2

What do you wish to accomplish here exactly? What is the expected result array after the intended operation in your 3x3 example?

#3 
this is input array ,

$arr = [ [1,2,4], [4,2,6], [2,1,10] ];


and this is output array ,

$arr = [ [1,2,4], [4,2,10], [2,1,18] ];

objective :

first sub-array will not change,
every next-sub-array will change.
and each next-subarray's end value is sum of previous-sub-array's end value.

this to be accomplished with a for consruct.
#4

you get -1 index because your $i loop starts from 0, but then you access index $i - 1 in the array which is -1. Start your $i loop from 1.
Your $j loop is completely unnecessary, you need to change only the last item, not all in the row.

#5 
<?php
//$arr = [ [1,2,4], [4,2,10], [2,1,18] ];		//result

$arr = [ [1,2,4], [4,2,6], [2,1,8] ];

// in each sub-array's end value

//add previous sub-array's end-value



$arr_count = count($arr);

$sub_arr_count = count($arr[0]);

for($i = 1; $i < $arr_count; $i++){
    
    
        $arr[$i][2] = $arr[$i-1][2] + $arr[$i][2] ;
    
}


echo "<pre>";
print_r($arr);
echo "<pre>";
?>
#6

but how that ?

one dimensional array access ,
and two dimensional array access
are different !

#7

can we do it via $j loop ?