I have a txt file with around 5000 lines in it, I would like to print a random line from that file. I have tried using readline but cant figure out how to choose which line to print using integers. What would be the fastest way to do this without using much memory?

Here is my code

const readline = require("readline");
const fs = require("fs");

const file = readline.createInterface({
    input: fs.createReadStream('file.txt'),
    output: process.stdout,
    terminal: false
});

file.on('line', (line) => {
    console.log(line);
})

CodePudding user response：

Taken from this answer, here is a code snippet that should work.

const fs = require("fs");
// note this will be async
function getRandomLine(filename, callback){
  fs.readFile(filename, "utf-8", function(err, data){
    if(err) {
        throw err;
    }

    // note: this assumes `data` is a string - you may need
    //       to coerce it
    data  = '';
    var lines = data.split('\n');
    
    // choose one of the lines...
    var line = lines[Math.floor(Math.random()*lines.length)]

    // invoke the callback with our line
    callback(line);
 })
}```

CodePudding user response：

The straightforward solution would be:

1. read file
2. split by \n, save to array
3. get number of lines (== length of array)
4. generate random number between 0 and number of lines
5. return array[generated_number]

But it might use quite a bit of memory if your file is really large.

so without reading the entire file, off the top of my head - Maybe seek to a random position, then read until you hit \n, and then again read until \n, then split by \n and pick the middle part of the array?

Note- this will break if the last line isn't followed by a new line. (if the random position is in the last two lines)

CodePudding user response：

two approaches here:

1. read all file, choose random number between 1 and 5000 and return that line (not good for general solution)
2. pre-define random line and return it when getting to that line.

const readline = require("readline");
const fs = require("fs");

const file = readline.createInterface({
    input: fs.createReadStream('file.txt'),
    output: process.stdout,
    terminal: false
});
const radnomLineNumber = getRandomInt(5000);
let i = 0;
file.on('line', (line) => {
   if (randomLineNumber === i) {
      console.log(line);
      return;
   }
   i  ;
});