JavaCast 慷慨地四舍五入

Question

我计算总和并将它们放入我在 Coldfusion 中使用 POI 库生成的 Excel sheet。由于 Java 库需要类型化变量，所以我总是调用 setCellValue( JavaCast( "float", myVar ) )。 .03 让我意识到舍入错误。比转换为浮动后通常已知的差异要大得多。

<cfset s = 601761.66>
<cfoutput>
#s#<br>
#JavaCast( "float", s )#<br>
#LSNumberFormat( JavaCast( "float", s ), ".0000000" )#<br><br>
</cfoutput>

第一行打印601761.66
第二轮到601761.7
然而，第三个打印：601761,6875000 等于 601761,69 并且比我输入的值大 .03。

我知道，LSNumberFormat returns一个字符串。我叫它只是为了比较。 POI 似乎存储浮点值，Excel 最终像 LSNumberFormat 那样显示该值。

如何将一个值传递给 setCellValue，该值非常接近我的值，至少小数后的第二位数字被正确舍入？

Answer 1

简答：

使用 double 类型而不是 float，即 javacast("double", value)

更长的答案：

Cell.setCellValue() method actually expects type Double (not Float). Double is also what CF uses for most numeric operations and functions。当您将 Float 传递给这些方法时，它会隐式转换为 Double。该转换（间接）导致意外结果。

原因是两者Float and Double are approximate types。但是，Double 具有更高的精度：

float: The float data type is a single-precision 32-bit IEEE 754 floating point. ...

double: The double data type is a double-precision 64-bit IEEE 754 floating point. ...

因此 this thread 指出（强调我的）：

It's not that you're actually getting extra precision - it's that the float didn't accurately represent the number you were aiming for originally. The double is representing the original float accurately; toString is showing the "extra" data which was already present. ... [When converted to a double, it] will have exactly the same value, but when you convert it to a string it will "trust" that it's accurate to a higher precision, so won't round off as early, and you'll see the "extra digits" which were already there, but hidden from you

这就是为什么“601761.66”和“601761.6875”似乎在转换为浮点数时四舍五入为“601761.7”，但在转换为双精度数时显示为预期。

<cfscript>
    value1 = "601761.66";
    value2 = "601761.6875";

    WriteOutput("<br>[Float] "& value1 &" = "& javacast("float", value1));
    WriteOutput("<br>[Float] "& value2 &" = "& javacast("float", value2));
    WriteOutput("<br>[Float=>Double] "& value1 &" = "& javacast("double", javacast("float", value1)));
    WriteOutput("<br>[Double] "& value1 &" = "& javacast("double", value1));
    WriteOutput("<br>[Double] "& value2 &" = "& javacast("double", value2));
</cfscript>

输出：

[Float] 601761.66 = 601761.7
[Float] 601761.6875 = 601761.7
[Float=>Double] 601761.66 = 601761.6875
[Double] 601761.66 = 601761.66
[Double] 601761.6875 = 601761.6875

NB: CF 使用 Float.toString() 和 Double.ToString() 通过 cfoutput/writeOutput/cfdump.

显示值

JavaCast 慷慨地四舍五入

JavaCast float rounds generously

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