I'm trying to make a BMI calculator as a beginner project, as I am learning Dart. However, when I'm trying to take input, it outputs an error
import 'dart:io';
void main() {
print('What is your weight in kgs?');
var weight = Stdin.readLineSync();
print("Your weight is $weight");
}
You're trying to call readLineSync on the class, not on an instance. Since readLineSync isn't static, you'll need to call this on an instance of Stdin.
There's a global instance of Stdin named stdin exposed through dart:io that you can use for this.
Related
I'm new to LLVM and I'm stuck on something that might seem basic.
I'm writing a LLVM pass to apply some transformations to global variables before they are use.
I would like to detect somehow when is the first usage of a global variable to only apply the transformation there, and not in all places where the global variable is used. But it must be the first time it is used otherwise the program crashes.
I have been reading about the AnalysisManager, and I would say that I want something similar to DominatorTree which is used for basic blocks in a function.
So the idea is to get the DominatorTree of a GlobalVariable to get the first time it is used in the code and apply there my transformation.
Given the following example
int MyGlobal = 30;
void foo()
{
printf("%s\n", MyGlobal);
}
int main()
{
printf("%s\n", MyGlobal);
foo();
}
In the example above, I only want to apply the transformation just before the first printf in the main function
Given the following example
int MyGlobal = 30;
void foo()
{
printf("%s\n", MyGlobal);
}
int main()
{
foo();
printf("%s\n", MyGlobal);
}
For the example above I would like to apply the transformation inside the foo function.
I want to avoid to create a stub function at the beginning of the program to process all globals before start running (This is what actually Im doing)
Does LLVM provide something that can help me doing this? or what should be the best approach to implement it?
I'm writing a discord bot using the nyxx library and want use dynamic file import for load command info and handler. But, after 5 hours of searching with Google, I didn't find anything to help me do that.
In Node.js, I can use require() or import() for it: Does the dart have something like that?
A small code snippet, showing what I want do:
this.commands = new Collection();
fs.readdirSync('./src/commands').filter(( f ) => f.endsWith( '.js' )).forEach((file) => {
const command = require(`../commands/${file}`);
this.commands.set( command.info.name, command );
});
Is it possible to do this or not? (I don't like to write many imports for commands and register it in lib.)
You can in theory use Isolate.spawnUri to spawn external Dart programs to run in its own Isolate instances that can then communicate back to the main program using SendPort.
It does, however, come with some limitations. E.g. it is very limited what types of objects you can send though SendPort when using spawnUri since the two programs does not share any type information (compared to Isolate.spawn which does allow you to send your own custom types). The documented types you can send can be found here:
Null
bool
int
double
String
List or Map (whose elements are any of these)
TransferableTypedData
SendPort
Capability
https://api.dart.dev/stable/2.17.6/dart-isolate/SendPort/send.html
But it does allow us to make some kind of protocol and you can create some helper class around this to handle the conversion of a known object structure into e.g. Map<String, Object>.
A small example that works with Dart VM would be:
Your command implemented as: command.dart
import 'dart:isolate';
void main(List<String> arguments, Map<String, Object> message) {
final userName = message['username'] as String;
final sendPort = message['port'] as SendPort;
sendPort.send('Hi $userName. '
'You got a message from my external command program!');
}
Your server that calls your command: server.dart
import 'dart:isolate';
void main() {
final recievePort = ReceivePort();
recievePort.listen((message) {
print('Got the following message: $message');
recievePort.close();
});
Isolate.spawnUri(Uri.file('command.dart'), [], {
'username': 'julemand101',
'port': recievePort.sendPort,
});
}
If running this with: dart server.dart you, hopefully, get:
Got the following message: Hi julemand101. You got a message from my external command program!
If you want to compile your application, you can do so by doing the following. You need to compile the command.dart, since a compiled Dart program does not understand how to read Dart code.
dart compile exe server.dart
dart compile aot-snapshot command.dart
You should here change Uri.file('command.dart') to Uri.file('command.aot') since the file-extension for aot-snapshot are .aot.
If everything works, you should be able to see:
> .\server.exe
Got the following message: Hi julemand101. You got a message from my external command program!
I am moving java script to dart, in java script I create dynamic variable like
window["text" + pageNumber] = 123;
alert(window["text" + pageNumber]);
How can I do it with dart?
In Dart Window (the type of window) is a class. You can't dynamically add properties to a Dart class.
window["text" + pageNumber] = 123; would work with a Map. Object representation in JS is quite similar to a map and therefore this works there.
If another class implements the [] operator you could call it on instances of that class as well but it would still not add properties. What it actually does just depends on the implementation of the [] operator.
There are probably different ways in Dart to achieve what you want, but you didn't add details about what actual problem you try to solve.
You can use normal global variables in Dart like explained in
Global Variables in Dart.
For your use case you can create a global Map variable this way
final Map<String,int> myGlobals = <String,int>{};
to create a map that stores integer values with string names.
Set values with myGlobals['someName'] = 123; and read them with print(myGlobals['someName']);.
If you need to set a global value that is also available for JS libraries you might use, you can use dart-js-interop
import 'dart:js';
import 'dart:html';
main() {
int pagenumber = 5;
context['Window']['text$pagenumber'] = 123;
window.alert('${context['Window']['text$pagenumber']}');
}
Try it on DartPad.
Hint:
"text" + pageNumber doesn't work when pageNumber is not a string.
In Dart you can't add string and numbers.
"text" + pageNumber.toString() would work but 'text$pagenumber' is a more darty way to do this. In string interpolation toString() is called automatically for you.
See also Dart js-interop not working if .dart file isn't included.
What is the difference between show and as in an import statement?
For example, what's the difference between
import 'dart:convert' show JSON;
and
import 'package:google_maps/google_maps.dart' as GoogleMap;
When do I use show and when should I use as?
If I switch to show GoogleMap all references to GoogleMap (e.g. GoogleMap.LatLng) objects are reported as undefined.
as and show are two different concepts.
With as you are giving the imported library a name. It's usually done to prevent a library from polluting your namespace if it has a lot of global functions. If you use as you can access all functions and classes of said library by accessing them the way you did in your example: GoogleMap.LatLng.
With show (and hide) you can pick specific classes you want to be visible in your application. For your example it would be:
import 'package:google_maps/google_maps.dart' show LatLng;
With this you would be able to access LatLng but nothing else from that library. The opposite of this is:
import 'package:google_maps/google_maps.dart' hide LatLng;
With this you would be able to access everything from that library except for LatLng.
If you want to use multiple classes with the same name you'd need to use as. You also can combine both approaches:
import 'package:google_maps/google_maps.dart' as GoogleMap show LatLng;
show case:
import 'dart:async' show Stream;
This way you only import Stream class from dart:async, so if you try to use another class from dart:async other than Stream it will throw an error.
void main() {
List data = [1, 2, 3];
Stream stream = new Stream.fromIterable(data); // doable
StreamController controller = new StreamController(); // not doable
// because you only show Stream
}
as case:
import 'dart:async' as async;
This way you import all class from dart:async and namespaced it with async keyword.
void main() {
async.StreamController controller = new async.StreamController(); // doable
List data = [1, 2, 3];
Stream stream = new Stream.fromIterable(data); // not doable
// because you namespaced it with 'async'
}
as is usually used when there are conflicting classes in your imported library, for example if you have a library 'my_library.dart' that contains a class named Stream and you also want to use Stream class from dart:async and then:
import 'dart:async';
import 'my_library.dart';
void main() {
Stream stream = new Stream.fromIterable([1, 2]);
}
This way, we don't know whether this Stream class is from async library or your own library. We have to use as :
import 'dart:async';
import 'my_library.dart' as myLib;
void main() {
Stream stream = new Stream.fromIterable([1, 2]); // from async
myLib.Stream myCustomStream = new myLib.Stream(); // from your library
}
For show, I guess this is used when we know we only need a specific class. Also can be used when there are conflicting classes in your imported library. Let's say in your own library you have a class named CustomStream and Stream and you also want to use dart:async, but in this case you only need CustomStream from your own library.
import 'dart:async';
import 'my_library.dart';
void main() {
Stream stream = new Stream.fromIterable([1, 2]); // not doable
// we don't know whether Stream
// is from async lib ir your own
CustomStream customStream = new CustomStream();// doable
}
Some workaround:
import 'dart:async';
import 'my_library.dart' show CustomStream;
void main() {
Stream stream = new Stream.fromIterable([1, 2]); // doable, since we only import Stream
// async lib
CustomStream customStream = new CustomStream();// doable
}
as and show keywords used with library import statement. These two keywords are optional with import keyword, But using these keywords you can provide convenience and additional information about your library importing.
show
show give restrictions to access only specific class of that library.
import 'dart:convert' show JSON;
Above dart:convert library contains more than 5 types of converters. (ascii,Base64,Latin1,Utf8 & json are some of them).
But with using show keyword you will give your application source file to access only that JSON converter class only.
warning !! :- if you try to access any other converters like ascii, Base64 or Latin1, you will get an exception.
Because using show keyword you give an restriction for only access Json class in that library api.
So if your source file want to access all the class in that library, you cannot define show keyword for that library importing.
as
Provide additional namespace for library members.
This as keyword is mostly used when a library that contains lot of global functions.
You will access static members of a library by Using the class name and . (dot operator).
eg:- ClassName.staticFun()
And also you will access instance methods and variables by using object name and . (dot operator) eg:- obj.instanceFunc()
And also library source file can have global functions. and we will access them by their name without any parental membership. eg:- func()
So when we access global functions of a different library inside our source file, we didnt have a way to seperatly identified that global function as seperate function of a different library.
But using as keyword, we can add namespace before accessing global functions of that library.
See below example to understanding real benefit of as keyword. 👇
import 'package:http/http.dart' as http;
http library contains lot of global functions. Below shows list of global functions in http library.
Accessing above http library global functions without http namespace.( import 'package:http/http.dart'; )
eg:-
1. get("url")
2. post("url")
Accessing above http library global functions with http namespace. ( import 'package:http/http.dart'as http; )
eg:-
1. http.get("url")
2. http.post("url")
So using as keyword , makes it easy to identify global functions of a different library separated from our source files' global functions.
I prefer the dart document, it's described in Libraries and visibility section.
import as: Specifying a library prefix, for example when import two libraries which has the same function name, then we can give them a prefix to specify the library.
import show: This is used to import part of the library, show only import one name of the library.
import hide: This is another one which is the opposite of the show, hide import all names except the name specified in the hide.
The following example (1) reads a file and prints the contents without explicitly assigning the file contents to a variable (ie. “.then(stdout.write)”). However, if I want to do more than just print the contents (2), I need to assign the contents to a variable (I think).
Is it possible to achieve that (print the contents and do more), without assigning the text of the file to a variable?
In the first example, is an implicit variable created? Or, put another way, does example1 use less resources by not creating an explicit variable?
//Example 1:
import 'dart:io';
void main() {
new File(new Options().script)
.readAsString(encoding: Encoding.ASCII)
.then(stdout.write)
.catchError((oError) => print(oError));
print("Reading file ...\n");
}
//Example 2:
import 'dart:io';
void main() {
new File(new Options().script)
.readAsString(encoding: Encoding.ASCII)
.then((String sText) {
stdout.write(sText+"\n\n");
print ('Completed');
})
.catchError((oError) => print(oError));
print("Reading file ...\n");
}
In the first example, this:
.then(stdout.write)
is equivalent to this:
.then((String sText) {
stdout.write(sText);
})
Technically there's one more function call, and you have one more variable, which should cost you a few bytes (I'm not sure on the exact implementation). Strings are immutable; you are only receiving a reference to the String, so you are not saving resources (other than the function call and a few bytes of memory) by using second version.
Whatever it is you want to do with the contents of the String probably will involve using resources, of course, but that shouldn't be an issue unless the file is huge.