String length

In Rust programs, strings have a byte length. This is separate from the char count—some chars may have multiple bytes.

With the len function, we get the number of bytes in a string. For ASCII strings this is sufficient to return the length of a string.

First example

Here we compute the length of 3 separate strings. We have a string literal, a String built with from, and a string concatenation.

fn main() {
    // Get length of string literal.
    let value = "test";
    println!("Length = {}", value.len());

    // Get length of string.
    let value2 = String::from("x");
    println!("Length = {}", value2.len());

    // Get length of mutable string built from concat.
    let mut value3 = String::from("x");
    value3 += "y";
    println!("Length = {}", value3.len());
}
Length = 4
Length = 1
Length = 2

Empty strings

Some strings may have length of 0—these are empty strings. The is_empty function returns true if called on zero length strings.

fn main() {
    // This string is not empty.
    let value = "?";
    if value.is_empty() {
        // Not reached.
        println!("...")
    }

    // This string is empty (it has length of 0).
    let value2 = "";
    if value2.is_empty() {
        println!("IS EMPTY")
    }
}
IS EMPTY

Substring length

It is possible to get a slice (substring) from an existing string, and then get its length. We use the len() function here.

fn main() {
    let value = "xyza";

    // Get substring and print length.
    let result1 = &value[1..3];
    println!("Substring = {}", result1);
    println!("Length = {}", result1.len());

    // Get string from slice and print length.
    let result2 = String::from(result1);
    println!("Length = {}", result2.len());
}
Substring = yz
Length = 2
Length = 2

Summary

We use the len() function to get the byte count of a string in Rust. This tells us the number of bytes, which is the same as the string length for ASCII strings.