Rust - String First Words

First words. It is sometimes helpful to get a string containing the first words from another string. This can be accomplished in Rust with a for-loop over chars.

With the chars iterator, we access each char in a string in order. And with enumerate() we can get each char and its index. We can count the spaces in this way.

Example. Consider the first_words() example function. This function receives a source str reference, and a count of words (as an isize) we want to keep.

Step 1 We call the enumerate function over the chars iterator. We use a for-loop to iterate over these values.

iter

Loop, String Chars

Step 2 When each space is encountered, we reduce the word count required. This is how we count words.

Step 3 We return a new string up to the point where zero further words are required. We use unwrap on a substring.

Substring

Step 4 If we do not have enough words in the source string to keep, we just return the entire string.

fn first_words(source: &str, mut count: isize) -> String { // Step 1: use enumerate over chars iterator to get indexes and chars. for (i, c) in source.chars().enumerate() { // Step 2: reduce count on space char. if c == ' ' { count -= 1; // Step 3: return new string up to this point when no more words needed. if count == 0 { return source.get(0..i).unwrap().to_string(); } } } // Step 4: just return entire string. source.to_string() } fn main() { let value = "there are many reasons"; // Test our first words method. let result1 = first_words(value, 2); println!("[{result1}]"); let result2 = first_words(value, 3); println!("{result2}"); let result3 = first_words(value, 100); println!("{result3}"); }

[there are] there are many there are many reasons

Performance notes. The function first_words will allocate and copy the initial words into a string. It would be possible to use indexes to avoid this copy if needed.

Summary. Simple string-processing functions can be written in safe Rust code. And with the safety in Rust, we can be assured the code will not cause any future memory-related issues.

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This page was last updated on Sep 12, 2023 (new).

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