How to fetch a random line from a text file in C++

In this tutorial, we are going to learn how to fetch a random line from a text file in C++. This is very easy to do. We have to find a random number using rand() function and get a line from the text file.

Fetch a random line from a text file in C++

To fetch a random line from a text file in C++, we need to follow the given steps.

• Save the file in the same directory as the program.
• Use ifstream to operate on the file. Include fstream header to use this.
• Call the function srand() with argument ‘time(0)’ or ‘time(NULL)’ to make sure rand() function returns different random number for each call.
• Count the number of total lines in the file using getline() function of C++. If you don’t know how to do it. See here: Count the number of lines in a text file in C++
• Store the lines of the text file in a string vector using push_back() function. If you don’t know about push_back() function, See this: push_back() and pop_back() function in C++
• Generate a random number lying between 0 and count of total lines using rand() function and modulus operator.
• Fetch the line for which the line index is equal to the random number. Assuming the indexing starts from 0.

Here is the example program for fetching a random line from a text file. See the code.

#include<iostream>
#include<stdlib.h>
#include<fstream>
#include<time.h>
#include<vector>
using namespace std;

int main()
{
    string line;
    vector<string> lines;
    
    srand(time(0));
    
    //input file stream
    ifstream file("A_file.txt");
    
    //count number of total lines in the file and store the lines in the string vector
    int total_lines=0;
    while(getline(file,line))
    {
   		total_lines++; 
    lines.push_back(line);	
  }
   

    //generate a random number between 0 and count of total lines
    int random_number=rand()%total_lines;
    
    
    
    //fetch the line where line index (starting from 0) matches with the random number
    
    cout<<lines[random_number];
  
    
    return 0;
}

The output is like this:

Second

The text file in the given program has ten lines and every line has a string specifying the line number. If I run the program again, I get a different random number, hence different outputs. For example,

Eighth

Thank you.

3 responses to “How to fetch a random line from a text file in C++”

1. jones says:

   January 19, 2021 at 11:46 pm

   Why am I unable to use this more than once? If I use it a second time i get “conversion from ‘time_t’ to ‘unsigned int’, possible loss of data” from srand(time(0));

   Reply
   • Ranjeet V says:

     January 20, 2021 at 9:07 pm

     The function time() returns time_t and srand() takes unsigned int as input. The statement “srand(time(0))” involves an implicit cast from time_t to unsigned_int. For some reason, your system is facing a loss of data while this conversion takes place. Try the following statement that does explicit casting– “srand((unsigned int)time(0))”

     Reply
2. CodyIs says:

   January 25, 2021 at 7:35 am

   How can I do the same but make the program read a line from 4 text files (randomly) instead of 1?

   Reply