Function to check if number is multiple of another (including decimals)

Question

Hello I am trying to build a function to check if a number is multiple of another:

What I am trying to get:

my_function (10,2) = true
my_function (8,3) = false
my_function (9.5,0.5) = true
my_function (6,1.5) = true
my_function (1.1,0.1) = true

My function is:

//Check if $number1 is a multiple of $number2
function my_function( $number1, $number2 ){
    $_number = $number1 / $number2;  

    if( $_number == floor( $_number ) ) :
        return true;
    else :
        return false;
    endif;
}

It is working like a charm, except that for some reason when there are values 3,7,11, etc it is returning false when it should clearly return true:

my_function (5.4,0.3) = false (it is retruning false, it should be true!)
my_function (7.7,0.7) = false (it is retruning false, it should be true!)

Any idea why? Is it a compiler error? I run out of ideas. Thank you

You should try printing out the result of `floor($_number) - $_number` It is possible that the floats are not exactly equal after the division and have something like `5.4 / 0.3 = 18.00000000000012` — Easton Bornemeier, Jun 09 '17 at 22:13
I echo'd $_number and floor( $_number ) and both are the same. Let me try printing floor($_number) - $_number — karlosuccess, Jun 09 '17 at 22:15
@EastonBornemeier you are right. After printing out `floor($_number) - $_number` the floats are not the same. What could i do? — karlosuccess, Jun 09 '17 at 22:19
Start with https://stackoverflow.com/questions/3148937/compare-floats-in-php — u_mulder, Jun 09 '17 at 22:21
[should I do float comporison](https://stackoverflow.com/q/4915462/6521116) — LF00, Jun 09 '17 at 22:25
This whole function can be replaced with: `return fmod($number1, $number2) < 0.00001` — rickdenhaan, Jun 09 '17 at 22:27
The above is the best answer, the marked answer will not always work *see my comment* — Easton Bornemeier, Jun 09 '17 at 22:29
This is not about comparing float numbers, question is to know if 1st number is integer multiple of 2nd number or not. — anubhava, Jun 09 '17 at 22:35
@anubhava what's your point? If the remainder of the division is 0, I am pretty sure that the first number is integer multiple of the 2nd. — GramThanos, Jun 10 '17 at 00:27

score 0 · Accepted Answer · answered Jun 09 '17 at 22:23

0

Since the resulting type of floor is double and $_number is also double, the safest way it to have a relative comparison < instead of absolute comparison.

Floating precision might vary. Thus, this is the safest way:

if (abs(floor( $_number ) - $_number) < 0.00001) {
    return true;
}

return false;

answered Jun 09 '17 at 22:23

Joseph D.

10,071
3
21
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1

This is dangerous and I beleive will still cause erorrs. What if the value of number is slightly below the desired number? i.e. `4/.5 = 7.999999999` this result will make it so it floors to 7, and then it will return false – Easton Bornemeier Jun 09 '17 at 22:26
2

@karlosuccess You may be interested in fmod later, check it out: http://php.net/manual/en/function.fmod.php – leoap Jun 09 '17 at 22:26
ok so if I use `fmod`, I still have to use `fmod( $number1, $number2 ) < 0.00001`, because even `fmod` also returns tiny decimals. So what is the answer then? – karlosuccess Jun 09 '17 at 22:50
@karlosuccess my area isn't math, so i can't help with the floating precision problem. I've asked some friends and they recommended me to check the BC Math extension, and linked me this: http://floating-point-gui.de/ and this: http://floating-point-gui.de/languages/php/ – leoap Jun 10 '17 at 01:50
@karlosuccess Yes, use `fmod( $number1, $number2 ) < 0.00001` – Easton Bornemeier Jun 10 '17 at 07:40

Function to check if number is multiple of another (including decimals)

1 Answers1