shannon碼不是最佳編碼，是香農(nóng)提出編碼理論后自己提出的一種編碼方式

shannon編碼主要分三個(gè)步驟

1.將字符按概率從大到小排序

2.將概率累加（除最小概率之外）

3.根據(jù)概率算出其信息熵確定碼長(zhǎng)，根據(jù)碼長(zhǎng)將其累加概率轉(zhuǎn)化為2進(jìn)制碼即shannon碼

栗子：

碼字符：	A	B	C	D
概率：	0.4	0.3	0.2	0.1
累加：	0	0.4	0.7	0.9
碼：	00	01	101	1110

// Shannon.cpp : 定義控制臺(tái)應(yīng)用程序的入口點(diǎn)。 //#include "iostream" #include "math.h" #include "map" #include "fstream" #include <sstream>using namespace std;class shannon{ public:shannon(string file_name);void show();//TODO://string to_shannon_code(string str = "");//string to_word(string str = "");private:shannon(){};int make_code();//編碼string decimals_to_binary(double decimals, int figures);//10進(jìn)制小數(shù)轉(zhuǎn)2進(jìn)制數(shù)（小數(shù)，位數(shù)）int get_probability(string file_name);//從文本中分析各個(gè)字符概率multimap<double, string> word;//原始字符、概率multimap<string, string> already;//編碼后字符、shannon碼string get_text; };shannon::shannon(string file_name){word.clear();get_text.clear();get_probability(file_name);make_code(); }int shannon::get_probability(string file_name){double word_amount = 0;double probability = 0;ifstream infile;ofstream outfile;map<char, double> word_count;infile.open(file_name, ios::in);if (!infile){cout << "can not open word text" << endl;return 0;}else{getline(infile, get_text);}for (auto &s : get_text){++word_count[s];++word_amount;}outfile.open("probability.txt", ios::out);for (auto &w : word_count){ostringstream stream;probability = w.second / word_amount;stream << w.first;outfile << stream.str() << "\t" << probability << endl;word.insert(make_pair(probability, stream.str()));already.insert(make_pair(stream.str(), ""));}return 1; }int shannon::make_code(){double temp = 1.0;string code = "";int figures = 0;for (auto &w : word){temp -= w.first;figures = -(log(w.first) / log(2));code = decimals_to_binary(temp, figures);already.find(w.second)->second = code;}return 1; }string shannon::decimals_to_binary(double decimals, int figures){string code = "";for (int i = 0; i < figures; ++i){decimals *= 2;ostringstream stream;stream << int(decimals);code += stream.str();if (decimals >= 1){decimals -= 1; }else{continue;}}return code; }void shannon::show(){for (auto s : already){cout << s.first << "\t" << s.second << endl;} }int main(){shannon a("demo.txt");a.show();system("pause"); }

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