How to use the same iterator twice, once for counting and once for iteration?

Question

It seems that an iterator is consumed when counting. How can I use the same iterator for counting and then iterate on it?

I'm trying to count the lines in a file and then print them. I am able to read the file content, I'm able to count the lines count, but then I'm no longer able to iterate over the lines as if the internal cursor was at the end of the iterator.

use std::fs::File;
use std::io::prelude::*;

fn main() {
    let log_file_name = "/home/myuser/test.log";
    let mut log_file = File::open(log_file_name).unwrap();
    let mut log_content: String = String::from("");
    //Reads the log file.
    log_file.read_to_string(&mut log_content).unwrap();
    //Gets all the lines in a Lines struct.
    let mut lines = log_content.lines();
    //Uses by_ref() in order to not take ownership
    let count = lines.by_ref().count();
    println!("{} lines", count); //Prints the count
                                 //Doesn't enter in the loop
    for value in lines {
        println!("{}", value);
    }
}

Iterator doesn't have a reset method, but it seems the internal cursor is at the end of the iterator after the count. Is it mandatory to create a new Lines by calling log_content.lines() again or can I reset the internal cursor?

For now, the workaround that I found is create a new iterator:

use std::fs::File;
use std::io::prelude::*;

fn main() {
    let log_file_name = "/home/myuser/test.log";
    let mut log_file = File::open(log_file_name).unwrap();
    let mut log_content: String = String::from("");
    //Reads the log file.
    log_file.read_to_string(&mut log_content).unwrap();
    //Counts all and consume the iterator
    let count = log_content.lines().count();
    println!("{} lines", count);
    //Creates a pretty new iterator
    let lines = log_content.lines();
    for value in lines {
        println!("{}", value);
    }
}

Answer 1

Calling count consumes the iterator, because it actually iterates until it is done (i.e. next() returns None).

You can prevent consuming the iterator by using by_ref, but the iterator is still driven to its completion (by_ref actually just returns the mutable reference to the iterator, and Iterator is also implemented for the mutable reference: impl<'a, I> Iterator for &'a mut I).

This still can be useful if the iterator contains other state you want to reuse after it is done, but not in this case.

You could simply try forking the iterator (they often implement Clone if they don't have side effects), although in this case recreating it is just as good (most of the time creating an iterator is cheap; the real work is usually only done when you drive it by calling next directly or indirectly).

So no, (in this case) you can't reset it, and yes, you need to create a new one (or clone it before using it).