Question

c++的题，求大神解答

1..The STL generate algorithm is defined as void generate(ForwardIterator start, For-wardIterator end, NullaryFunction fn) and iterates over the specified range storing the return value of the zero-parameter function fn as it goes. For example, calling generate(v.be-gin(), v.end(), rand) would fill the range [v.begin() to v.end()) with random values.
Write a function FillAscending that accepts an iterator range, then sets the first element in the range to zero, the second to one,etc. Do not use any loops.

2.Write a function StandardDeviation that accepts an input range of iterators over doubles (or values implicitly convertible to doubles) and returns its standard deviation. Do not use any loops – instead use the accumulate function to compute the average, then use accumulate once more to compute the sum.(Hint :To get the number of elements in the range, you can use the distance function)
3. Write a main function to test them..

Answer 1

#include <vector>
#include <iostream>
#include <algorithm>
#include <numeric>
using namespace std;

int  FillAscending()
{
	static int i = 0;
	return i++;
}

//  计算差的平方
class DiffSquare
{
private:
	float m_fAvarage;
public:
	DiffSquare(float fAvarage) : m_fAvarage(fAvarage){}
	float operator()(float fVal1, float fVal2)
	{
		float fDiff = fVal2 - m_fAvarage;
		return fVal1 + fDiff * fDiff;
	}
};


// 计算均方差
float StandardDeviation(vector<float>::iterator itBegin, vector<float>::iterator itEnd)
{
	int nSize = std::distance(itBegin, itEnd);
	// 1 计算平均值
	float fAvarage = (float)std::accumulate(itBegin, itEnd, 0.0f) / nSize;
	// 2 计算每个元素与均值的差的平方
	float f1 = accumulate(itBegin, itEnd, 0.0f, DiffSquare(fAvarage));
	return sqrt( f1 / nSize);
}

// 第三问
int main()
{
        // 第一问
	vector<float> vecFloat(10);
	generate(vecFloat.begin(), vecFloat.end(), FillAscending);
	// 第二问	
	float fStanderDeviation = StandardDeviation(vecFloat.begin(), vecFloat.end());
	cout << "The StanderDeviation is : " << fStanderDeviation << endl;
	return 0;
}

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Answer 2

/**************************************************************************
 *
 *   Copyright (c) 2016 www.bjfz.cc. All rights reserved.
 *
 * @file main.cc
 *
 * 绠楁硶缁冧范
 *
 * 1. FillAsscending
 * 2. StandardDeviation
 *
 * @author zhanghb zhaingbo@foxmail.com
 *
 *************************************************************************/
#include <iostream>
#include <algorithm>
#include <numeric>
#include <vector>
#include <cmath>


using namespace std;

struct sq_of_dvi {
    sq_of_dvi(double avg){
        _avg = avg;
    }
        
    double operator()(double y, double x) {
        return y + (x-_avg)*(x-_avg);
    }
        
    double _avg;
};

int increment()
{
	static int i = 0;
    return i++;
}

template<typename T>
void FillAsscending(T beg, T end)
{
    generate(beg, end, increment);
}

template<typename T>
double StandardDeviation(T beg, T end)
{
    // 1. calc the avg
    double avg = accumulate(beg, end, 0.0) / (end - beg);

    // 2. calc the sum
    // 2.1 squares_of_deviations
    struct sq_of_dvi sod(avg);
    
    // calc the sum of squares_of_deviations
    double sum = accumulate(beg, end, 0.0, sod);

    // 3. standard deviation
    return sqrt(sum);
}


int main(int argc, char *argv[])
{
    vector<int> ivec(10);
    vector<int>::iterator beg;
    beg = ivec.begin();
    while (beg != ivec.end()) {
        std::cout << *beg++ << "\t";
    }
    std::cout << std::endl;
    
    FillAsscending(ivec.begin(), ivec.end());

    beg = ivec.begin();
    while (beg != ivec.end()) {
        std::cout << *beg++ << "\t";
    }
    std::cout << std::endl;

    std::cout << StandardDeviation(ivec.begin(), ivec.end()) << std::endl;

    
    return 0;
}


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