Dart Reading Bytes from File and Converting to Double

Question

In Dart/Flutter, I'm trying to read bytes from a data file (2 bytes per value; on a mobile device) and convert to a double.

I have the below code:

fileToData(File filename)  {
  var bytes = filename.readAsBytesSync();
  return bytes.buffer.asInt16List().map((e) {
    print("Value: ${e.toDouble()}");
    return e.toDouble();
  }).toList();
}

The return List values are all 0.0; however, in the print statement I see the e.toDouble() is not 0.0 and is indeed the correct value expected. I just can't seem to return it.

For example:

I/flutter (10636): Value: 404.0
I/flutter (10636): Value: 356.0
I/flutter (10636): Value: 345.0
I/flutter (10636): Value: 297.0
I/flutter (10636): Value: 200.0
I/flutter (10636): Value: 164.0

Back the returned List<double> is:

I/flutter (10636): SIGNAL: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,.....

Any suggestions? Thanks.

I don't see anything wrong with your code. Are you absolutely certain you're printing the correct list? — jamesdlin
Very sure. Rather than returning the list I've also just print it and always a list of all zeros. — user2264327
Please post a minimal, complete, reproducible example. I copied and pasted your fileToData function, added appropriate imports and void main() => print(fileToData(File('/path/to/some/file'))); ran it from the command-line, and it worked fine. — jamesdlin
Also, it might be useful to know how long your lists are and if you're seeing the same number of elements printed out. If you're using print, be aware that Android and iOS can throttle and truncate excessive console spew. — jamesdlin
Evidently the code runs as expected on a desktop Linux (with the same data file) but not on an Android device in Flutter. — user2264327

Kohls Kohls · Accepted Answer · 2021-02-10T16:31:33

Always add explicit return data types, not adding a data type (AKA dynamic) can not always expect the result you want.

List<double> fileToData(File filename)  {
  var bytes = filename.readAsBytesSync();
  return bytes.buffer.asInt16List().map((e) {
    print("Value: ${e.toDouble()}");
    return e.toDouble();
  }).toList();
}

List<double> data = fileToData(file);

Dart Reading Bytes from File and Converting to Double

1 Answers