CODEWITHSUNDEEP
phpdecimalmultiplication
Jeff 131313
Jeff 131313

Reputation: 37

PHP multiplication decimals

I have a strange bug when multiplying decimals. When I multiply a decimal such as 2.1 by a number such as 2 (to get the product 4.2), PHP displays them identically, and, on comparison, shows them as equal. But when I multiply 2.1 by 7 (14.7), though PHP still displays them as equal, upon comparison it declares the products unequal.

Here's code illustrating the issue (note what PHP returns):

$num_1 = 4.2;
$num_2 = 2.1*2;
if($num_1 == $num_2){
  $eq = "Equal";
}else{
  $eq = "Unequal";
}
echo $num_1.", ".$num_2.", ".$eq."<br>";
// Returns  4.2, 4.2, Equal

$num_1 = 14.7;
$num_2 = 2.1*7;
if($num_1 == $num_2){
  $eq = "Equal";
}else{
  $eq = "Unequal";
}
echo $num_1.", ".$num_2.", ".$eq."<br>";
// Returns  14.7, 14.7, Unequal

Thanks.

Upvotes: 1

Views: 8409

Answers (3)

MH2K9
MH2K9

Reputation: 12039

I suggest you to use round().

$num_1 = round(14.7, 2);
$num_2 = round(2.1 * 7, 2);
echo ($num_1 == $num_2) ? 'equal' : 'uequal';

Now notice the result here what exactly occurred.

$num_1 = 14.7;
$num_2 = 2.1 * 7;
echo abs($num_1-$num_2); //Output: 1.7763568394003E-15

Upvotes: 1

RussianVodka
RussianVodka

Reputation: 440

This is PHP, bro! Just convert your floats to strings and compare them...

$num_1 = 14.7;
$num_2 = 2.1*7;
if((string) $num_1 == (string) $num_2){
  $eq = "Equal";
}else{
  $eq = "Unequal";
}
echo $num_1.", ".$num_2.", ".$eq."<br>";

Or use that bcmul as shown above...

Upvotes: -1

Machavity
Machavity

Reputation: 31614

The problem is that some floating point numbers can't be represented accurately. If you need to compare them, or a higher level of precision, use bcmul

$num_1 = 14.7;
$num_2 = bcmul(2.1, 7, 1);
if((string)$num_1 == $num_2){
  $eq = "Equal";
}else{
  $eq = "Unequal";
}
echo $num_1.", ".$num_2.", ".$eq."<br>";

Upvotes: 2

Related Questions