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Apr 6th, 2007

Reading in a random line from a file

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Hi. I am new to the forum and I had a question. I have looked around and haven't really been able to find exactly what I am looking for.

I am looking for the code in C++ to read in a random single line with spaces from a file.

This is the file I have...(example.txt)

the big green
forrest gump
blades of glory
glory road
animal house
pirates of the carribean
the seven little foys
the lord of the rings
the green mile
cast away
star wars
it
now and then
the princess bride

This is the code for a function I have so far, but it just prints out the entire file.

c++ Syntax (Toggle Plain Text)
 
void file(){
string line;
ifstream myfile ("example.txt");
if (myfile.is_open())
{
while (! myfile.eof())
{
getline (myfile,line);
cout << line << endl;
}
myfile.close();
}
else cout << "Unable to open file";
}
 
void file(){
string line;
ifstream myfile ("example.txt");
if (myfile.is_open())
{
while (! myfile.eof())
{
getline (myfile,line);
cout << line << endl;
}
myfile.close();
}
else cout << "Unable to open file";
}

how do i change this so that it reads in just a random single line?

Last edited by kylcrow; Apr 6th, 2007 at 2:22 pm.

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kylcrow

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Apr 6th, 2007

Re: Reading in a random line from a file

just count the lines as they are read and stop when it has read the line you need. Toss all lines but the last one away.

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Apr 6th, 2007

Re: Reading in a random line from a file

1. Read in each line into a string array or std::vector.

2. Whilst at the same time incrementing a counter, which counts each line.

3. Using srand pick a number which is less than the max line count.

4. Use that number as the index to the array of strings/ vector and output to the screen.

Alternatively you can do something similar without the need for a temporary storage device (vector etc).

Last edited by iamthwee; Apr 6th, 2007 at 2:25 pm.

iamthwee

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Apr 6th, 2007

Re: Reading in a random line from a file

I think I understand, but I am new to C++ so could you show me an example of the actual code? It was be greatly appreciated.

kylcrow

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Apr 6th, 2007

Re: Reading in a random line from a file

Well the first thing you need to do is figure out how to generate random numbers.

iamthwee

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Apr 6th, 2007

Re: Reading in a random line from a file

I don't have my compiler so this is not tested.

C++ Syntax (Toggle Plain Text)
#include <cstdlib> 
#include <iostream>
#include <fstream>
#include <vector>
#include <string>
#include <ctime> 
 
class RandomCrap
{
public:
   int show(int p)
   {
      srand( ( unsigned )time( 0 ) ); 
      int random_integer; 
 
      random_integer = ( rand ( ) % p ) + 1;
      return random_integer;
    }
};
 
int main() 
{ 
    std::ifstream read ("example.txt");
 
    std::string line;
    std::vector < std::string > stuff;
 
    int counter = 0;
    while (getline(read, line, '\n'))
    {
      stuff.push_back( line );
      counter++;
    }
 
 
    read.close();
 
    RandomCrap a;
    std::cout << stuff[a.show( counter - 1 )];
 
    std::cin.get(); 
    return 0;
}
#include <cstdlib> 
#include <iostream>
#include <fstream>
#include <vector>
#include <string>
#include <ctime> 

class RandomCrap
{
public:
   int show(int p)
   {
      srand( ( unsigned )time( 0 ) ); 
      int random_integer; 
    
      random_integer = ( rand ( ) % p ) + 1;
      return random_integer;
    }
};

int main() 
{ 
    std::ifstream read ("example.txt");
    
    std::string line;
    std::vector < std::string > stuff;
    
    int counter = 0;
    while (getline(read, line, '\n'))
    {
      stuff.push_back( line );
      counter++;
    }
    

    read.close();

    RandomCrap a;
    std::cout << stuff[a.show( counter - 1 )];
    
    std::cin.get(); 
    return 0;
}

I think it might always miss the first line. Meh?

Last edited by iamthwee; Apr 6th, 2007 at 3:30 pm.

iamthwee

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Apr 6th, 2007

Re: Reading in a random line from a file

the basic problem here is that we need to select one item at random
when we do not know beforehand how many items there are. the most
common example of this in real code is when a random element has to
be picked from a list (size unknown). there is a well known algorithm to
do this without having to pass through the entire sequence:

C++ Syntax (Toggle Plain Text)
string random_line( ifstream file )
{
  string line ;
  int nlines = 0 ;
  while( getline( file, line ) )
     if(  ( rand() % ++nlines  ) == 0 ) break ;
  return line ;
}
string random_line( ifstream file )
{
  string line ;
  int nlines = 0 ;
  while( getline( file, line ) )
     if(  ( rand() % ++nlines  ) == 0 ) break ;
  return line ;
}

the variable nlines counts the number of lines as they are read.
for any line, rand() % ++nlines == 0 is true with probability
1 / nlines. the first line is initially selected (with probability 1),
the second line replaces it with probability 1/2, the third line will
replace the survivor (either line one or line 2, each of which are
equally likely) with probability 1/3, and so on.therefore, for each
of the nlines lines seen so far is selected with probability
1/nlines.

if i remember right, this was first seen widely in some code
written by brian kernighan.

Last edited by vijayan121; Apr 6th, 2007 at 8:55 pm. Reason: add the last 2 lines

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Apr 7th, 2007

Re: Reading in a random line from a file

THERE IS A SERIOUS ERROR IN THE PREVIOUS POST.

Quote ...

there is a well known algorithm to
do this without having to pass through the entire sequence:
C++ Syntax (Toggle Plain Text)
 string random_l[ine( ifstream file )
{
   string line ;
   int nlines = 0 ;
   while( getline( file, line ) )
      if(  ( rand() % ++nlines  ) == 0 ) break ;
   return line ;
}
string random_l[ine( ifstream file ) { string line ; int nlines = 0 ; while( getline( file, line ) ) if( ( rand() % ++nlines ) == 0 ) break ; return line ; }

IT SHOULD BE
there is a well known algorithm to
do this without having to copy the entire sequence
into a buffer having random access :

C++ Syntax (Toggle Plain Text)
 string random_line( ifstream file )
{
      string line ;
      string selected_line ; // added
      int nlines = 0 ;
      while( getline( file, line ) )
         if(  ( rand() % ++nlines  ) == 0 ) // break ;
                  selected_line = line ;      
    return selected_line ;
}
 string random_line( ifstream file )
{
      string line ;
      string selected_line ; // added
      int nlines = 0 ;
      while( getline( file, line ) )
         if(  ( rand() % ++nlines  ) == 0 ) // break ;
                  selected_line = line ;      
    return selected_line ;
}

Last edited by vijayan121; Apr 7th, 2007 at 2:11 am. Reason: add code tag

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Mar 13th, 2011

Problems with the method

Both of the previous algorithms are seriously flawed. They never choose the first line, while the second line has a 50% probability of being picked up. For any subsequent line, the probability of being picked up is decreasing.

Another idea would be to jump in the file (fseek) at a random offset and read the current line. However, this would favorize the large lines. We can pick a line pointed by the random offset with the probability of 1/string_length. This means, if the average length of a line is 100, we will read on average about 100 lines until picking one up. Even if the procedure is statistically correct, it is inefficient, as it reads much more lines than needed.

razvan1

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Mar 14th, 2011

Re: Reading in a random line from a file

Nice info, but 4 years too late.

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