Obtain data from a file dynamically - c

Question

I have text file emails.txt..this is how the entry in the text file..
Emails.txt
abc#gmail.com
sfs#yahoo.com
i have to obtain the data from file and select 2 entries from the data randomly..
could anyone suggest me the technique to do this.
Thanks

Answer 1

You can read the file twice, first time to count the number of lines, then generate two random numbers in the range of 0 to number_of_lines found and then read the file again while looking for the lines you are interested in or you can do it like this:
Filename: emails.c
#include
int main (int argc, char **argv)
{
// open a handler to your file (read)
FILE *fp = fopen("emails.txt", "r");
// check if we have successfully opened the file for reading
if (fp != NULL)
{
// in your case 256 characters is enough for line size
// since emails are not that long but if longer buffer overflow
// is very possible and its not helpful as stackoverflow.com is :p
char line_buffer[256];
// count the number of lines read
unsigned int lines_read = 0;
// read up to line size or until EOL (End of Line) or EOF (End of File)
// will return NULL on error or eof
while (fgets(line_buffer, sizeof(line_buffer), fp) != NULL) {
// use rand() and seed it with the number of lines read
if ((rand() % ++lines_read) == 0) {
// do something with this line, it was randomly picked
// for the example, will print it on the screen
printf("%s \n", line_buffer);
}
}
// close file handler as we don't need it anymore
fclose(fp);
}
// return to the OS
return 0;
}
NOTE: This is C implementation, so save as .c file.

Answer 2

If you are using c++, and not just c -- you can use something like the following code:
#include <iostream>
#include <fstream>
#include <time.h>
#include <vector>
using namespace std;
int getrand(int num, int notnum)
{
int result = 0;
while (true)
{
result = abs(rand()) % num + 1;
if (result != notnum)
{
return result;
}
}
}
int main()
{
ifstream emails;
srand(time(NULL));
emails.open("emails.txt");
string email;
vector<string> emailVector;
while (emails >> email)
{
emailVector.push_back(email);
}
int index1 = getrand(emailVector.size(), 0);
int index2 = getrand(emailVector.size(), index1);
cout << "email 1: " << emailVector[index1 - 1] << endl;
cout << "email 2: " << emailVector[index2 - 1] << endl;
}

Answer 3

This will work:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define BUF_SIZE 4096
#define RAND_COUNT 2
int count_lines(FILE *fp) {
char buf[BUF_SIZE];
int line_count=0;
fseek(fp, 0L, SEEK_SET);
while(fgets(buf, BUF_SIZE, fp) != NULL) {
line_count++;
}
fseek(fp, 0L, SEEK_SET);
return line_count;
}
int line_num(FILE *fp, char *buf, int line_num){
fseek(fp, 0L, SEEK_SET);
int i=0;
while(fgets(buf, BUF_SIZE, fp) != NULL) {
if (++i == line_num) {
return i;
}
}
return -1;
}
int main (int argc, const char * argv[]) {
FILE *fp=NULL;
char buf[BUF_SIZE];
char name[]="email.txt";
if((fp=fopen(name, "r"))==NULL){
printf("can't open: %s\n\n",name);
return -1;
}
int line_count=count_lines(fp);
printf("line count=%i\n",line_count);
srand ((unsigned int)time(NULL));
for (int i=1; i<=line_count; i++) {
line_num(fp,buf,i);
printf("%i = %s",i,buf);
}
for (int i=0; i<RAND_COUNT; i++) {
int a=(rand() % line_count);
line_num(fp,buf,a);
printf("line %i = %s\n",a,buf);
}
fclose(fp);
return 0;
}