In dart how add multiple list in a list? - flutter

Question

I want to add multiple List in a List. The Length of the Outer loop is 2
and the length of the inner loop is 2.
List matchName = [];
List a = [];
List b = [];
void getMatchName(GetUserMatchModelRes m) {
matchName.clear();
for (var i = 0; i < m.ObjectList.length; i++) {
matchName.clear();
matchName.add(
m.ObjectList[i].ContactName,
);
print("i:$i");
for (var j = 0; j < m.ObjectList[i].ContactPhones.length; j++) {
print("j:$j");
matchName.add(
m.ObjectList[i].ContactPhones[j].MatchedContactName,
);
}
a.add(matchName);
print(a);
}
}
Output when outer loop is 0: [[a,b,c]]
When outer loop is 1: [[d,e,f],[d,e,f]]
But I want [[a,b,c],[d,e,f]]
How I can achieve this?

Answer 1

You're essentially doing:
var outer = <List<String>>[];
var inner = ['foo'];
outer.add(inner);
outer.add(inner);
Now outer has two elements that refer to the same object:
+---+---+
outer: | | |
+-|-+-|-+
| |
v v
+-------+
inner: | 'foo' |
+-------+
If you modify inner, you'll see the change in both outer[0] and in outer[1].
To avoid this, you need to add separate objects so that they can be modified independently. Either create new lists to add instead of modifying an existing one:
var outer = <List<String>>[];
outer.add(['foo']);
outer.add(['foo']);
or add copies:
var outer = <List<String>>[];
var inner = ['foo'];
outer.add([...inner]);
outer.add([...inner]);