Trying to iterate 2 files in rust - rust

Question

I am trying to read 2 files and compare each item in each file to see if they are equal.
use std::fs::File;
use std::io::{BufRead, BufReader};
fn main() {
let filename1 = "file1.txt";
let filename2 = "file2.txt";
// Open the file in read-only mode (ignoring errors).
let file = File::open(filename1).unwrap();
let reader = BufReader::new(file);
let file2 = File::open(filename2).unwrap();
let mut reader2 = BufReader::new(file2);
// Read the file line by line using the lines() iterator from std::io::BufRead.
for line1 in reader.lines() {
let line = line.unwrap(); // Ignore errors.
for line2 in reader2.lines() {
let line2 = line2.unwrap(); // Ignore errors.
if line2 == line1 {
println!("{}",line2)
}
}
}
}
However, this doesn't work. How do I apply a loop over a loop with Buffers?

Answer 1

Your first problem is a duplicate of this question. TLDR: you need to call by_ref if you want to be able to reuse reader2 after calling its lines method (eg. in the next loop iteration).
With that your code will compile but won't work, because once you have processed the first line of the first file you are at the end of the second file, so the second file will appear empty when processing the subsequent lines. You can fix that by rewinding the second file for each line. The minimal set of changes that will make your code work is:
use std::io::Read;
use std::io::Seek;
use std::io::SeekFrom;
use std::fs::File;
use std::io::{BufRead, BufReader};
fn main() {
let filename1 = "file1.txt";
let filename2 = "file2.txt";
// Open the file in read-only mode (ignoring errors).
let file = File::open(filename1).unwrap();
let reader = BufReader::new(file);
let file2 = File::open(filename2).unwrap();
let mut reader2 = BufReader::new(file2);
// Read the file line by line using the lines() iterator from std::io::BufRead.
for line1 in reader.lines() {
let line1 = line1.unwrap(); // Ignore errors.
reader2.seek (SeekFrom::Start (0)).unwrap(); // <-- Add this line
for line2 in reader2.by_ref().lines() { // <-- Use by_ref here
let line2 = line2.unwrap(); // Ignore errors.
if line2 == line1 {
println!("{}",line2)
}
}
}
}
However this will be pretty slow. You can make it much faster by reading one of the files in a HashSet and checking if each line of the other file is in the set:
use std::collections::HashSet;
use std::fs::File;
use std::io::{BufRead, BufReader};
fn main() {
let filename1 = "file1.txt";
let filename2 = "file2.txt";
// Open the file in read-only mode (ignoring errors).
let file = File::open(filename1).unwrap();
let reader = BufReader::new(file);
let file2 = File::open(filename2).unwrap();
let reader2 = BufReader::new(file2);
let lines2 = reader2.lines().collect::<Result<HashSet<_>, _>>().unwrap();
// Read the file line by line using the lines() iterator from std::io::BufRead.
for line1 in reader.lines() {
let line1 = line1.unwrap(); // Ignore errors.
if lines2.contains (&line1) {
println!("{}", line1)
}
}
}
Finally you can also read both files into HashSets and print out the intersection:
use std::collections::HashSet;
use std::fs::File;
use std::io::{BufRead, BufReader};
fn main() {
let filename1 = "file1.txt";
let filename2 = "file2.txt";
// Open the file in read-only mode (ignoring errors).
let file = File::open(filename1).unwrap();
let reader = BufReader::new(file);
let lines1 = reader.lines().collect::<Result<HashSet<_>, _>>().unwrap();
let file2 = File::open(filename2).unwrap();
let reader2 = BufReader::new(file2);
let lines2 = reader2.lines().collect::<Result<HashSet<_>, _>>().unwrap();
for l in lines1.intersection (&lines2) {
println!("{}", l)
}
}
As a bonus this last solution will remove duplicate lines. OTOH it won't preserve the order of the lines.

Answer 2

Although I found a solution, it is horribly slow. If anyone has a better solution to find items similar in 2 files please let me know.
use std::fs::File;
use std::io::{BufRead, BufReader};
fn main() {
let mut vec2 = findvec("file1.txt".to_string());
let mut vec3 = &findvec("file2.txt".to_string());
for line in vec2 {
for line2 in vec3 {
if line.to_string() == line2.to_string() {
println!("{}",line.to_string());
}
}
}
}
fn findvec(filename: String) -> Vec<String> {
// Open the file in read-only mode (ignoring errors).
let file = File::open(filename).unwrap();
let reader = BufReader::new(file);
// blank vector
let mut myvec = Vec::new();
// Read the file line by line using the lines() iterator from std::io::BufRead.
for (index, line) in reader.lines().enumerate() {
let line = line.unwrap(); // Ignore errors.
// Show the line and its number.
myvec.push(line);
}
myvec
}