parse Examples: Convert String to Integer
This page was last reviewed on Feb 6, 2023.
Dot Net Perls
Parse. Strings can be converted to integers in Rust with the parse() function. We can parse just parts of strings with get(). With the turbofish operator, we can parse to specific types.
Round-trip parsing. With the to_string function, we can convert an integer into a string. And then parse() can convert the string back into a u32 integer.
First example. Here we want to parse to a specific type like a usize. We can use the special Rust turbofish operator in an if-let statement to parse to a usize.
Detail With parse(), we can unwrap() the result. But using if let Ok() is a more elegant and clean syntax for parse.
fn main() { let data = "1200"; // Parse to a specific type with turbofish. if let Ok(result) = data.parse::<usize>() { println!("USIZE RESULT: {}", result); } }
Get example. Suppose we have some digits inside of another string. We can extract a slice from the string with a function like get(). Then we can parse the number.
Detail We use len() to extract the range of characters up to 2 places before the end. We unwrap() the result.
String Length
fn main() { let value = "x 1234 y"; // Get inner part of string we want to parse. let inner_part = value.get(2..value.len() - 2).unwrap(); println!("INNER PART: {}", inner_part); // Parse the inner part. let result: u32 = inner_part.parse().unwrap(); println!("PARSED: {0}", result); }
Convert to str. Sometimes we want to convert a string to an integer, and other times we want the opposite conversion. Here we convert an integer into a str with to_string.
fn main() { // Convert an int into a string, and then back into an int. // ... Use u32 to mean 32-bit int. let value = 5678; let result = value.to_string(); let parsed: u32 = result.parse().unwrap(); // Results. println!("RESULT: {} {} {}", value, result, parsed); }
RESULT: 5678 5678 5678
String array. Often we are dealing with arrays of str in Rust programs. We can parse these with a for-loop over the array—the concepts remain the same.
String Array
fn main() { let values: [&str; 2] = ["123", "456"]; // Loop over string array, parsing each value. for value in &values { let parsed: u32 = value.parse().unwrap(); println!("PARSED+1: {}", parsed + 1); } }
PARSED+1: 124 PARSED+1: 457
Optimized function. If we do not need many features, we can write a simple loop-based parsing function in Rust. This just iterates over bytes and multiplies the result by 10.
Loop, String Chars
fn parse_fast(value: &str) -> i32 { // Parse with bytes loop. let mut result = 0; for b in value.bytes() { result = 10 * result + ((b as i32) - 48); } result } fn main() { println!("RESULT: {}", parse_fast("123") == 123); }
RESULT: true
Custom function benchmark. If we have a Rust program that parses i32 values excessively, should we use the custom optimized function? This benchmark tests the function.
Version 1 This code calls the std parse function and specifies the turbofish operator to get an i32 result.
Version 2 This version uses the parse_fast function, which returns a value directly and does not support error handling.
Result The custom function was 10 times faster. If a program does nothing but parse, this could make it 10 times faster.
use std::time::*; fn parse_fast(value: &str) -> i32 { // Parse with bytes loop. let mut result = 0; for b in value.bytes() { result = 10 * result + ((b as i32) - 48); } result } fn main() { if let Ok(max) = "100000000".parse::<usize>() { let number = String::from("123"); // Version 1: use parse function. let t0 = Instant::now(); for _ in 0..max { if let Ok(result) = number.parse::<i32>() { if result != 123 { return; } } } println!("{}", t0.elapsed().as_millis()); // Version 2: use simple parse fast function. let t1 = Instant::now(); for _ in 0..max { if parse_fast(&number) != 123 { return; } } println!("{}", t1.elapsed().as_millis()); } }
355 ms parse 31 ms parse_fast
A review. With parse() we can convert a string into numeric types like u32. We must unwrap the result of parse. To parse ranges of strings, we can first call get().
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Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Feb 6, 2023 (edit link).
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