Word count

Counting the words in a string can help determine if the string is appropriate for a specific usage in a program. For example, a summary may need to be a certain number of words.

In Rust, we are often focused on performance. Implementing a word counting function should use iteration over the chars in a string, and not allocate excessively.

Example program

To begin, we introduce the count_words function. This returns a usize, which is the number of words in the argument string.

Info We receive a str reference in count_words. This is ideal as a either a string literal or String can be passed to the function.

Next We iterate over the characters in the string by calling chars(). We store the previous char in a "mut" local variable.

Finally We test the chars for ASCII character ranges using the built-in "is_ascii" functions in Rust.

Tip A word is defined as a sequence of letters, digits and punctuation following a whitespace char.

fn count_words(s: &str) -> usize {
    let mut total = 0;
    let mut previous = char::MAX;
    for c in s.chars() {
        // If previous char is whitespace, we are on a new word.
        if previous.is_ascii_whitespace() {
            // New word has alphabetic, digit or punctuation start.
            if c.is_ascii_alphabetic() || c.is_ascii_digit() || c.is_ascii_punctuation() {
                total += 1;
            }
        }
        // Set previous.
        previous = c;
    }
    if s.len() >= 1 {
        total += 1
    }
    total
}

fn main() {
    let mut data = String::new();
    data.push_str("cat, ");
    data.push_str("bird, ");
    data.push_str("and dog");

    // Borrow String to call count_words.
    let count = count_words(&data);
    println!(" DATA: {}", data);
    println!("COUNT: {}", count);
}DATA: cat, bird, and dog
COUNT: 4

Function results

In our simple example, the number of words is correctly determined. When no whitespace ends the string, it is important we add the final word to the count.

Many functions in other languages can be ported to Rust with minimal changes. The borrow checker is not involved, so memory issues are not a concern.