Loops
This page shows how you can control the flow of your Dart code using loops and supporting statements:
-
for
loops -
while
anddo while
loops -
break
andcontinue
You can also manipulate control flow in Dart using:
-
Branching, like
if
andswitch
-
Exceptions, like
try
,catch
, andthrow
For loops
You can iterate with the standard for
loop. For example:
var message = StringBuffer('Dart is fun');
for (var i = 0; i < 5; i++) {
message.write('!');
}
Closures inside of Dart’s for
loops capture the value of the index.
This avoids a common pitfall found in JavaScript. For example, consider:
var callbacks = [];
for (var i = 0; i < 2; i++) {
callbacks.add(() => print(i));
}
for (final c in callbacks) {
c();
}
The output is 0
and then 1
, as expected. In contrast, the example
would print 2
and then 2
in JavaScript.
Sometimes you might not need to know the current iteration counter
when iterating over an Iterable
type, like List
or Set
.
In that case, use the for-in
loop for cleaner code:
for (final candidate in candidates) {
candidate.interview();
}
To process the values obtained from the iterable,
you can also use a pattern in a for-in
loop:
for (final Candidate(:name, :yearsExperience) in candidates) {
print('$name has $yearsExperience of experience.');
}
Iterable classes also have a forEach() method as another option:
var collection = [1, 2, 3];
collection.forEach(print); // 1 2 3
While and do-while
A while
loop evaluates the condition before the loop:
while (!isDone()) {
doSomething();
}
A do
-while
loop evaluates the condition after the loop:
do {
printLine();
} while (!atEndOfPage());
Break and continue
Use break
to stop looping:
while (true) {
if (shutDownRequested()) break;
processIncomingRequests();
}
Use continue
to skip to the next loop iteration:
for (int i = 0; i < candidates.length; i++) {
var candidate = candidates[i];
if (candidate.yearsExperience < 5) {
continue;
}
candidate.interview();
}
If you’re using an Iterable
such as a list or set,
how you write the previous example might differ:
candidates
.where((c) => c.yearsExperience >= 5)
.forEach((c) => c.interview());