Exercise – fixing the word counter

Armed with the basics, it's time to put our knowledge to use! Here, we have a program that counts instances of words in a text file, which is passed to it as an argument. It's almost complete, but has a few bugs that the compiler catches and a couple of subtle ones. Here's our incomplete program:

// word_counter.rs

use std::env;
use std::fs::File;
use std::io::prelude::BufRead;
use std::io::BufReader;

#[derive(Debug)]
struct WordCounter(HashMap<String, u64>);

impl WordCounter {
    fn new() -> WordCounter {
        WordCounter(HashMap::new());
    }

    fn increment(word: &str) {
        let key = word.to_string();
        let count = self.0.entry(key).or_insert(0);
        *count += 1;
    }

    fn display(self) {
        for (key, value) in self.0.iter() {
            println!("{}: {}", key, value);
        }
    }
}

fn main() {
    let arguments: Vec<String> = env::args().collect();
    let filename = arguments[1];
    println!("Processing file: {}", filename);
    
    let file = File::open(filenam).expect("Could not open file");
    let reader = BufReader::new(file);
    
    let mut word_counter = WordCounter::new();

    for line in reader.lines() {
        let line = line.expect("Could not read line");
        let words = line.split(" ");
        for word in words {
            if word == "" {
                continue
            } else {
                word_counter.increment(word);
            }
        }
    }
    
    word_counter.display();
}

Go ahead and type the program into a file; try to compile and fix all the bugs with the help of the compiler. Try to fix one bug at a time and get feedback from the compiler by recompiling the code. The point of this exercise, in addition to covering the topics of this chapter, is to make you more comfortable with the error messages from the compiler, which is an important mental exercise in getting to know more about the compiler and how it analyzes your code. You might also be surprised to see how the compiler is quite smart in helping you removing errors from the code.

Once you are done fixing the code, here are some exercises for you to try so that you can flex your muscles a bit further:

• Add a filter parameter to the display method of WordCounter for filtering the output based on the count. In other words, display a key/value pair only if the value is greater than that filtering value.
• Since HashMaps store their values randomly, the output is also random every time you run the program. Try to sort the output. The HashMap's values method may be useful.
• Take a look at the display method's self parameter. What happens if you remove the & operator before self?