如何將2Dint陣列分配給Dart中的位元組陣列？

我有一個 DART 二維陣列定義為

     var matrix = List.generate(10, (i) =>List.filled(4, 0, growable: false),
          growable: false);
 

我可以用亂數填充它

  for (int i = 0; i < 10; i  ) {
        for (int j = 0; j < 4; j  )
        {
        matrix[i][j] = random.nextInt(MaxRan);
        }
      }

我現在想使用各種 base64 和壓縮庫來處理這個陣列。如果我嘗試將其轉換為 Uint8List

Uint8List bytes = Uint8List.fromList(matrix);

我收到一條訊息說“ The argument type 'List<List<int>>' can't be assigned to the parameter type 'List<int>'”。

如何將串列分配給位元組結構？

謝謝你。

uj5u.com熱心網友回復：

如錯誤訊息所示，Uint8List可以從 a 構造List<int>，而不是從構造List<List<int>>。你有幾個選擇：

• 將所有子串列拼接成一個List<int>，然后將其轉換為Uint8List：

  var flattened = [for (var row in matrix) ...row];
  var bytes = Uint8List.fromList(flattened);
  

• 創建Uint8List具有總大小的 a 并從每個子串列中初始化其元素：

  var bytes = Uint8List(matrix.length * matrix[0].length);
  var byteOffset = 0;
  for (var row in matrix) {
    for (var cell in row) {
      bytes[byteOffset] = cell;
      byteOffset  = 1;
    }
  }
  

• 您還可以通過首先使用 aUint8List并提供輔助函式來訪問元素來避免額外的復制：

  class Matrix {
    final int rows;
    final int columns;
  
    final Uint8List data;
  
    Matrix({required this.rows, required this.columns})
        : data = Uint8List(rows * columns);
  
    int getIndex(int row, int column) {
      if (row < 0 || row >= rows) {
        throw RangeError('row $row not in range [0, $rows)');
      }
      if (column < 0 || column >= columns) {
        throw RangeError('column $column not in range [0, $columns)');
      }
  
      return row * columns   column;
    }
  
    int get(int row, int column) => data[getIndex(row, column)];
  
    void set(int row, int column, int value) =>
        data[getIndex(row, column)] = value;
  }
  
  void main() {
    var i = 0;
    var matrix = Matrix(rows: 10, columns: 4);
    for (var row = 0; row < matrix.rows; row  = 1) {
      for (var column = 0; column < matrix.columns; column  = 1) {
        matrix.set(row, column, i);
        i  = 1;
      }
    }
  
    print(matrix.data);
  }
  

uj5u.com熱心網友回復：

您想在位元組陣列 ( Uint8List) 中存盤多大的數字？

我在這里采取的基本方法是

1. 展平矩陣 ( List<List<int>>)
   • 這可以用 matrix.expand((v) => v)
2. 創建Uint8List有足夠空間來容納所有矩陣元素
   • 這將是rows倍cols倍然而，許多位元組需要存盤每個INT。
3. 用于setAll將矩陣中的元素放入Uint8List.

在下面的代碼中，我演示了如何使用幾種不同大小的整數來做到這一點。我還添加了一些代碼，顯示了如何可以將其重新從轉換Uint8List到List<List<int>>。

import 'dart:typed_data';
import 'dart:math';

void main() {
  // setup original matrix
  final random = Random();
  const rows = 10;
  const cols = 4;
  const maxRan = 255;
  
  var matrix = List.generate(rows, (i) => List.filled(cols, 0, growable: false),
      growable: false);
  
  for (int i = 0; i < rows; i  ) {
    for (int j = 0; j < cols; j  ) {
      matrix[i][j] = random.nextInt(maxRan);
    }
  }
  
  print(matrix);

  // convert matrix to Uint8list
  
  // only works if maxRan is between 0 and 255
  final asBytesInt8 = Uint8List(rows * cols)
    ..setAll(0, matrix.expand((v) => v));

  // only works if maxRan is between 0 and 65535 
  final asBytesInt16 = Uint8List(2 * rows * cols)
    ..buffer.asUint16List().setAll(0, matrix.expand((v) => v));

  // only works if maxRan is between 0 and 4294967295 
  final asBytesInt32 = Uint8List(4 * rows * cols)
    ..buffer.asUint32List().setAll(0, matrix.expand((v) => v));

  print(' 8: $asBytesInt8');
  print('16: $asBytesInt16');
  print('32: $asBytesInt32');

  // convert Uint8list back to matrix
  
  final matrixFromInt8 = List.generate(
      rows, (i) => List.filled(cols, 0, growable: false),
      growable: false);

  for (int i = 0; i < rows; i  ) {
    for (int j = 0; j < cols; j  ) {
      matrixFromInt8[i][j] = asBytesInt8[cols * i   j];
    }
  }

  final matrixFromInt16 = List.generate(
      rows, (i) => List.filled(cols, 0, growable: false),
      growable: false);

  for (int i = 0; i < rows; i  ) {
    for (int j = 0; j < cols; j  ) {
      matrixFromInt16[i][j] =
          asBytesInt16.buffer.asUint16List()[cols * i   j];
    }
  }

  final matrixFromInt32 = List.generate(
      rows, (i) => List.filled(cols, 0, growable: false),
      growable: false);

  for (int i = 0; i < rows; i  ) {
    for (int j = 0; j < cols; j  ) {
      matrixFromInt32[i][j] =
          asBytesInt32.buffer.asUint32List()[cols * i   j];
    }
  }

  print(' 8: $matrixFromInt8');
  print('16: $matrixFromInt16');
  print('32: $matrixFromInt32');
}

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