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.

using namespace std;

int main()
    string line;
    vector<string> lines;
    //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;

    //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
    return 0;

The output is like this:


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,


Thank you.

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

  1. jones says:

    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));

    • Ranjeet V says:

      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))”

  2. CodyIs says:

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

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