A list of Dart StringBuffers seem to interfere with each other - dart

Question

I'm doing a nested iteration over two lists, in which I am populating some StringBuffers, like this:
var int_list = [1, 2, 3];
var letters_list = ['a', 'b', 'c'];
var row_strings = List.filled(3, StringBuffer());
var single_buffer = StringBuffer();
int_list.asMap().forEach((int_index, column) {
letters_list.asMap().forEach((letter_index, letter) {
// debug the writing operation
print('writing $letter_index - $letter');
row_strings[letter_index].write(letter);
// try a buffer not in a list as a test
if (letter_index == 0) {
single_buffer.write(letter);
}
});
});
print(single_buffer);
print(row_strings);
What I expect to happen is that in the list of StringBuffers, buffer 0 gets all the 'a's, buffer 1 gets all the 'b's, and buffer 3 the 'c'.
The debug output confirms that the writing operation is doing the right thing:
writing 0 - a
writing 1 - b
writing 2 - c
writing 0 - a
writing 1 - b
writing 2 - c
writing 0 - a
writing 1 - b
writing 2 - c
and the single string buffer gets the right output:
aaa
But the output of the list is this:
[abcabcabc, abcabcabc, abcabcabc]
What is going on here? There seems to be some strange behaviour when the StringBuffers are in a list.

Answer 1

Your problem is this line:
var row_strings = List.filled(3, StringBuffer());
This constructor is documented as:
List.filled(int length, E fill, {bool growable: false})
Creates a list of the given length with fill at each position.
https://api.dart.dev/stable/2.10.5/dart-core/List/List.filled.html
So what you are doing is creating a single StringBuffer instance and uses that on every position in your row_strings list.
What you properly want, is to create a new StringBuffer for each position in the list. You need to use List.generate for that:
List.generate(int length,E generator(int index), {bool growable: true})
Generates a list of values.
Creates a list with length positions and fills it with values created by calling generator for each index in the range 0 .. length - 1 in increasing order.
https://api.dart.dev/stable/2.10.5/dart-core/List/List.generate.html
Using that, we end up with:
final row_strings = List.generate(3, (_) => StringBuffer());
We don't need the index argument in our generator so I have just called it _. The function (_) => StringBuffer() will be executed for each position in the list and saved. Since out function returns a new instance of StringBuffer each time it is executed, we will end up with a list of 3 separate StringBuffer instances.