Stopwatch
- Dart course
- Advanced
- Lesson 74/83
- 5 min
Learn time measurement with Stopwatch.
Stopwatch
Stopwatch is Dart's built-in timer class (in dart:core). It starts with start() and stops with stop(). elapsedMicroseconds gives the elapsed microseconds; elapsed gives a Duration.
Example: final sw = Stopwatch()..start(); /* work */ sw.stop(); print(sw.elapsedMicroseconds);. ..start() is the cascade operator; it creates the object and starts it at once. isRunning tells whether it is still running. Stopwatch measures the real time of a loop or operation; the result depends on the machine and load.
Timing a loop with Stopwatch
To time a loop, Stopwatch is started before the loop and stopped after. The elapsed time is printed. But the time is not deterministic; it depends on the machine and load. So in tests it is safer to print the deterministic result of the loop rather than the time itself.
Example: var sum = 0; for (var i = 0; i < 1000; i++) { sum += i; } print('Sum: $sum'); print('Running: ${sw.isRunning}');. Here sum (0+1+...+999 = 499500) is deterministic. sw.isRunning is always false after stop(). Together they show both that the timing happened and that the result is correct.
Practice A loop with Stopwatch
In main start the timer with final sw = Stopwatch()..start();. Write a for loop with var sum = 0;: i from 0 to 999 (exclusive 1000), sum += i. Then call sw.stop();. Print with print('Sum: $sum') and print('Running: ${sw.isRunning}').
Starter code
void main() {
// create and start the Stopwatch
// compute sum in a 0..999 loop
// stop the Stopwatch
// print sum and timer status
}Sample solution
void main() {
final sw = Stopwatch()..start();
var sum = 0;
for (var i = 0; i < 1000; i++) {
sum += i;
}
sw.stop();
print('Sum: $sum');
print('Running: ${sw.isRunning}');
}Expected output
Sum: 499500 Running: false
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Starting the Stopwatch
final sw = Stopwatch()..____();final sw = Stopwatch()..start();Explanation: Stopwatch()..start() starts the timer.
2 What does a Stopwatch measure?
- Memory
- Time Correct answer
- Disk
- Network
Explanation: a Stopwatch measures time.
3 Match the Stopwatch parts
| Term | Meaning |
|---|---|
start |
Starts |
stop |
Stops |
elapsed |
Elapsed |
isRunning |
Is running |