PHP 果然是世界上最好的语言，再也不相信 var_dump() 了

[laoyuan]

<?php
$a = 57; //或者 $a = ‘57’;
$b = '0.57' * 100;
$c = round('0.57' * 100);
var_dump( $b, $c, $a == $b, $a == $c);
?>

float(57)
float(57)
bool(false)
bool(true)

谁能解释为什么加 round 就 true 了？PHP Version 5.5.20

[laoyuan]

@huson 重点是 round

[huson]

@laoyuan
http://php.net/manual/en/language.types.float.php#warn.float-precision
这页有讲 栗子在下面


$x = 8 - 6.4; // which is equal to 1.6
$y = 1.6;
var_dump($x == $y); // is not true

PHP thinks that 1.6 (coming from a difference) is not equal to 1.6. To make it work, use round()

var_dump(round($x, 2) == round($y, 2)); // this is true

This happens probably because $x is not really 1.6, but 1.599999.. and var_dump shows it to you as being 1.6.

[laoyuan]

@huson 哎呀坑爹啊，我看的中文版手册倒是带评论，但不是按投票排序的，所以评论都略过了！

[bombless]

@Kilerd 在C语言中这叫promotion，在此处是不需要区分单精度和双精度的