I would like to have a list of functions and call one of them by using a number n to call the nth function of the list.
I thought that the best way to do this would be by using a map, but I can immediately spot some problems that I don't know how to solve in this example:
import 'dart:math';
void functionOne(x) => print("the randomly generated number is $x");
void functionTwo(x) => print("the inverse of the randomly generated number is ${1/x}");
var example = {0:functionOne(), 1:functionTwo()};
var intValue = Random().nextInt(2);
var biggerIntValue = Random().nextInt(100) + 1;
void main() {
example[0](biggerIntValue);
}
I would like to be able to pass parameters into said function and be able to access the functions by using a number. I know that I could make one giant function and use if statements to do certain actions, but I was wondering if there was any predetermined syntax that would look better than this:
import 'dart:math';
void largeFunction(funcNumber, x) {
if (funcNumber == 0) {functionOne(x);}
if (funcNumber == 1) {functionTwo(x);}
}
void functionOne(x) => print("the randomly generated number is $x");
void functionTwo(x) => print("the inverse of the randomly generated number is ${1/x}");
var intValue = Random().nextInt(2);
var biggerIntValue = Random().nextInt(100) + 1;
void main() {
largeFunction(intValue, biggerIntValue);
}
Thanks for reading my question!
Your problem with the map is that you call the function expressions. Try:
var example = {0:functionOne, 1:functionTwo};
The next problem will be that
example[0](biggerIntValue);
complains that example[0] is nullable.
I'd change that to:
example[0]?.call(biggerIntValue);
so that it only calls something if the integer is a key in the map.
Related
Is there a way to return several values in a function return statement (other than returning an object) like we can do in Go (or some other languages)?
For example, in Go we can do:
func vals() (int, int) {
return 3, 7
}
Can this be done in Dart? Something like this:
int, String foo() {
return 42, "foobar";
}
Dart doesn't support multiple return values.
You can return an array,
List foo() {
return [42, "foobar"];
}
or if you want the values be typed use a Tuple class like the package https://pub.dartlang.org/packages/tuple provides.
See also either for a way to return a value or an error.
I'd like to add that one of the main use-cases for multiple return values in Go is error handling which Dart handle's in its own way with Exceptions and failed promises.
Of course this leaves a few other use-cases, so let's see how code looks when using explicit tuples:
import 'package:tuple/tuple.dart';
Tuple2<int, String> demo() {
return new Tuple2(42, "life is good");
}
void main() {
final result = demo();
if (result.item1 > 20) {
print(result.item2);
}
}
Not quite as concise, but it's clean and expressive code. What I like most about it is that it doesn't need to change much once your quick experimental project really takes off and you start adding features and need to add more structure to keep on top of things.
class FormatResult {
bool changed;
String result;
FormatResult(this.changed, this.result);
}
FormatResult powerFormatter(String text) {
bool changed = false;
String result = text;
// secret implementation magic
// ...
return new FormatResult(changed, result);
}
void main() {
String draftCode = "print('Hello World.');";
final reformatted = powerFormatter(draftCode);
if (reformatted.changed) {
// some expensive operation involving servers in the cloud.
}
}
So, yes, it's not much of an improvement over Java, but it works, it is clear, and reasonably efficient for building UIs. And I really like how I can quickly hack things together (sometimes starting on DartPad in a break at work) and then add structure later when I know that the project will live on and grow.
Create a class:
import 'dart:core';
class Tuple<T1, T2> {
final T1 item1;
final T2 item2;
Tuple({
this.item1,
this.item2,
});
factory Tuple.fromJson(Map<String, dynamic> json) {
return Tuple(
item1: json['item1'],
item2: json['item2'],
);
}
}
Call it however you want!
Tuple<double, double>(i1, i2);
or
Tuple<double, double>.fromJson(jsonData);
You can create a class to return multiple values
Ej:
class NewClass {
final int number;
final String text;
NewClass(this.number, this.text);
}
Function that generates the values:
NewClass buildValues() {
return NewClass(42, 'foobar');
}
Print:
void printValues() {
print('${this.buildValues().number} ${this.buildValues().text}');
// 42 foobar
}
The proper way to return multiple values would be to store those values in a class, whether your own custom class or a Tuple. However, defining a separate class for every function is very inconvenient, and using Tuples can be error-prone since the members won't have meaningful names.
Another (admittedly gross and not very Dart-istic) approach is try to mimic the output-parameter approach typically used by C and C++. For example:
class OutputParameter<T> {
T value;
OutputParameter(this.value);
}
void foo(
OutputParameter<int> intOut,
OutputParameter<String>? optionalStringOut,
) {
intOut.value = 42;
optionalStringOut?.value = 'foobar';
}
void main() {
var theInt = OutputParameter(0);
var theString = OutputParameter('');
foo(theInt, theString);
print(theInt.value); // Prints: 42
print(theString.value); // Prints: foobar
}
It certainly can be a bit inconvenient for callers to have to use variable.value everywhere, but in some cases it might be worth the trade-off.
you can use dartz package for Returning multiple data types
https://www.youtube.com/watch?v=8yMXUC4W1cc&t=110s
Dart is finalizing records, a fancier tuple essentially.
Should be in a stable release a month from the time of writing.
I'll try to update, it's already available with experiments flags.
you can use Set<Object> for returning multiple values,
Set<object> foo() {
return {'my string',0}
}
print(foo().first) //prints 'my string'
print(foo().last) //prints 0
In this type of situation in Dart, an easy solution could return a list then accessing the returned list as per your requirement. You can access the specific value by the index or the whole list by a simple for loop.
List func() {
return [false, 30, "Ashraful"];
}
void main() {
final list = func();
// to access specific list item
var item = list[2];
// to check runtime type
print(item.runtimeType);
// to access the whole list
for(int i=0; i<list.length; i++) {
print(list[i]);
}
}
I am trying to understand the rational behind using writeByte(3) in the write method in Hive TypeAdapter.
Please consider the class:
#HiveType()
class Person{
#HiveField(0)
String name;
#HiveField(1)
int age;
}
In the TypeAdapter below It is easy to understand the read method, since it is just reads sequentially each field.
However, I'm trying to figure out why the same mechanism does not apply to the write, instead of using ..writeByte(...) just before each field. And, what is the meaning of the first ..writeByte(2)?
class PersonAdapter extends TypeAdapter<Person> {
#override
Person read(BinaryReader reader) {
var numOfFields = reader.readByte();
var fields = <int, dynamic>{
for (var i = 0; i < numOfFields; i++) reader.readByte(): reader.read(),
};
return Trips()
..name = fields[0] as String
..age = fields[1] as int;
}
#override
void write(BinaryWriter writer, Person obj) {
writer
..writeByte(2) // Why this here? (sometimes I see writeByte(3) !! )
..writeByte(0)
..write(obj.name)
..writeByte(1)
..write(obj.age);
}
}
Thanks for any clarification.
I know nothing about Hive but if you take a look at the builder which create this write method you can see the following:
String buildWrite() {
var code = StringBuffer();
code.writeln('writer');
code.writeln('..writeByte(${getters.length})');
for (var field in getters) {
var value = _convertIterable(field.type, 'obj.${field.name}');
code.writeln('''
..writeByte(${field.index})
..write($value)''');
}
code.writeln(';');
return code.toString();
}
https://github.com/hivedb/hive/blob/59ad5403593283233d922f62f76832c64fa33a3b/hive_generator/lib/src/class_builder.dart#L122
So based on this we can conclude the first writeByte is the length of getters. The next one is the index of the first getter (0) following by the value and next getter (1) with value and so on.
This makes sense since the protocol properly needs to know how many fields it can expect to get.
I am very new to Dart so excuse me if I didnt see this part.
I want to make a union type e.g. for a function input. In TS this would be:
let variableInput: string | number
typedef doesnt really define types but functions and enums dont really help too.
On the other side how should it look like when a function return either one or the other of two types? There must be something I dont see here.
There are no union types in Dart.
The way to do this in Dart is returning/accepting dynamic as a type:
dynamic stringOrNumber() { ... }
void main() {
final value = stringOrNumber();
if (value is String) {
// Handle a string value.
} else if (value is num) {
// Handle a number.
} else {
throw ArgumentError.value(value);
}
}
See also: https://dart.dev/guides/language/sound-dart
How can I use existing Flux operators to make a Flux return incoming values into multiple Lists with a minimum delay between returns?
This can be achieved with a non-trivial set of composed operators.
import java.time.Duration;
import java.util.*;
import reactor.core.publisher.*;
public class DelayedBuffer {
public static void main(String[] args) {
Flux.just(1, 2, 3, 6, 7, 10)
.flatMap(v -> Mono.delayMillis(v * 1000)
.doOnNext(w -> System.out.println("T=" + v))
.map(w -> v)
)
.compose(f -> delayedBufferAfterFirst(f, Duration.ofSeconds(2)))
.doOnNext(System.out::println)
.blockLast();
}
public static <T> Flux<List<T>> delayedBufferAfterFirst(Flux<T> source, Duration d) {
return source
.publish(f -> {
return f.take(1).collectList()
.concatWith(f.buffer(d).take(1))
.repeatWhen(r -> r.takeUntilOther(f.ignoreElements()));
});
}
}
(Note however, that the expected emission pattern may be better matched with a custom operator due to time being involved.)
I thought buffer(Duration) would fit your need, but it doesn't.
edit: leaving this in case someone with your exact same need is tempted to use that operator. This variant of buffer splits the sequence into consecutive time windows (that each produce a buffer). That is, the new delay starts at the end of the previous one, not whenever a new out-of-delay element is emitted.
I would like to pass a primitive (int, bool, ...) by reference. I found a discussion about it (paragraph "Passing value types by reference") here: value types in Dart, but I still wonder if there is a way to do it in Dart (except using an object wrapper) ? Any development ?
The Dart language does not support this and I doubt it ever will, but the future will tell.
Primitives will be passed by value, and as already mentioned here, the only way to 'pass primitives by reference' is by wrapping them like:
class PrimitiveWrapper {
var value;
PrimitiveWrapper(this.value);
}
void alter(PrimitiveWrapper data) {
data.value++;
}
main() {
var data = new PrimitiveWrapper(5);
print(data.value); // 5
alter(data);
print(data.value); // 6
}
If you don't want to do that, then you need to find another way around your problem.
One case where I see people needing to pass by reference is that they have some sort of value they want to pass to functions in a class:
class Foo {
void doFoo() {
var i = 0;
...
doBar(i); // We want to alter i in doBar().
...
i++;
}
void doBar(i) {
i++;
}
}
In this case you could just make i a class member instead.
No, wrappers are the only way.
They are passed by reference. It just doesn't matter because the "primitive" types don't have methods to change their internal value.
Correct me if I'm wrong, but maybe you are misunderstanding what "passing by reference" means? I'm assuming you want to do something like param1 = 10 and want this value to still be 10 when you return from your method. But references aren't pointers. When you assign the parameter a new value (with = operator), this change won't be reflected in the calling method. This is still true with non-primitive types (classes).
Example:
class Test {
int val;
Test(this.val);
}
void main() {
Test t = new Test(1);
fn1(t);
print(t.val); // 2
fn2(t);
print(t.val); // still 2, because "t" has been assigned a new instance in fn2()
}
void fn1(Test t) {
print(t.val); // 1
t.val = 2;
}
void fn2(Test t) {
t = new Test(10);
print(t.val); // 10
}
EDIT
I tried to make my answer more clear, based on the comments, but somehow I can't seem to phrase it right without causing more confusion. Basically, when someone coming from Java says "parameters are passed by reference", they mean what a C/C++ developer would mean by saying "parameters are passed as pointers".
As dart is compiled into JavaScript, I tried something that works for JS, and guess what!? It worked for dart!
Basically, what you can do is put your value inside an object, and then any changes made on that field value inside that function will change the value outside that function as well.
Code (You can run this on dartpad.dev)
main() {
var a = {"b": false};
print("Before passing: " + a["b"].toString());
trial(a);
print("After passing: " + a["b"].toString());
}
trial(param) {
param["b"] = true;
}
Output
Before passing: false
After passing: true
One of the way to pass the variables by reference by using the values in List. As arrays or lists are Pass by reference by default.
void main() {
List<String> name=['ali' ,'fana'];
updatename(name);
print(name);
}
updatename(List<String> name){
name[0]='gufran';
}
Try this one, This one of the simplest way to pass by reference.
You can use ValueNotifier
And, you can pass it as ValueListenable to classes or methods that needs to know up-to-date value, but should not edit it:
class Owner {
final theValue = ValueNotifier(true);
final user = User(theValue);
...
}
class User {
final ValueListeneble<bool> theValue;
User(this.theValue);
...
}
It provides more functionality than actually needed, but solves the problem.
If ValueNotifier + ValueListenable do not work for you (you want to make sure the client does not listen to every change of the value, or your package is pure Dart package and thus cannot reference Flutter libraries), use a function:
class Owner {
int _value = 0;
int getValue() => _value;
void increase() => _value++;
}
void main() {
final owner = Owner();
int Function() obtainer = owner.getValue;
print(obtainer());
owner.increase();
print(obtainer());
}
Output will be:
0
1
This approach has memory usage related downside: the obtainer will hold the reference to the owner, and this, even if owner is already not referenced, but obtainer is still reachable, owner will be also reachable
and thus will not be garbage collected.
If you do not want the downside, pass the smaller container than the entire owner:
import 'package:flutter/foundation.dart';
class ListenableAsObtainer<T> implements ValueObtainer<T> {
ListenableAsObtainer(this._listenable);
final ValueListenable<T> _listenable;
#override
T get value => _listenable.value;
}
class FunctionAsObtainer<T> implements ValueObtainer<T> {
FunctionAsObtainer(this._function);
final T Function() _function;
#override
T get value => _function();
}
class ValueAsObtainer<T> implements ValueObtainer<T> {
ValueAsObtainer(this.value);
#override
T value;
}
/// Use this interface when the client needs
/// access to the current value, but does not need the value to be listenable,
/// i.e. [ValueListenable] would be too strong requirement.
abstract class ValueObtainer<T> {
T get value;
}
The usage of FunctionAsObtainer will still result in holding the owner from garbage collection, but two other options will not.
Just to make it clear:
void main() {
var list1 = [0,1,2];
var modifiedList1 = addMutable(list1, 3);
var list2 = [0,1,2];
var modifiedList2 = addImmutable(list2, 3);
print(list1);
print(modifiedList1);
print(list2);
print(modifiedList2);
}
List<int> addMutable(List<int> list, int element){
return list..add(element);
}
List<int> addImmutable(List<int> list, int element){
return [...list, element];
}
Output:
[0, 1, 2, 3]
[0, 1, 2, 3]
[0, 1, 2]
[0, 1, 2, 3]
All variables are passed by value. If a variable contains a primitive (int, bool, etc.), that's it. You got its value. You can do with it whatever you want, it won't affect the source value. If a variable contains an object, what it really contains is a reference to that object.
The reference itself is also passed by value, but the object it references is not passed at all. It just stayed where it was. This means that you can actually make changes to this very object.
Therefore, if you pass a List and if you .add() something to it, you have internally changed it, like it is passed by reference. But if you use the spread operator [...list], you are creating a fresh new copy of it. In most cases that is what you really want to do.
Sounds complicated. Isn't really. Dart is cool.