Float 1有时转换为0，有时为1 [duplicate]

Question

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I am trying to get the first decimal place of a float number as an integer by subtracting the integer part, multiplying the remainder with 10 and then casting the result to int or using intval(). I noticed that the result for numbers with x.1 is correctly 1 as float, but after converting it to integer, it becomes sometimes 0, sometimes 1.

I tried to test it with numbers from 1.1 to 9.1:

for ($number = 1; $number < 10; $number++) {
    $result = 10 * ($number + 0.1 - $number);
    echo "<br/> number = " . ($number + 0.1) . ", result: ";
    var_dump($result);
    $result_int = intval($result);
    var_dump($result_int);
}

Starting with 4.1 as input, the 1 oddly gets converted to 0:

number = 1.1, result: float(1) int(1)
number = 2.1, result: float(1) int(1)
number = 3.1, result: float(1) int(1)
number = 4.1, result: float(1) int(0)
number = 5.1, result: float(1) int(0)
number = 6.1, result: float(1) int(0)
number = 7.1, result: float(1) int(0)
number = 8.1, result: float(1) int(0)
number = 9.1, result: float(1) int(0)  

Why at 4.1? That doesn't make any sense to me. Can anyone give me a hint what I am doing wrong?

PS: also tested at http://ideone.com/hr7M0A

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Answer 1

You are seeing these results because floating point arithmetic is not perfectly accurate.

Instead of trying to manually get the first decimal point use fmod:

$result = substr(fmod($number, 1) * 10, 0, 1)