ryu 0.2.8

Fast floating point to string conversion


Build Status Latest Version Rust Documentation Rustc Version 1.15+

Pure Rust implementation of Ryū, an algorithm to quickly convert floating point numbers to decimal strings.

The PLDI'18 paper Ryū: fast float-to-string conversion by Ulf Adams includes a complete correctness proof of the algorithm. The paper is available under the creative commons CC-BY-SA license.

This Rust implementation is a line-by-line port of Ulf Adams' implementation in C, https://github.com/ulfjack/ryu. The ryu::raw module exposes exactly the API and formatting of the C implementation as unsafe pure Rust functions. There is additionally a safe API as demonstrated in the example code below. The safe API uses the same underlying Ryū algorithm but diverges from the formatting of the C implementation to produce more human-readable output, for example 0.3 rather than 3E-1.

Requirements: this crate supports any compiler version back to rustc 1.15; it uses nothing from the Rust standard library so is usable from no_std crates.

ryu = "0.2"


fn main() {
    let mut buffer = ryu::Buffer::new();
    let printed = buffer.format(1.234);
    assert_eq!(printed, "1.234");


You can run upstream's benchmarks with:

$ git clone https://github.com/ulfjack/ryu c-ryu
$ cd c-ryu
$ bazel run -c opt //ryu/benchmark

And the same benchmark against our implementation with:

$ git clone https://github.com/dtolnay/ryu rust-ryu
$ cd rust-ryu
$ cargo run --example upstream_benchmark --release

These benchmarks measure the average time to print a 32-bit float and average time to print a 64-bit float, where the inputs are distributed as uniform random bit patterns 32 and 64 bits wide.

The upstream C code, the unsafe direct Rust port, and the safe pretty Rust API all perform the same, taking around 21 nanoseconds to format a 32-bit float and 31 nanoseconds to format a 64-bit float.

There is also a Rust-specific benchmark comparing this implementation to the standard library which you can run with:

$ cargo bench

The benchmark shows Ryu approximately 4-10x faster than the standard library across a range of f32 and f64 inputs. Measurements are in nanoseconds per iteration; smaller is better.

type=f32 0.0 0.1234 2.718281828459045 f32::MAX
RYU 3ns 28ns 23ns 22ns
STD 40ns 106ns 128ns 110ns
type=f64 0.0 0.1234 2.718281828459045 f64::MAX
RYU 3ns 50ns 35ns 32ns
STD 39ns 105ns 128ns 202ns


This library tends to produce more human-readable output than the standard library's to_string, which never uses scientific notation. Here are two examples:

  • ryu: 1.23e40, std: 12300000000000000000000000000000000000000
  • ryu: 1.23e-40, std: 0.000000000000000000000000000000000000000123

Both libraries print short decimals such as 0.0000123 without scientific notation.


Licensed under either of the following at your option.