jqgrid - What's the difference between 2 lines in PHP? -


i have found in jqgrid server side php example next line

$start = $limit*$page - $limit; // not put $limit*($page - 1)

what's difference between 

$start = $limit*$page - $limit;

and

$start = $limit*($page - 1);

why authors not recommend second way ?

i suspect numerical math issue. suppose $page huge, in 6.02e231. 

($page - 1) = 6.02e231

(the exact same floating point representation), 

$limit*($page - 1) = $limit*$page

from point of view of processor. compare

$limit*$page = [some huge value] $limit*$page - $limit = [subtraction of 2 huge values]

which yield different result, more accurate "minus one" notation.

so although in perfect world of mathematics, 2 notations equal, in non-perfect world of computing, first 1 more prone 'catastrophic cancellation' (wiki that) second.


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