IEEE 754：是否始终保证v * = -1与v = -v相同？

编程问答 2022-06-09

问题描述

标准（IEEE 754 / C）是否保证在以下代码声明中将永远不会失败？

int main()
{
    for ( /* all possible float / double values */ )
    {
        v_neg1 = v * -1;
        v_neg2 = -v;
        assert( v_neg1 == v_neg2 );
    }
    return 0;
}

UPD。

  1. 询问时，我的意思是all possible float / double values 不包括NaNs 。
  2. 看到类似的问题：.NET decimal.Negate vs multiplying by -1

解决方法

不，这不一定是正确的，因为 NaNs 从不相互比较。

除此之外，考虑到符号位与指数和绝对有效位无关，我认为对于所有普通值来说都是正确的。 C指ISO / IEC 60559 / IEEE 754附录;不幸的是，我本人没有这个标准。

 flutter doctor --android-licenses
Error: Unknown argument --licenses
Usage: 
  sdkmanager [--uninstall] [<common args>] \
    [--package_file <package-file>] [<packages>...]
  sdkmanager --update [<common args>]
  sdkmanager --list [<common args>]

In its first form,installs,or uninstalls,or updates packages.
    <package> is a sdk-style path (e.g. "build-tools;23.0.0" or 
             "platforms;android-23").
    <package-file> is a text file where each line is a sdk-style path
                   of a package to install or uninstall.
    Multiple --package_file arguments may be specified in combination
     with explicit paths.
In its second form (with --update),currently installed packages are
    updated to the latest version.
In its third form,all installed and available packages are printed out.

Common Arguments:
    --sdk_root=<sdkRootPath>: Use the specified SDK root instead of the SDK containing this tool
    --channel=<channelId>: Include packages in channels up to <channelId>.
                           Common channels are:
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    --include_obsolete: With --list,show obsolete packages in the
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    --proxy_port=<port #>: Proxy port to connect to.                                                                                                                                                             
                                                                                                                                                                                                                 
* If the env var REPO_OS_OVERRIDE is set to "windows","macosx",or "linux",packages will be downloaded for that OS.
cdoublefloating-pointieee-754