Generate N different random numbers (C + +, range 0~n-1)

The process of doing a project often has a random selection process. This note mainly gives two methods of randomly generating n different random numbers, and then simply introduces the function Srand,rand and time used in the random numbers in C + +. In the end, a simple example is given, which is to randomly select K images from a folder containing n images to be stored in another folder.

A: A method of generating n different random numbers

#include <stdio.h> #include <stdlib.h> #include <time.h> #define N 20#define K 10void swap (int *a, int *b)    {if (*a! = *b) {//XOR/Operation interchange two digit position *a ^= *b;*b ^= *a;*a ^= *b;} }/******************************************************************************* function name: void GenerateDiffRandV1 ( int a[], int n) * Function function: produce different random number * Entry parameters: * return value: None * Note: space change time ************************************************************    /void generateDiffRandV1 (int a[], int n, int k) {int i;    time_t T;    for (i = 0; i < n; i++) {a[i] = i;    } srand ((int) time (&t));    for (i = 0; i < K; i++) {swap (&a[i], &a[i+rand ()% (n-i)]); }}/******************************************************************************* function name: void GenerateDiffRandV2 ( int a[], int n) * Function function: Produces a different random number (the range that produces the random number is 1~n-1) * Entry parameter: * return value: no * * idea: Sir into a set of the array, and then randomly extracted, after extraction to prevent duplication, immediately zero. *: Each time you generate a seat, just determine whether it is 0, greatly improving the efficiency of program execution. ***********************************************************/void generateDiffRandV2 (int a[], int n) {int *flag = (int *) malloc (sizeof (int) * n);    static int flag_once = 0;        int I, index;    for (i = 0; i < n; i++) flag[i] = i+1;        if (!flag_once) {Srand (time (0));    Flag_once = 1;        } for (i = 0; i < n;) {index = rand ()% n;            if (Flag[index]! = 0) {a[i++] = flag[index]-1;        Flag[index] = 0; }} free (flag);}    void PrintArray (int a[], int n) {int i;    for (i = 0; i < n; i++) {printf ("%d", a[i]); } printf ("\ n");}    int main () {int a[n];    GenerateDiffRandV1 (A, N, K);        PrintArray (A, N);    GenerateDiffRandV2 (A, N);        PrintArray (A, N); return 0;}

II: Introduction to Srand,rand and time functions

The 1:srand function is the initialization function of the random number generator. It needs to provide a seed, which will correspond to a random number, and if the same seed is used, the following rand () function will appear the same random number. such as: Srand (1); Srand (1) is also the default seed to initialize seeds directly using one. Included in the library <stdlib.h>.

Such as:

#include <iostream> #include <time.h>using namespace Std;int main () {//Srand equivalent to a random number of initialization functions, sowing seeds for random numbers,//srand ((unsigned) time (0));    Time returned is time_t type Srand (1),//time_t t= time (0), for (int i = 0; i < i++) {cout << rand () << Endl;} return 0;}

no matter how many times this code runs, it's always from the seed 1 start generating random numbers, so the results are the same.

to avoid this situation, we used the Srand ((unsigned) time (0)); 0 Representative null,time (NULL) is to get the current time meaning , this will produce a different random number for each run . Time () Returns the result of time_t, cast for unsigned type, contained in library <time.h>.

The 2:rand function is a pseudo-random number generator that needs to call Srand initialization, typically initializing the random number seed with the current calendar time, so that each line of code can produce a different random number.

Three: Example (a folder containing n pictures randomly selected K pictures into another folder)

/* Randomly select 1w images from the gallery */#include <stdio.h> #include <stdlib.h>//Use the Srand function, so this header file # include <io stream> #include "browsedir.hpp" #include <cv.h> #include 
Directory Operation code BROWSEDIR.HPP:
#pragma once#include <stdio.h> #include <stdlib.h> #include <direct.h> #include <string.h># Include <iostream> #include <io.h> #include <vector> #include <fstream> #include <string> Using namespace Std;class cbrowsedir{protected://holds the absolute path to the initial directory, ending with ' \ ' char M_szinitdir[_max_path];p ublic:// The default constructor Cbrowsedir ();//Set the initial directory to Dir, if return False, indicates that the directory is not available bool Setinitdir (const char *dir);//start traversing files of the type specified by filespec under the initial directory and its subdirectories Filespec You can use wildcard characters *?, you cannot include a path. If False, indicates that the traversal process was aborted by the user virtual bool Beginbrowse (const char *filespec);p rotected://traverse directory dir under the file specified by filespec//For subdirectories, Use iterative method//If return FALSE, indicates abort traversal file bool Browsedir (const char *dir,const char *filespec);//function Browsedir every file found, call Processfile and pass the file name as a parameter passed past//if return false, indicates abort traverse file//user can overwrite the function, add their own processing code virtual BOOL Processfile (const char *filename);// The function Browsedir each entry into a directory, invokes processdir//and passes the directory name being processed and the previous directory name as a parameter pass///If the initial directory is being processed, the parentdir=null//user can override the function to add their own processing code For example, the user can count the number of subdirectories here virtual void Processdir (const char *currentdir,const char *PARENTDIR);}; Cbrowsedir::cbrowsedir () {///Initialize M_SZINITDIRGETCWD (M_szinitdir,_max_path) with the current directory;//If the last letter of the directory is not ' \ ', add a ' \ ' int at the end Len=strlen (M_szinitdir); if (m_szinitdir[len-1]! = ' \ \ ') strcat (m_szinitdir, "\ \");} BOOL Cbrowsedir::setinitdir (const char *dir) {//dir is first converted to absolute path if (_fullpath (m_szinitdir,dir,_max_path) = = NULL) return false;//determine if the directory exists if (_chdir (m_szinitdir)! = 0) Return false;//if the last letter of the directory is not ' \ ', then at the end add a ' \ ' int len=strlen (M_SZINITDIR) if (m_szinitdir[len-1]! = ' \ \ ') strcat (m_szinitdir, "\ \"); return true;} BOOL Cbrowsedir::beginbrowse (const char *filespec) {processdir (m_szinitdir,null); return Browsedir (M_szinitdir, filespec);} BOOL Cbrowsedir::browsedir (const char *dir,const char *filespec) {_chdir (dir);//First Look for the files in dir that meet the requirements long hfile;_finddata_t Fileinfo;if ((Hfile=_findfirst (filespec,&fileinfo))! =-1) {do{//Check is not a directory//if not, then process if (! ( Fileinfo.attrib & _a_subdir) {char filename[_max_path];strcpy (filename,dir); strcat (filename,fileinfo.name);// cout << filename << endl;if (! Processfile (FilenaMe)) return false;}} while (_findnext (hfile,&fileinfo) = = 0); _findclose (hfile);} Find subdirectories in Dir//Because the processfile of the derived class may have changed//current directory when processing the file in Dir, so also reset the current directory to dir. Once the _findfirst has been performed, the relevant information may be recorded, so changing the directory//has no effect on _findnext. _chdir (dir); if ((Hfile=_findfirst ("* *", &fileinfo))! =-1) {do{//Check is not a directory//If yes, then check if it is not. Or, if not, iterate if ( Fileinfo.attrib & _a_subdir) {if (strcmp (Fileinfo.name, ".")! = 0 && strcmp (fileinfo.name, "...")! = 0) {char subdir[_max_path];strcpy (subdir,dir); strcat (subdir,fileinfo.name); Strcat (subdir, "\ \"); Processdir (Subdir,dir); if (! Browsedir (Subdir,filespec)) return false;}} while (_findnext (hfile,&fileinfo) = = 0); _findclose (hfile);} return true;} BOOL Cbrowsedir::P rocessfile (const char *filename) {return true;} void Cbrowsedir::P rocessdir (const char *currentdir,const char *parentdir) {}//a subclass derived from Cbrowsedir,   Used to count the number of files and subdirectories in the directory class Cstatdir:public Cbrowsedir{protected:int m_nfilecount; Save file number int m_nsubdircount; Save subdirectory number public:vector<string> vec_files;public://default constructor CStatdir () {//Initialize data members M_nfilecount and m_nsubdircountm_nfilecount=m_nsubdircount=0;} Returns the number of files int getfilecount () {return m_nfilecount;} Returns the number of subdirectories int getsubdircount () {///Because the function is also called when the initial directory is entered processdir,//so minus 1 is the true number of subdirectories. return m_nsubdircount-1;} void Visitedfiles (Ostream&out) {for (Vector<string>::iterator iter=vec_files.begin (); iter!=vec_files.end (); iter++) {out<<*iter<< "\ n";}} virtual bool Beginbrowse (const char *filespec) {string file_exts=filespec;vector<string> vec_exts;size_t Pos_ start=0;size_t Npos=file_exts.find ("|"); String Exts;while (Npos!=string::npos) {exts=file_exts.substr (Pos_start,npos-pos_start); Vec_exts.push_back (exts); Pos_start=npos+1;npos=file_exts.find ("|", Pos_start);} Exts=file_exts.substr (Pos_start); Vec_exts.push_back (exts); bool Sign=true;for (Vector<string>::iterator ITER =vec_exts.begin (); Iter!=vec_exts.end (); iter++) {exts=*iter;if (! Cbrowsedir::beginbrowse (Exts.c_str ())) {Sign=false;}} return sign;} protected://Overwrite virtual function processfile, each call, the number of files plus 1virtual bool ProceSsfile (const char *filename) {m_nfilecount++;//cout<<filename<<endl;vec_files.push_back (filename); Return Cbrowsedir::P rocessfile (filename);} Overwrite virtual function Processdir, per call, number of subdirectories plus 1virtual void processdir (const char *currentdir,const char *parentdir) {m_nsubdircount+ +; Cbrowsedir::P rocessdir (Currentdir,parentdir);};


Reference documents:
1:http://blog.chinaunix.net/uid-21228455-id-2406483.html generates a distinct random number of k less than n
2:http://blog.csdn.net/kongfanyu/article/details/6387642
3:http://wenku.baidu.com/link?url=eyvsyysirpdfidgav7begz16gciinu_ Azcpdtpphl4v5y9tvvqzxweviyb74aqvuw2lnpshtwrsxa05t0bz3yqgxui_hvbyd5usn1pmu_m_
Generate N different random numbers (C + +, range 0~n-1)