Write 32 bit integers to file?

Question

I have a list of 32-bit unsigned integers:

  var unixTimestamps = [
    1573613134,
    1573623934,
    1573631134,
    1573659934,
    1573746334,
    1573731934,
    1573764334,
    1573789534,
    1573847134,
    1573933534,
    1573893934,
    1573980334,
    1574153134,
    1574178334
  ];

I would like to save these to a file.

The list needs to be written iteratively as it depends on the user's interaction with the app.

final file = await _localFile(filename);
for (var i=0; i<unixTimestamps.length; i++){
  file.writeAsBytesSync(unixTimestamps[i], mode: FileMode.append)
}

with

Future _localFile(String filename) async {
  final path = await _localPath();
  return File('$path/$filename');
}

Future _localPath() async {
  final directory = await getApplicationDocumentsDirectory();
  return directory.path;
}

When I read back the data from that file, I noticed that it's unsigned 8-bit integers. (Values are between 0 and 255)

Below is the readData script:

  Future readData(filename, i) async {
    try {
      final file = await _localFile(filename);
      List contents = await file.readAsBytesSync();
      return contents;
    } catch (e) {
      print(e);
      return 0;
    }
  }

Is there a way for files to be saved and read as 32 bit integers?

afaik, you should use int8 to write and read but you can cast data type while writing and reading, for example https://stackoverflow.com/a/57536472/2395656 — Tarık Yılmaz, Dec 22 '19 at 19:36

mezoni · Accepted Answer · 2019-12-22T20:56:16.397

If you do not want to use standard solutions, you can use a non-standard approach to solving the problem.
Example of non-standard approach (easy to understand and easy to maintain):

void main() {
  var list32 = [0x40302010, 0x10203040];
  final bytes = toBytes(list32, 4);
  print(bytes);
  final newList32 = fromBytes(bytes, 4);
  print(list32);
  print(newList32);
}

List<int> toBytes(List<int> list, int size) {
  final result = <int>[];
  for (var i = 0; i < list.length; i++) {
    final value = list[i];
    for (var j = 0; j < size; j++) {
      final byte = value >> (j * 8) & 0xff;
      result.add(byte);
    }
  }

  return result;
}

List<int> fromBytes(List<int> list, int size) {
  if (list.length % size != 0) {
    throw ArgumentError('Wrong size');
  }

  final result = <int>[];
  for (var i = 0; i < list.length; i += size) {
    var value = 0;
    for (var j = 0; j < size; j++) {
      var byte = list[i + j];
      final val = (byte & 0xff) << (j * 8);
      value |= val;
    }

    result.add(value);
  }

  return result;
}

Result:

[16, 32, 48, 64, 64, 48, 32, 16]
[1076895760, 270544960]
[1076895760, 270544960]

P.S.

You can add an overflow check to both methods to check the correctness of the data if you need such a check.

Write 32 bit integers to file?

1 Answers1