for_each（）算法 [英] for_each() algorithm
问题描述
我有一个计算标准差的函数。我想用
用STL算法重写它。
我正在考虑使用for_each（）算法，但这需要我的
函数传入有状态（例如标准差的值
sum）。我读了一些在线文章，说状态不是很好的想法。那么有更好的解决方案吗？
谢谢。
/ **
计算标准偏差，给出数字的平均值
* /
double sd（const vector< int>& v，
const double mean）
{
double stdDev = 0.0;
int N = v.size（）;
const double mean_ = mean ;
//计算标准差金额
double stdDevSum = 0;
double x;
for（int i = 0; i< N; i ++）{
x = v [i] - mean_;
stdDevSum = stdDevSum +（x * x） ;
}
double variance = stdDevSum / static_cast< double>（N-1）;
stdDev = sqrt（variance）;
返回stdDev;
} // sd
I have a function which calculate standard deviation. I am trying to
re-write it using STL algorithm.
I am thinking of using for_each() algorithm, but that will require my
function passing in to have state (e.g. the value of standard deviation
sum). I read some online article that having state is not a good
idea. So is there a better solution?
Thank you.
/**
Calculate the standard deviation, given the mean of the numbers
*/
double sd( const vector<int>& v,
const double mean )
{
double stdDev = 0.0;
int N = v.size();
const double mean_ = mean;
// calculate the standard deviation sum
double stdDevSum = 0;
double x;
for (int i = 0; i < N; i++) {
x = v[i] - mean_;
stdDevSum = stdDevSum + (x * x);
}
double variance = stdDevSum / static_cast<double>(N-1);
stdDev = sqrt( variance );
return stdDev;
} // sd
推荐答案
si *************** @ gmail.com 写道：
我有一个计算标准差的函数。我正在尝试使用STL算法重新编写它。
我正在考虑使用for_each（）算法，但这需要我的
函数传入以获得状态（例如，标准差
总和）。我读了一些在线文章，说状态不是一个好主意。那么有更好的解决方案吗？
I have a function which calculate standard deviation. I am trying to
re-write it using STL algorithm.
I am thinking of using for_each() algorithm, but that will require my
function passing in to have state (e.g. the value of standard deviation
sum). I read some online article that having state is not a good
idea. So is there a better solution?
std :: for_each（）是一般规则的例外：函数对象
与之一起使用算法可能有状态，这就是
算法返回函数对象的原因。
祝你好运，
Tom
std::for_each() is an exception to the general rule: Function objects
used with that algorithm may have state, and that is the reason that
algorithm returns the function object.
Best regards,
Tom
si *************** @ gmail.com 写道：
我有一个计算标准差的函数。我正在尝试使用STL算法重新编写它。
我正在考虑使用for_each（）算法，但这需要我的
函数传入以获得状态（例如，标准差
总和）。我读了一些在线文章，说状态不是一个好主意。那么有更好的解决方案吗？
I have a function which calculate standard deviation. I am trying to
re-write it using STL algorithm.
I am thinking of using for_each() algorithm, but that will require my
function passing in to have state (e.g. the value of standard deviation
sum). I read some online article that having state is not a good
idea. So is there a better solution?
函数对象。使用operator（）声明一个类StdDeviation，以便
这个类的对象可以作为第三个参数传递给for_each。
stdDevSum将是这样一个类的数据成员，通过no-arg构造函数初始化为零
。
Function Objects. Declare a class StdDeviation with operator() so that
an object of this class can be passed as third parameter to for_each.
stdDevSum will be a data member of such a class, initialized to zero
via no-arg constructor.
si *************** @ gmail.com 写道：
我有一个计算标准差的函数。我正在尝试使用STL算法重新编写它。
我正在考虑使用for_each（）算法，但这需要我的
函数传入以获得状态（例如，标准差
总和）。我读了一些在线文章，说状态不是一个好主意。那么有更好的解决方案吗？
I have a function which calculate standard deviation. I am trying to
re-write it using STL algorithm.
I am thinking of using for_each() algorithm, but that will require my
function passing in to have state (e.g. the value of standard deviation
sum). I read some online article that having state is not a good
idea. So is there a better solution?
是的。采用你拥有的算法，并用适当的算法替换for循环和
。在这种情况下，你总结了
（x [i] -xavg）*（x [i] -xavg）。
现在，总结一些值最好由std :: accumulate完成。
HTH，
Michiel Salters
Yes. Take the algorithm you have, and replace just the for-loop with
the appropriate algorithm. In this case, you''re summing
(x[i]-xavg)*(x[i]-xavg).
Now, summing a number of values is best done by std::accumulate.
HTH,
Michiel Salters
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