Fixed List is not updating with for loop - Flutter - flutter

Question

I've a fixed list reservedGuest. After checking the condition in for loop I want to update the seats if the membership date has expired. The list is not updating. The code is as follows. PS. The List is filled through API on init().
class MyClubController extends GetxController {
List goldLane = List.filled(3, null, growable: false);
void _alterLanesOnContractEnds() {
for (var i in goldLane) {
print("\n\n I: $i");
if (i == null ||
DateTime.parse(i['contractEnds']).isBefore(
DateTime.now(),
)) {
i = null;
print('Can be removed');
} else {
print('Cannot be removed');
}
}
update();
}
}

Answer 1

A for-in loop will not allow you to reassign elements of the List. When you do:
for (var i in goldLane) {
// ...
i = null;
}
you are reassigning what the local i variable refers to, not mutating the goldLane List.
You instead can iterate with an index:
void _alterLanesOnContractEnds() {
for (var i = 0; i < goldLane.length; i += 1) {
var element = goldLane[i];
print("\n\n I: $element");
if (element == null ||
DateTime.parse(element['contractEnds']).isBefore(
DateTime.now(),
)) {
goldLane[i] = null;
print('Can be removed');
} else {
print('Cannot be removed');
}
}
update();
}

Answer 2

You can just create a new List where unqualified guests are nullified. For example,
void _alterLanesOnContractEnds() {
goldLane = goldLane.map(
(guest) => guest == null || DateTime.parse(guest['contractEnds']).isBefore(DateTime.now()) ? null: guest
).toList(growable: false);
update();
}
You should not and cannot modify a list while iterating with its iterator.
Elaborated by Jamesdlin,
Modifying the elements of a List while iterating is fine. Modifying
the length of the List while iterating is not, but that won't be a
problem for a non-growable List.
The bottom line is you should not mutate the size of the list while iterating.

Answer 3

I solved it by using
goldLane.forEach(
(element) {
print('\n\n ELEMENT: $element');
if (element == null ||
DateTime.parse(element['contractEnds']).isBefore(
DateTime.now(),
)) {
int ix = goldLane.indexWhere(
(element) => element != null
? DateTime.parse(element['contractEnds']).isBefore(
DateTime.now(),
)
: true,
);
goldLane[ix] = null;
} else {
print('Cannot be removed');
}
},
);
Yet I'll test the other answers. Thank You.