在某些情况下，acos(1)返回NAN [英] acos(1) returns NAN in some conditions

问题描述

这是我的代码:

<?php

$lat1 = 35.697959;
$lat2 = 35.697959;
$lon1 = 139.707085;
$lon2 = 139.707085;
$theta = $lon1 - $lon2;
$dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) +  cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta));
var_dump($dist); // returns 1

$dist = acos($dist); 
var_dump($dist); // returns NAN

从注释中可以看到，

计算后$dist等于1，但是当我应用acos()时，它返回NAN.

As you can see from the comments, $dist is equal to 1 after the calculation, but when I apply acos() it returns NAN.

如果我尝试手动复制它，则效果很好:

If I try to replicate it manually, it works just fine:

$x = 1;
$x = acos($x);
var_dump($x);  // returns 0 as expected

这是怎么回事，我该如何解决?

What's going on here and how can I fix it?

在线外壳中的代码与此相同，因此您可以看到它实际上返回了这些值.

Here's the same code in an online shell, so you can see that it actually returns those values.

推荐答案

找到了解决方案.看来问题与精度有关.如果我确定$dist在[-1,1]范围内，则可以正常运行:

Found the solution. It seems that the problem is related to precision. If I make sure that $dist is within [-1,1] range, it works fine:

$dist = acos(min(max($dist,-1.0),1.0)); 

工作示例: https://3v4l.org/dlunK

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