php integer and float comparison mismatch

I have the following code

$amount1 = 7299;
$amount2 = 72.9875;

$amount2_in_cents = round($amount2, 2) * 100;

if ($amount1 != $amount2_in_cents) {
    echo "Amount $amount1 != $amount2_in_cents\n";

    var_dump($amount1);
    var_dump($amount2_in_cents);    

} else {
    echo "Amounts matched";
}

and this is the output

Amount 7299 != 7299
int(7299)
float(7299)

Now I realise that floats and int are different, but given the rounding i would have expected the two values to match. And I have solved it by casting to int.

So my question is why does this comparison not work as i would have expected (both values matching)?

Solution 1:

Notice the big red warning in the PHP Manual!

Never expect anything when comparing floats. The result of round, even if the precision is 0, is still a float. In your particular case it happened that the result was a little bigger than expected, so casting to int resulted in equality, but for other numbers it might as well happen for it to be a little smaller than expected and casting to int won't round it, but truncate it, so you can't use casting as a workaround. (As a note, a better solution than yours would be casting to string :), but still a lousy option.)

If you need to work with amounts of money always use the BC Math extension.

For rounding with BC Math you can use this technique:

$x = '211.9452';
$x = bcadd($x, '0.005', 2);

Good luck,
Alin

Solution 2:

Use

round()

$float_val = 4.5;
$float_val = round($float_val);

now Compare

Solution 3:

For example a nasty case: We want to check if our ratting is greater or equal than 3.3 out of 5 while in a loop. ( This is all weird example to show a "flaw")

$a="3.3";
for($i=0; $i<5 ; $i=$i+0.15){
    echo "\nTesting if $i>=$a\n";
    var_dump($i,$a);
    if($i>=$a){ 
        echo "$i>=$a is TRUE\n";
    }else{
        echo "$i>=$a is FALSE\n";

    }
}

Now the output will be this one:

Testing if 0>=3.3
int(0)
string(3) "3.3"
0>=3.3 is FALSE

Testing if 0.15>=3.3
float(0.15)
string(3) "3.3"
0.15>=3.3 is FALSE

Testing if 0.3>=3.3
float(0.3)
string(3) "3.3"
0.3>=3.3 is FALSE

Testing if 0.45>=3.3
float(0.45)
string(3) "3.3"
0.45>=3.3 is FALSE

Testing if 0.6>=3.3
float(0.6)
string(3) "3.3"
0.6>=3.3 is FALSE

Testing if 0.75>=3.3
float(0.75)
string(3) "3.3"
0.75>=3.3 is FALSE

Testing if 0.9>=3.3
float(0.9)
string(3) "3.3"
0.9>=3.3 is FALSE

Testing if 1.05>=3.3
float(1.05)
string(3) "3.3"
1.05>=3.3 is FALSE

Testing if 1.2>=3.3
float(1.2)
string(3) "3.3"
1.2>=3.3 is FALSE

Testing if 1.35>=3.3
float(1.35)
string(3) "3.3"
1.35>=3.3 is FALSE

Testing if 1.5>=3.3
float(1.5)
string(3) "3.3"
1.5>=3.3 is FALSE

Testing if 1.65>=3.3
float(1.65)
string(3) "3.3"
1.65>=3.3 is FALSE

Testing if 1.8>=3.3
float(1.8)
string(3) "3.3"
1.8>=3.3 is FALSE

Testing if 1.95>=3.3
float(1.95)
string(3) "3.3"
1.95>=3.3 is FALSE

Testing if 2.1>=3.3
float(2.1)
string(3) "3.3"
2.1>=3.3 is FALSE

Testing if 2.25>=3.3
float(2.25)
string(3) "3.3"
2.25>=3.3 is FALSE

Testing if 2.4>=3.3
float(2.4)
string(3) "3.3"
2.4>=3.3 is FALSE

Testing if 2.55>=3.3
float(2.55)
string(3) "3.3"
2.55>=3.3 is FALSE

Testing if 2.7>=3.3
float(2.7)
string(3) "3.3"
2.7>=3.3 is FALSE

Testing if 2.85>=3.3
float(2.85)
string(3) "3.3"
2.85>=3.3 is FALSE

Testing if 3>=3.3
float(3)
string(3) "3.3"
3>=3.3 is FALSE

Testing if 3.15>=3.3
float(3.15)
string(3) "3.3"
3.15>=3.3 is FALSE

Testing if 3.3>=3.3
float(3.3)
string(3) "3.3"
3.3>=3.3 is FALSE

Testing if 3.45>=3.3
float(3.45)
string(3) "3.3"
3.45>=3.3 is TRUE

Testing if 3.6>=3.3
float(3.6)
string(3) "3.3"
3.6>=3.3 is TRUE

Testing if 3.75>=3.3
float(3.75)
string(3) "3.3"
3.75>=3.3 is TRUE

Testing if 3.9>=3.3
float(3.9)
string(3) "3.3"
3.9>=3.3 is TRUE

Testing if 4.05>=3.3
float(4.05)
string(3) "3.3"
4.05>=3.3 is TRUE

Testing if 4.2>=3.3
float(4.2)
string(3) "3.3"
4.2>=3.3 is TRUE

Testing if 4.35>=3.3
float(4.35)
string(3) "3.3"
4.35>=3.3 is TRUE

Testing if 4.5>=3.3
float(4.5)
string(3) "3.3"
4.5>=3.3 is TRUE

Testing if 4.65>=3.3
float(4.65)
string(3) "3.3"
4.65>=3.3 is TRUE

Testing if 4.8>=3.3
float(4.8)
string(3) "3.3"
4.8>=3.3 is TRUE

Testing if 4.95>=3.3
float(4.95)
string(3) "3.3"
4.95>=3.3 is TRUE

And the nasty part:

 Testing if 3.3>=3.3
    float(3.3)
    string(3) "3.3"
    3.3>=3.3 is FALSE

3.3 is greater or equal than 3.3 but php things not! Makes no sense right

Now if you put a ini_set('precision', 18); before the code you can see that the evaluation actually was:

Testing if 3.29999999999999893>=3.3
float(3.29999999999999893)
string(3) "3.3"
3.29999999999999893>=3.3 is FALSE

So the $i=$i+=0.15 implicitly converts the $i to a float, which will cause this issue.

For this case the $i+=0.15 should be changed to $i=number_format($i+=0.15, 2)