使用float作为STL映射中的索引？ [英] using a float as the index in an STL map?
问题描述
似乎使用花车作为STL地图的第一个tupelo（显示为
以下）可能会导致问题，但我不知道记住问题是什么。
使用如下结构是否常见？如果你能分享你使用这种结构的经验，我将不胜感激
。谢谢。
std :: map< float，intm;
JD
Hi,
It seems that using floats as the first tupelo for an STL map (shown
below) can cause problems but I don''t remember what the problems were.
Is it common practice to use a structure like below? I would appreciate
if you can share your experience using such a structure. Thanks.
std::map<float, intm;
JD
推荐答案
JDT写道：
JDT wrote:
似乎使用浮动作为STL地图的第一个tupelo（显示
以下）可能会导致问题，但我不记得是什么问题。
It seems that using floats as the first tupelo for an STL map (shown
below) can cause problems but I don''t remember what the problems were.
我唯一能想到的是，如果你期望两个不同的
计算结果导致相同的数字（和最终
与地图值中存储的相同），您可能会感到意外。
由于FPU中的舍入错误，数学上相当于
计算可以在内部导致不同的数字。
例如，假设''''是1.f而''b''是2.f，表达式
（a + 2.f / 3）和（b - 1.f / 3）_can_给你不同的结果。
The only thing I can think of is that if you expect two different
results of a calculation to lead to the same number (and ultimately
to the same stored in the map value), you may be in for a surprise.
Due to rounding errors in the FPU the mathematically equivalent
calculations can lead to different numbers internally.
Example, given that ''a'' is 1.f and ''b'' is 2.f, the expressions
(a + 2.f/3) and (b - 1.f/3) _can_ give you different results.
是吗使用如下结构的常见做法？如果你可以分享你使用这种结构的经验，我会很高兴。
谢谢。
std :: map< float，intm;
Is it common practice to use a structure like below? I would
appreciate if you can share your experience using such a structure.
Thanks.
std::map<float, intm;
我_never_看到''map''其中''float''将是Key类型。我不能说b $ b声称已经看到了世界上所有的代码，甚至是一个重要的部分
，所以我无法证明共性。练习。
V
-
请在通过电子邮件回复时删除资金'A'
我没有回复最热门的回复，请不要问
I _never_ saw a ''map'' where ''float'' would be the Key type. I cannot
claim to have seen all code in the world, and even a significant part
of it, so I cannot attest to "commonality" of the practice.
V
--
Please remove capital ''A''s when replying by e-mail
I do not respond to top-posted replies, please don''t ask
Victor Bazarov写道：
Victor Bazarov wrote:
例如，假设''''是1.f而''b''是2.f，表达式
（a + 2.f / 3）和（b - 1.f / 3）_can_给你不同的结果。
Example, given that ''a'' is 1.f and ''b'' is 2.f, the expressions
(a + 2.f/3) and (b - 1.f/3) _can_ give you different results.
具体示例：
double d1 = 1.0;
double d2 = 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1;
std :: cout<< d1<< " " << d2<< " " << （d1 == d2）<< std :: endl;
打印（至少在普通电脑中）：
1 1 0
打印带有更多小数位的双打将显示
的差异。它非常小，但它就在那里，因此（d1 == d2）是
false。
Concrete example:
double d1 = 1.0;
double d2 = 0.1+0.1+0.1+0.1+0.1+0.1+0.1+0.1+0.1+0.1;
std::cout << d1 << " " << d2 << " " << (d1 == d2) << std::endl;
That prints (at least in a regular PC):
1 1 0
Printing the doubles with more decimal places would show the
difference. It''s very small, but it''s there, thus (d1 == d2) is
false.
Victor Bazarov写道：
Victor Bazarov wrote:
JDT写道：
JDT wrote:
>>似乎使用浮动作为第一个tupelo STL地图（如下所示）可能会导致问题，但我不记得问题是什么。
>>It seems that using floats as the first tupelo for an STL map (shown
below) can cause problems but I don''t remember what the problems were.
我唯一能想到的是，如果你期望两个不同的
计算结果导致相同的数字（最终
与地图值中存储的相同），您可能会感到意外。
由于FPU中的舍入错误，数学上等效
计算可以在内部产生不同的数字。
例如，假设''''是1.f而''b''是2.f ，表达式
（a + 2.f / 3）和（b - 1.f / 3）_can_给你不同的结果。
The only thing I can think of is that if you expect two different
results of a calculation to lead to the same number (and ultimately
to the same stored in the map value), you may be in for a surprise.
Due to rounding errors in the FPU the mathematically equivalent
calculations can lead to different numbers internally.
Example, given that ''a'' is 1.f and ''b'' is 2.f, the expressions
(a + 2.f/3) and (b - 1.f/3) _can_ give you different results.
>>通常的做法是使用如下所示的结构吗？如果你能用这样的结构分享你的经验，我会很感激。
谢谢。
std :: map< float，intm;
>>Is it common practice to use a structure like below? I would
appreciate if you can share your experience using such a structure.
Thanks.
std::map<float, intm;
我_never_看到''map''''float''将是Key类型。我不能说b $ b声称已经看到了世界上所有的代码，甚至是一个重要的部分
，所以我无法证明共性。这个练习。
V
I _never_ saw a ''map'' where ''float'' would be the Key type. I cannot
claim to have seen all code in the world, and even a significant part
of it, so I cannot attest to "commonality" of the practice.
V
你好Victor：
下面的场景，我认为使用float作为地图的关键使得
感觉。例如，下表需要按第1个元组的
升序排序（即值）。所以我可以简单地将两个对子插入地图中，然后自动获得一个排序对的列表。
人们经常有类似的需求，还是有其他更好的方法
能达到这个目的吗？非常感谢任何进一步的帮助。
价值物品数量
3.5 5
4.7 9
9.3 7
......
JD
Hi Victor:
In the following scenario, I think using float as the key to a map makes
sense. For example, the following table needs to be sorted in the
ascending order of the 1st tuple (i.e. Values). So I can simply insert
the pairs into a map and then get a list of sorted pairs automatically.
Do people often have similar needs or are there other better ways to
accomplish this purpose? Any further help is much appreciated.
Values # of items
3.5 5
4.7 9
9.3 7
......
JD
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