Dart - problem with multi-dimensional lists-CodePudding

I am programming in Dart and I need a multi-dimensional array to be filled with some data. But when I tried to run it, data at previous indexes were overwritten.

Here is my code:

List<List<String>> list = List.filled(4, List.filled(4, ""));
String toFill = "abcdefghijklmnop";

for (int i = 0; i < 4; i  ) {
    for (int j = 0; j < 4; j  ) {
      list[i][j] = toFill[i * 4   j];
    }
}

print(list);

I expected output of print() function to be

[[a, b, c, d], [e, f, g, h], [i, j, k, l], [m, n, o, p]]

but when I run it, it printed

[[m, n, o, p], [m, n, o, p], [m, n, o, p], [m, n, o, p]]

I don't know, why the last part is duplicating over to previous values.

I'd be glad for any help.

Thanks

CodePudding user response：

I was able to solve this use List.generate which makes a new list from an iterable.

List<List<String>> list = List.generate(4, (int index) => List.generate(4, (int index) => ""));

Which gives me

> [[a, b, c, d], [e, f, g, h], [i, j, k, l], [m, n, o, p]]

This seems a little verbose, so there may be a shortcut to do the same thing.

Edit: Since you don't care about the index variable, you can shorten it somewhat but ignoring it:

List<List<String>> list = List.generate(4, (_) => List.generate(4, (_) => ""));

CodePudding user response：

You could also combine the creating and filling the list:

import 'dart:math';

void main() {
  String toFill = "abcdefghijklmnopqrstuvwxyz";
  List<List<String>> list = List.generate(
    (toFill.length/4).ceil(),
    (i) => toFill.substring(i*4, min(toFill.length, (i 1)*4)).split(''),
  );
  print(list);
}

[[a, b, c, d], [e, f, g, h], [i, j, k, l], [m, n, o, p], [q, r, s, t], [u, v, w, x], [y, z]]

Which, if your string is always 16 characters long, can be simplified to:

void main() {
  String toFill = "abcdefghijklmnop";
  List<List<String>> list = List.generate(
    4,
    (i) => toFill.substring(i*4, (i 1)*4).split(''),
  );
  print(list);
}