KNN in c++

Pseudo Code of KNN

We can implement a KNN model by following the below steps:

1. Load the data
2. Initialise the value of k
3. For getting the predicted class, iterate from 1 to total number of training data points
1. Calculate the distance between test data and each row of training data. Here we will use Euclidean distance as our distance metric since it’s the most popular method. The other metrics that can be used are Chebyshev, cosine, etc.
2. Sort the calculated distances in ascending order based on distance values
3. Get top k rows from the sorted array
4. Get the most frequent class of these rows
5. Return the predicted class

Iris Data Set

把数据作为string类型处理，进行string和double类型转换。

#include <iostream>
#include <string>
#include <fstream>
#include <sstream>
#include <numeric>
#include <functional>
#include <vector>
#include <algorithm>
#include <cmath>
#include <map>

template <class T1, class T2>
double ManhattanDistance(std::vector<T1> &inst1, std::vector<T2> &inst2) {
    if(inst1.size() != inst2.size()) {
        std::cout<<"the size of the vectors is not the same ";
        return -1;
    }
    std::vector<double> temp;
    for(size_t i=0;i<inst1.size();++i) {
        temp.push_back(std::abs(inst1.at(i)-inst2.at(i)));
    }
    double distance=accumulate(temp.begin(), temp.end(), 0.0);

    return distance;
}

template <class DataType1, class DataType2>
double EuclideanDistance(const std::vector<DataType1> &inst1, const std::vector<DataType2> &inst2) {
    if(inst1.size() != inst2.size()) {
    std::cout<<"the size of the vectors is not the same ";
        return -1;
    }
    std::vector<double> temp;
    for(size_t i=0; i<inst1.size(); ++i) {
        temp.push_back(pow(inst1.at(i)-inst2.at(i), 2.0));
    }
    double distance=accumulate(temp.begin(), temp.end(), 0.0);
    distance=sqrt(distance);

    return distance;
}

void vstr2vdouble(std::vector<std::string>::const_iterator beg, std::vector<std::string>::const_iterator end, std::vector<double> &vdouble) {
    for(std::vector<std::string>::const_iterator it=beg; it!=end; ++it) {
        double d;
        std::stringstream ss;
        ss<<*it;
        ss>>d;
        vdouble.push_back(d);
    }
}

void knn(std::vector<std::vector<std::string> > &trainset, std::vector<std::string> &testdata, int &k) {
    std::vector<double> testitem;
    vstr2vdouble(testdata.begin(), testdata.end(), testitem);
    std::multimap<std::string, std::string> mmap;

    for(size_t i=0;i<trainset.size();++i) {
        std::vector<double> trainitem;
        vstr2vdouble(trainset[i].begin(), trainset[i].end()-1, trainitem);
        double distance=EuclideanDistance(testitem, trainitem);
        std::string strdis;
        std::stringstream ss;
        ss<<distance;
        ss>>strdis;
        mmap.insert(std::pair<std::string, std::string>(strdis, trainset[i].back()));
    }
    size_t i=0;
    for(std::multimap<std::string, std::string>::const_iterator it=mmap.begin(); i<k; ++i,++it) {
        std::cout<<it->first<<" "<<it->second<<" ";
    }
}

template <class DataType>
void ReadDataFromFile(std::string &filename, std::vector<std::vector<DataType> > &lines_feat) {
    std::ifstream vm_info(filename.c_str());
    std::string lines, var;
    std::vector<std::string> row;

    lines_feat.clear();

    while(!vm_info.eof()) {
        getline(vm_info, lines);
        if(lines.empty())
            break;
        std::replace(lines.begin(), lines.end(), ',', ' ');
        std::stringstream stringin(lines);
        row.clear();

        while(stringin >> var) {
            row.push_back(var);
        }
        lines_feat.push_back(row);
    }
}

template <class DataType>
void Display2DVector(std::vector<std::vector<DataType> > &vv) {
    std::cout<<"the total rows of 2d vector_data: "<<vv.size()<<std::endl;

    for(size_t i=0;i<vv.size();++i) {
        for(typename::std::vector<DataType>::const_iterator it=vv[i].begin();it!=vv[i].end();++it) {
            std::cout<<*it<<" ";
        }
        std::cout<<" ";
    }
    std::cout<<"--------the end of the Display2DVector()-------- ";
}

int main() {
    std::string trainpath="Iris.data", testpath="knntest.data";
    std::vector<std::vector<std::string> > knn_data, test_data;

    ReadDataFromFile(trainpath, knn_data);
    ReadDataFromFile(testpath, test_data);

    Display2DVector(test_data);

    int k=3;
    for(size_t i=0;i<test_data.size();++i) {
        knn(knn_data, test_data[i], k);
    }

    return 0;
}