Time

SystemTime::now

The clock on the wall is SystemTime, and now reads it. The value is opaque on purpose, because wall clocks can jump backwards:

use std::time::SystemTime; fn main() { let start = SystemTime::now(); println!("{:?}", start); }

Debug output shows the raw timestamp. SystemTime is for talking to the outside world, never for measuring speed.

Instant stopwatches

For measuring speed use Instant, a stopwatch that only moves forward. Read it once, do the work, then ask how much time passed:

use std::time::Instant; fn main() { let t0 = Instant::now(); let mut sum: u64 = 0; for i in 0..1000 { sum += i; } println!("sum: {} in {:?}", sum, t0.elapsed()); }

Duration math

A Duration is a span of time with friendly constructors. Build values, add them, and read them back in any unit you need:

use std::time::Duration; fn main() { let wait = Duration::from_secs(2) + Duration::from_millis(500); println!("secs: {}", wait.as_secs()); println!("millis: {}", wait.as_millis()); }

Adding durations is plain addition with plus. Convert with as_secs or as_millis depending on the precision you need.

Elapsed helper

Most programs repeat the same stopwatch dance, so wrap it in a helper. Pass the start time by reference and get seconds back as a float:

use std::time::Instant; fn elapsed_secs(t0: &Instant) -> f64 { t0.elapsed().as_secs_f64() } fn main() { let t0 = Instant::now(); println!("lap: {}", elapsed_secs(&t0)); }

One helper keeps every benchmark in the program consistent. Parsing lessons next give the stopwatch something to measure.

Next: Parsing strings

Article author: Arthur Isaev