Swift：双转换不一致。如何正确比较双打？ [英] Swift: Double conversion inconsistency. How to correctly compare Doubles?
问题描述
我有一个非常简单的功能，将温度从˚C转换为˚K。
I have a very simple function to convert temperature from ˚C TO ˚K.
func convertKelvinToCelsius(temp:Double) ->Double {
return temp - 273.15
}
我有一个单元测试来驱动此功能。这是问题所在：
And I have a unit test to drive this function. This is where the problem is:
func testKelvinToCelsius(){
var check1 = conv.convertKelvinToCelsius(200.00) // -73.149999999999977
var check2 = 200.00 - 273.15 // -73.149999999999977
var check3 = Double(-73.15) // -73.150000000000006
//Passes
XCTAssert(conv.convertKelvinToCelsius(200.00).description == Double(-73.15).description, "Shoud convert from celsius kelvin")
//Fails
XCTAssert(conv.convertKelvinToCelsius(200.00) == Double(-73.15), "Shoud convert from celsius kelvin")
}
当你添加一个断点并检查check1，check2和check3的值，他们非常有趣：
When you add a breakpoint and check the values of check1, check2 and check3, they are very interesting:
check1 Double -73.149999999999977
check2 Double -73.149999999999977
check3 Double -73.150000000000006
问题：
-
为什么Swift为check1 / check2和check3返回不同的值
Why does Swift return different values for check1/check2 and check3
如何获得第二个测试通过，因为写作就像我做了一个测试一样的气味。为什么我必须将双打转换成字符串才能对比？
How can I get the second test to pass, because writing it like I did the test1 smells. Why should I have to convert Doubles to Strings to be able to compare them?
最后，当我 println check1，check2和check3，它们都打印为'-73.15'。为什么？为什么不准确打印，不要混淆程序员！
Finally, when I println check1, check2 and check3, they all print to be '-73.15'. Why? Why not print accurately, and not confuse the programmers!?
要复制：
只需在操场上输入 200 - 273.15 == -73.15 ，然后观看 false !!
Just type 200 - 273.15 == -73.15 in you playground and watch it go false!!
推荐答案
这是浮点值的预期行为。 它们不能100％准确地表示。
This is expected behavior for floating point values. They cannot be 100% accurately represented.
您可以使用 XCTAssertEqualWithAccuracy 函数来断言浮点值在彼此的给定范围内。
You can use the XCTAssertEqualWithAccuracy function to assert floating point values are within a given range of each other.
原因 println 打印所有的相同值是因为它内部将它们四舍五入为小数（我假设）。
The reason println prints the same value for all is because it internally rounds them to two decimals (I assume).
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