I'm new to Dart. I'm calculating the price of a pizza order. In my current solution, I'm using the assertion operator. What do you think about it?
I've read many times that you shouldn't use it. Do you think my code is ok, or would you do something better/different?
void main() {
const List<String> order = ['margherita', 'pepperoni', 'pineapple'];
calcTotalPrice(order: order);
}
calcTotalPrice({required List<String> order}) {
final Map<String, double> pizzaPrices = {
'margherita': 5.5,
'pepperoni': 7.5,
'vegetarian': 6.5
};
double total = 0.0;
for (var item in order) {
pizzaPrices[item] ??= 0.0;
total += pizzaPrices[item]!; // assertion operator (!)
}
print(total);
}
Your code is fine but you can avoid collecting unknown keys in the map pizzaPrices with:
for (var item in order) {
total += pizzaPrices[item] ?? 0.0;
}
Related
I'm working through an exercise from this course. This code:
void main() {
const order = ['pepperoni', 'margherita', 'pineapple'];
print("Total: ${calculateTotal(order)}");
}
double calculateTotal(List<String> order) {
var total = 0.0;
const pizzaPrices = {
'margherita': 5.5,
'pepperoni': 7.5,
'vegetarian': 6.5,
};
for (var item in order) {
if (pizzaPrices[item]!=null) {
total += pizzaPrices[item];
}
}
return total;
}
Produces the error message The argument type 'double?' can't be assigned to the parameter type 'num'. pointing to the line total += pizzaPrices[item];
total += pizzaPrices[item]! compiles as expected, without errors.
I don't understand why the compiler would need the !, since it already knows pizzaPrices[item] cannot be null.
The reason is that the [] operator on Map is defined to return a nullable type since if the element you search for are not in the map, the [] operator will return null.
It might look obvious to you, but the compiler cannot know for sure that just because you checked the returned value from pizzaPrices[item] once, it will return the same value again the second time you ask (e.g. in some custom made Map implementation).
A solution is instead to save the value in a local variable which you can then check for null. Dart will in this case promote the variable as expected:
void main() {
const order = ['pepperoni', 'margherita', 'pineapple'];
print("Total: ${calculateTotal(order)}");
}
double calculateTotal(List<String> order) {
var total = 0.0;
const pizzaPrices = {
'margherita': 5.5,
'pepperoni': 7.5,
'vegetarian': 6.5,
};
for (var item in order) {
final pizzaPrice = pizzaPrices[item];
if (pizzaPrice != null) {
total += pizzaPrice;
}
}
return total;
}
In Vaadin 7, is there an easy way to calculate the numeric total for selected columns? I know how to do it for Vaadin 8, defined here. But since Vaadin 7 uses a container, I am trying to think of the best way to do it. Currently, this is the best way I can think of, based on the documentation here. Code is a rough draft, so I expect there are some syntax problems. Treat it more as pseudo code, if possible.
Map<Object,Double> totals = new HashMap();
for (Iterator<?> i = container.getItemIds().iterator(); i.hasNext();) {
Object itemId = i.next();
Item item = container.getItem(itemId);
for(Object totalCol : totalColumns)
{
Object columnVal = item.getItemProperty(totalCol);
Double total = totals.get(totalCol);
if(!(total instanceof Double))
total = 0.0;
if(columnVal instanceof Double)
{
total += (Double)columnVal;
}
else if(columnVal instanceof Long)
{
total += (Long)columnVal;
}
else if(columnVal instanceof Integer)
{
total += (Integer)columnVal;
}
else if(columnVal instanceof String)
{
try {
Long value = Long.parseLong((String) columnVal);
total += value;
} catch (NumberFormatException e) {
try {
Double value = Double.parseDouble((String) columnVal);
total += value;
} catch (NumberFormatException e1) {
}
}
}
totals.put(totalCol, total);
}
/* At this point, go through totals Map, and set value to correct footer column with correct
* text formatting. This part is easy, and clearly documented, so leaving it off this
* code example.
*/
}
By the way, the above idea works, my question is more if this is the best idea or not?
I have a requirement of converting Stream<List<T>> to Stream<Map<K,T>>
I have a class
class Order
{
int id;
DateTime date;
}
I want to convert Stream<List<Order>> to Stream<Map<DateTime, List<Order>>
I want to display orders as below
12-Dec-2020
Order 1
Order 2
13-Dec-2020
Order 3
Order 4
14-Dec-2020
Order 5
Order 6
Suggestions for a better DS are welcome.
How do I do this?
Please suggest.
Thanks in advance.
Using groupListsBy from collection: ^1.15.0
K keySelector(T t) { ... }
final Stream<List<T>> source$ = ...;
final Stream<Map<K, List<T>>> result$ = source$.map((list) => list.groupListsBy(keySelector));
You can do this without any package.
First, you have to create an empty Map, then map a List<Order> to a map with a key as a date from the list item (single order) then add it to That Empty map.
Check if the date is available as a key in Map. If the date exists as a key in that map then simply add a new order to that value or else simply add a new key to the Map.
Code:
Stream<List<Order>> orders = ...;
Stream<Map<DateTime, List<Order>>> ordersByDate = {};
orders.forEach((Order order){
ordersByDate.keys.contains(order.date)
? ordersByDate.update(order.date, (oldValue) => oldValue + <Order>[order])
: ordersByDate[order.date] = [order];
});
print(ordersByDate);
//At here your ordersByDate
Let me know if it works for you.
Fold method is what you're looking for.
Example:
void main() async {
final today = DateTime.now();
final tomorrow = today.add(const Duration(days: 1));
final order1 = Order(1, today);
final order2 = Order(2, today);
final order3 = Order(3, tomorrow);
final x = await Stream
.fromIterable([order1, order2, order3])
.fold<Map<DateTime, List<Order>>>({}, (val, element) {
(val[element.date] ??= []).add(element);
// Same as:
// if (val[element.date] == null) {
// val[element.date] = [];
// }
// val[element.date]!.add(element);
return val;
});
print(x);
}
class Order
{
int id;
DateTime date;
Order(this.id, this.date);
#override
String toString() {
return 'Order(id: $id, date: $date)';
}
}
Is there a native (language supported) lazy evaluation syntax? Something like lazy val in Scala.
I've gone through the docs, and could not find anything. There is only a chapter about "lazily loading a library", but it's not what I am asking.
Based on this research I incline to believe (please correct me if I'm wrong) that currently there is no such thing. But maybe you know of any plans or feature requests which will provide the functionality? Or maybe it was considered and rejected by the Dart team?
If indeed there is no native support for this, then what is the best practice (best syntax) for implementing lazy evaluation? An example would be appreciated.
Edit:
The benefits of the feature that I am looking for are mostly the same as in implementation in other languages: Scala's lazy val or C#'s Lazy<T> or Hack's __Memorize attribute:
concise syntax
delayed computation until the value is needed
cache the result (the by-need laziness)
don't break pure functional paradigm (explanation below)
A simple example:
class Fibonacci {
final int n;
int _res = null;
int get result {
if (null == _res) {
_res = _compute(this.n);
}
return _res;
}
Fibonacci(this.n);
int _compute(n) {
// ...
}
}
main(List<String> args) async {
print(new Fibonacci(5).result);
print(new Fibonacci(9).result);
}
The getter is very verbose and has a repetitive code. Moreover I can't make the constructor const because the caching variable _res has to be computed on demand. I imagine that if I had a Scala-like lazy feature then I would also have language support for having a constant constructor. That's thanks to the fact, that the lazy evaluated _res is referentially transparent, and would not be in the way.
class Fibonacci {
final int n;
int lazy result => _compute(this.n);
const Fibonacci(this.n); // notice the `const`
int _compute(n) {
// ...
}
}
main(List<String> args) async {
// now these makes more sense:
print(const Fibonacci(5).result);
print(const Fibonacci(9).result);
}
Update 2021
Lazy initialization is now part of dart from the release 2.12.
Simply add late modifier to the variable declaration
late MyClass obj = MyClass();
And this object will be initialized only when it is first used.
From the docs:
Dart 2.12 added the late modifier, which has two use cases:
Declaring a non-nullable variable that’s initialized after its
declaration.
Lazily initializing a variable.
Checkout the example here:
https://dartpad.dev/?id=50f143391193a2d0b8dc74a5b85e79e3&null_safety=true
class A {
String text = "Hello";
A() {
print("Lazily initialized");
}
sayHello() {
print(text);
}
}
class Runner {
late A a = A();
run() async {
await Future.delayed(Duration(seconds: 3));
print("First message");
a.sayHello();
}
}
Here class A will be initialized only after "First message" has been displayed.
update2
From #lrn s comment - using an Expando for caching makes it work with const:
class Lazy<T> {
static final _cache = new Expando();
final Function _func;
const Lazy(this._func);
T call() {
var result = _cache[this];
if (identical(this, result)) return null;
if (result != null) return result;
result = _func();
_cache[this] = (result == null) ? this : result;
return result;
}
}
defaultFunc() {
print("Default Function Called");
return 42;
}
main([args, function = const Lazy(defaultFunc)]) {
print(function());
print(function());
}
Try it in DartPad
update
A reusable Lazy<T> could look like below in Dart but that also doesn't work with const and can't be used in field initializers if the calculation needs to refer instance members (this.xxx).
void main() {
var sc = new SomeClass();
print('new');
print(sc.v);
}
class SomeClass {
var _v = new Lazy<int>(() {
print('x');
return 10;
});
int get v => _v();
}
class Lazy<T> {
final Function _func;
bool _isEvaluated = false;
Lazy(this._func);
T _value;
T call() {
if(!_isEvaluated) {
if(_func != null) {
_value = _func();
}
_isEvaluated = true;
}
return _value;
}
}
Try it in DartPad
original
Dart version of http://matt.might.net/articles/implementing-laziness/ using a closure to lazy evaluate:
void main() {
var x = () {
print ("foo");
return 10;
}();
print("bar");
print(x);
// will print foo, then bar then 10.
print('===');
// But, the following Scala program:
x = () {
print("foo");
return 10;
};
print ("bar");
print (x());
// will print bar, then foo, then 10, since it delays the computation of x until it’s actually needed.
}
Try it in DartPad
Update
int _val;
int get val => _val ??= 9;
Thanks #Nightscape
Old
I think this little snippet might help you...
int _val;
int get val => _val ?? _val = 9;
Hey StackOverflow Community,
So, I have this line of information from a txt file that I need to parse.
Here is an example lines:
-> date & time AC Power Insolation Temperature Wind Speed
-> mm/dd/yyyy hh:mm.ss kw W/m^2 deg F mph
Using a scanner.nextLine() gives me a String with a whole line in it, and then I pass this off into StringTokenizer, which then separates them into individual Strings using whitespace as a separator.
so for the first line it would break up into:
date
&
time
AC
Power
Insolation
etc...
I need things like "date & time" together, and "AC Power" together. Is there anyway I can specify this using a method already defined in StringTokenizer or Scanner? Or would I have to develop my own algorithm to do this?
Would you guys suggest I use some other form of parsing lines instead of Scanner? Or, is Scanner sufficient enough for my needs?
ejay
oh, this one was tricky, maybe you could build up some Trie structure with your tokens, i was bored and wrote a little class which solves your problem. Warning: it's a bit hacky, but was fun to implement.
The Trie class:
class Trie extends HashMap<String, Trie> {
private static final long serialVersionUID = 1L;
boolean end = false;
public void addToken(String strings) {
addToken(strings.split("\\s+"), 0);
}
private void addToken(String[] strings, int begin) {
if (begin == strings.length) {
end = true;
return;
}
String key = strings[begin];
Trie t = get(key);
if (t == null) {
t = new Trie();
put(key, t);
}
t.addToken(strings, begin + 1);
}
public List<String> tokenize(String data) {
String[] split = data.split("\\s+");
List<String> tokens = new ArrayList<String>();
int pos = 0;
while (pos < split.length) {
int tokenLength = getToken(split, pos, 0);
tokens.add(glue(split, pos, tokenLength));
pos += tokenLength;
}
return tokens;
}
public String glue(String[] parts, int pos, int length) {
StringBuilder sb = new StringBuilder();
sb.append(parts[pos]);
for (int i = pos + 1; i < pos + length; i++) {
sb.append(" ");
sb.append(parts[i]);
}
return sb.toString();
}
private int getToken(String[] tokens, int begin, int length) {
if (end) {
return length;
}
if (begin == tokens.length) {
return 1;
}
String key = tokens[begin];
Trie t = get(key);
if (t != null) {
return t.getToken(tokens, begin + 1, length + 1);
}
return 1;
}
}
and how to use it:
Trie t = new Trie();
t.addToken("AC Power");
t.addToken("date & time");
t.addToken("date & foo");
t.addToken("Speed & fun");
String data = "date & time AC Power Insolation Temperature Wind Speed";
List<String> tokens = t.tokenize(data);
for (String s : tokens) {
System.out.println(s);
}