The function below works even though I intentionally deleted 'return' command:
main() {
add(i) => i + 2; //I intentionally deleted 'return'
print(add(3)); //5
}
But, the function below doesn't work after I intentionally deleted the 'return' command.
main() {
makeAdder(num addBy) {
return (num i) {
addBy + i; //I intentionally deleted 'return'
};
}
var add2 = makeAdder(2);
print(add2(3) ); //expected 5, but null.
}
Edited to clarify my question.
The last sentence in the latter function above, add2(3) doesn't return a value(I expect 5) but just null returns.
My question is why 'addBy + i' of the latter function doesn't return contrary to the fact that 'add(i) => i + 2' of the first function returns 'i + 2'.
Edited again.
The answer is in the fact of '=>' being {return }, not just {}.
main() {
makeAdder(num addBy) => (num i) { return addBy + i; };
var add2 = makeAdder(2);
print(add2(3) ); // 5
}
Even the code below works as '=>' has 'return' command in it.
main() {
makeAdder(num addBy) => (num i) => addBy + i; ;
var add2 = makeAdder(2);
print(add2(3) ); //5
}
In Dart each function without an explicit return someValue; returns null;
The null object does not have a method 'call'.
makeAdder (add2) without return returns null and null(3) leads to the exception.
I would like to quote two important note here. It might help others:
Though Dart is Optionally typed ( meaning, specifying the return type of a function such as int or void is optional ), it always recommended to specify type wherever possible. In your code, as a sign of good programming practice, do mention the return type.
If your function does not return a value then specify void. If you omit the return type then it will by default return null.
All functions return a value. If no return value is specified, the statement return null; is implicitly appended to the function body.
Related
I'm studying recursion and I wrote this method to calculate the N° number of the Fibonacci series:
fibonacci(int n, Map memo) {
if (memo.containsKey(n)) return memo[n]; // Memo check
if (n <= 2) return 1; // base case
// calculation
memo[n] = fibonacci(n - 1, memo) + fibonacci((n - 2), memo);
return memo[n];
}
I think it doesn't need to be explained, my problem is just how to call this function from the main, avoiding providing an empty Map.
this is how I call the function now:
fibonacci(n, {});
But I would rather prefer to call it just like this:
fibonacci(n);
The canonical approach is to make memo optional, and use a fresh map if the memo argument is omitted. Because you want to change and update the map, you can't use a default value for the parameter, because default values must be constant, and constant maps are not mutable.
So, written very concisely:
int fibonacci(int n, [Map<int, int>? memo]) {
if (n <= 2) return 1;
return (memo ??= {})[n] ??= fibonacci(n - 1, memo) + fibonacci(n - 2, memo);
}
The ??= operator assigns to the right-hand side if the value is null.
It's used both to initialize memo to a new map if the argument was omitted,
and to update the map if a cached value wasn't present.
I'd actually reconsider using a map. We know that the Fibonacci computation will compute a value for every prior number down to 1, so I'd just use a list instead:
int fibonacci(int n, [List<int?>? memo]) {
if (n <= 2) return 1;
return (memo ??= List<int?>.filled(n - 2))[n - 3] ??=
fibonacci(n - 1, memo) + fibonacci(n - 2, memo);
}
That should work just like the map.
(I subtract 3 from n when doing the lookup because no value below 3 needs the list - it's handled by the prior if).
There are multiple ways to do it. This is my personal favorite, because it also limits the function that is only used for internal means and it doesn't have the need to check every recursion, as you already know there is a map provided:
int fibonacci(int n) {
return _fibonacci(n, {});
}
int _fibonacci(int n, Map<int, int> memo) {
if (n <= 2) return 1; // base case
final previouslyCalculated = memo[n]; // Memo check
if(previouslyCalculated != null) {
return previouslyCalculated;
}
// calculation
final next = _fibonacci(n - 1, memo) + _fibonacci((n - 2), memo);
memo[n] = next;
return next;
}
void main() {
print(fibonacci(4));
}
As Dart does not support overloading, if you actually need both versions to be publicly available (or want both private) you would have to pick different names.
Please note that I added proper types to your methods and cleaned them up a bit for everything that would not compile once proper types are used. Make sure you always use proper types and don't rely on dynamic to somehow works it's magic. The compiler can only help you, if you are explicit about what you want to do. Otherwise they can only nod and let you run into any mistake you may have made. Be smart, let your compiler help, it will catch a lot of errors for you at compile time that you would otherwise have to spent countless hours on debugging.
This is the solution I've found so far but looks very verbose and inelegant:
fibonacci(int n, [Map<int, int>? memo]) {
memo == null ? memo = {} : null; // null check
if (memo.containsKey(n)) return memo[n];
if (n <= 2) return 1;
memo[n] = fibonacci(n - 1, memo) + fibonacci((n - 2), memo);
return memo[n];
}
In this way I can call just:
fibonacci(n);
The code Dart is complaining about:
Map<String,int> get_frequency(String text) {
Map<String,int> retval = {};
for (int i = 0; i<text.length; ++i) {
retval[text[i]] = retval[text[i]] ?? 0;
retval[text[i]]++; //<---- this is where dart is complaining.*
}
return retval;
}
void main() {
const paragraphOfText = 'Once upon a time there was a Dart programmer who '
'had a challenging challenge to solve. Though the challenge was great, '
'a solution did come. The end.';
var data = get_frequency(paragraphOfText);
print(data);
}
Obviously the line marked with (*) can not be null, so how do I tell that to Dart? I tried the null assertion operator (!), but that didn't work.
Null Safety is enabled.
Error message:
challenge2.dart:5:20: Error: Operator '+' cannot be called on 'int?' because it is potentially null.
retval[text[i]]++;
^
challenge2.dart:5:20: Error: A value of type 'num' can't be assigned to a variable of type 'int'.
retval[text[i]]++;
^
A possible solution is to change the line
retval[text[i]]++; //<---- this is where dart is complaining.*
into
retval[text[i]] = retval[text[i]]! + 1;
Just figured it out.
Functions in Dart are first-class objects, allowing you to pass them to other objects or functions.
void main() {
var shout = (msg) => ' ${msg.toUpperCase()} ';
print(shout("yo"));
}
This made me wonder if there was a way to modify a function a run time, just like an object, prior to passing it to something else. For example:
Function add(int input) {
return add + 2;
}
If I wanted to make the function a generic addition function, then I would do:
Function add(int input, int increment) {
return add + increment;
}
But then the problem would be that the object I am passing the function to would need to specify the increment. I would like to pass the add function to another object, with the increment specified at run time, and declared within the function body so that the increment cannot be changed by the recipient of the function object.
The answer seems to be to use a lexical closure.
From here: https://dart.dev/guides/language/language-tour#built-in-types
A closure is a function object that has access to variables in its
lexical scope, even when the function is used outside of its original
scope.
Functions can close over variables defined in surrounding scopes. In
the following example, makeAdder() captures the variable addBy.
Wherever the returned function goes, it remembers addBy.
/// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(int addBy) {
return (int i) => addBy + i;
}
void main() {
// Create a function that adds 2.
var add2 = makeAdder(2);
// Create a function that adds 4.
var add4 = makeAdder(4);
assert(add2(3) == 5);
assert(add4(3) == 7);
}
In the above cases, we pass 2 or 4 into the makeAdder function. The makeAdder function uses the parameter to create and return a function object that can be passed to other objects.
You most likely don't need to modify a closure, just the ability to create customized closures.
The latter is simple:
int Function(int) makeAdder(int increment) => (int value) => value + increment;
...
foo(makeAdder(1)); // Adds 1.
foo(makeAdder(4)); // Adds 2.
You can't change which variables a closure is referencing, but you can change their values ... if you an access the variable. For local variables, that's actually hard.
Mutating state which makes an existing closure change behavior can sometimes be appropriate, but those functions should be very precise about how they change and where they are being used. For a function like add which is used for its behavior, changing the behavior is rarely a good idea. It's better to replace the closure in the specific places that need to change behavior, and not risk changing the behavior in other places which happen to depend on the same closure. Otherwise it becomes very important to control where the closure actually flows.
If you still want to change the behavior of an existing global, you need to change a variable that it depends on.
Globals are easy:
int increment = 1;
int globalAdder(int value) => value + increment;
...
foo(globalAdd); // Adds 1.
increment = 2;
foo(globalAdd); // Adds 2.
I really can't recommend mutating global variables. It scales rather badly. You have no control over anything.
Another option is to use an instance variable to hold the modifiable value.
class MakeAdder {
int increment = 1;
int instanceAdd(int value) => value + increment;
}
...
var makeAdder = MakeAdder();
var adder = makeAdder.instanceAdd;
...
foo(adder); // Adds 1.
makeAdder.increment = 2;
foo(adder); // Adds 2.
That gives you much more control over who can access the increment variable. You can create multiple independent mutaable adders without them stepping on each other's toes.
To modify a local variable, you need someone to give you access to it, from inside the function where the variable is visible.
int Function(int) makeAdder(void Function(void Function(int)) setIncrementCallback) {
var increment = 1;
setIncrementCallback((v) {
increment = v;
});
return (value) => value + increment;
}
...
void Function(int) setIncrement;
int Function(int) localAdd = makeAdder((inc) { setIncrement = inc; });
...
foo(localAdd); // Adds 1.
setIncrement(2);
foo(localAdd); // Adds 2.
This is one way of passing back a way to modify the local increment variable.
It's almost always far too complicated an approach for what it gives you, I'd go with the instance variable instead.
Often, the instance variable will actually represent something in your model, some state which can meaningfully change, and then it becomes predictable and understandable when and how the state of the entire model changes, including the functions referring to that model.
Using partial function application
You can use a partial function application to bind arguments to functions.
If you have something like:
int add(int input, int increment) => input + increment;
and want to pass it to another function that expects to supply fewer arguments:
int foo(int Function(int input) applyIncrement) => applyIncrement(10);
then you could do:
foo((input) => add(input, 2); // `increment` is fixed to 2
foo((input) => add(input, 4); // `increment` is fixed to 4
Using callable objects
Another approach would be to make a callable object:
class Adder {
int increment = 0;
int call(int input) => input + increment;
}
which could be used with the same foo function above:
var adder = Adder()..increment = 2;
print(foo(adder)); // Prints: 12
adder.increment = 4;
print(foo(adder)); // Prints: 14
According to the documentation, the function identical checks whether two references are to the same object.
With that in mind, I don't understand why the following is the case:
int a = 1;
int b = 1;
print(identical(a, b)); // prints 'true'
Map c = { 1: 'y' };
Map d = { 1: 'y' };
print(identical(c, d)); // prints 'false'
I'd expect both calls to return 'false'.
identical compares references. a and b are references to a compile time literal 1. Thus they are identical.
I am using the JSON2 library in order to use JSON.stringify to send some JSON data to my MVC controller.
When I include another script in my view (Telerik MVC) I start to get script conflicts when using IE7.
When I click the refresh button in the grid, I get the following error:
Line: 191
Error: Object doesn't support this property or method
String.prototype.toJSON =
Number.prototype.toJSON =
Boolean.prototype.toJSON = function (key) {
return this.valueOf();
};
The error occurs on the following line specifically:
return this.valueOf();
Does anyone have any insight into why this conflict is occurring and how to resolve it? Specifically, why would this work in IE8/Chrome but fail in IE7. What would cause the error? Are both scripts trying to define the same method and that's why it is failing or is it impossible to tell without digging through tons of code?
Edit:
This is the json2.js library I am speaking of: https://github.com/douglascrockford/JSON-js
Probably the reply is too late, but I thought it's worth replying as this might save some valuable lives ;)
The JSON2 script won't initialize/extend the JSON object if there is an existing implementation(Native or Included). However if the JSON object does not exist, the script will create that object and attach few methods to it (JSON.stringify and JSON.parse to be precise). However in order to make those methods to work, there are other objects (like Date, String, Number and Boolean objects) which need to be extended to support certain methods (like toJSON method). The JSON2 script takes care of extending the required objects as well.
Now coming to the specific issue here (Telerik MVC). I faced the same problem while working with Telerik for one of the Projects. However I was able to trace it. The probable cause is the conflict between Telerik scripts and the current JSON2 script. The Date and Boolean Objects' toJSON method somehow conflicts with Telerik's implmentation of the same method for those two objects which breaks the Telerik script at some places. I have modified the JSON2 library for a more robust check which doesn't fail in any scenario (even on use of Telerik MVC on the page). I have tested the script and it works fine for me, however in case someone finds any further conflicts, please reply back.
var JSON;
if (!JSON) {
JSON = {};
}
(function () {
'use strict';
function f(n) {
// Format integers to have at least two digits.
return n < 10 ? '0' + n : n;
}
if (typeof Date.prototype.toJSON !== 'function') {
Date.prototype.toJSON = function (key) {
return isFinite(this.valueOf())
? this.getUTCFullYear() + '-' +
f(this.getUTCMonth() + 1) + '-' +
f(this.getUTCDate()) + 'T' +
f(this.getUTCHours()) + ':' +
f(this.getUTCMinutes()) + ':' +
f(this.getUTCSeconds()) + /*added - start*/ '.'+
f(this.getUTCMilliseconds()) + /*added - end*/ 'Z'
: null;
};
//pushed the below code outside current if block
// String.prototype.toJSON =
// Number.prototype.toJSON =
// Boolean.prototype.toJSON = function (key) {
// return this.valueOf();
// };
}
/*added - start*/
if (typeof String.prototype.toJSON !== 'function') {
String.prototype.toJSON = function (key) {
return ((typeof this.valueOf === 'function') ? this.valueOf(): this.toString());
};
}
if (typeof Number.prototype.toJSON !== 'function') {
Number.prototype.toJSON = function (key) {
return ((typeof this.valueOf === 'function') ? this.valueOf(): this.toString());
};
}
if (typeof Boolean.prototype.toJSON !== 'function') {
Boolean.prototype.toJSON = function (key) {
return ((typeof this.valueOf === 'function') ? this.valueOf(): this.toString());
};
}
/*added - end*/
var cx = /[\u0000\u00ad\u0600-\u0604\u070f\u17b4\u17b5\u200c-\u200f\u2028-\u202f\u2060-\u206f\ufeff\ufff0-\uffff]/g,
escapable = /[\\\"\x00-\x1f\x7f-\x9f\u00ad\u0600-\u0604\u070f\u17b4\u17b5\u200c-\u200f\u2028-\u202f\u2060-\u206f\ufeff\ufff0-\uffff]/g,
gap,
indent,
meta = { // table of character substitutions
'\b': '\\b',
'\t': '\\t',
'\n': '\\n',
'\f': '\\f',
'\r': '\\r',
'"' : '\\"',
'\\': '\\\\'
},
rep;
function quote(string) {
// If the string contains no control characters, no quote characters, and no
// backslash characters, then we can safely slap some quotes around it.
// Otherwise we must also replace the offending characters with safe escape
// sequences.
escapable.lastIndex = 0;
return escapable.test(string) ? '"' + string.replace(escapable, function (a) {
var c = meta[a];
return typeof c === 'string'
? c
: '\\u' + ('0000' + a.charCodeAt(0).toString(16)).slice(-4);
}) + '"' : '"' + string + '"';
}
function str(key, holder) {
// Produce a string from holder[key].
var i, // The loop counter.
k, // The member key.
v, // The member value.
length,
mind = gap,
partial,
value = holder[key];
// If the value has a toJSON method, call it to obtain a replacement value.
if (value && typeof value === 'object' &&
typeof value.toJSON === 'function') {
value = value.toJSON(key);
}
// If we were called with a replacer function, then call the replacer to
// obtain a replacement value.
if (typeof rep === 'function') {
value = rep.call(holder, key, value);
}
// What happens next depends on the value's type.
switch (typeof value) {
case 'string':
return quote(value);
case 'number':
// JSON numbers must be finite. Encode non-finite numbers as null.
return isFinite(value) ? String(value) : 'null';
case 'boolean':
case 'null':
// If the value is a boolean or null, convert it to a string. Note:
// typeof null does not produce 'null'. The case is included here in
// the remote chance that this gets fixed someday.
return String(value);
// If the type is 'object', we might be dealing with an object or an array or
// null.
case 'object':
// Due to a specification blunder in ECMAScript, typeof null is 'object',
// so watch out for that case.
if (!value) {
return 'null';
}
// Make an array to hold the partial results of stringifying this object value.
gap += indent;
partial = [];
// Is the value an array?
if (Object.prototype.toString.apply(value) === '[object Array]') {
// The value is an array. Stringify every element. Use null as a placeholder
// for non-JSON values.
length = value.length;
for (i = 0; i < length; i += 1) {
partial[i] = str(i, value) || 'null';
}
// Join all of the elements together, separated with commas, and wrap them in
// brackets.
v = partial.length === 0
? '[]'
: gap
? '[\n' + gap + partial.join(',\n' + gap) + '\n' + mind + ']'
: '[' + partial.join(',') + ']';
gap = mind;
return v;
}
// If the replacer is an array, use it to select the members to be stringified.
if (rep && typeof rep === 'object') {
length = rep.length;
for (i = 0; i < length; i += 1) {
if (typeof rep[i] === 'string') {
k = rep[i];
v = str(k, value);
if (v) {
partial.push(quote(k) + (gap ? ': ' : ':') + v);
}
}
}
} else {
// Otherwise, iterate through all of the keys in the object.
for (k in value) {
if (Object.prototype.hasOwnProperty.call(value, k)) {
v = str(k, value);
if (v) {
partial.push(quote(k) + (gap ? ': ' : ':') + v);
}
}
}
}
// Join all of the member texts together, separated with commas,
// and wrap them in braces.
v = partial.length === 0
? '{}'
: gap
? '{\n' + gap + partial.join(',\n' + gap) + '\n' + mind + '}'
: '{' + partial.join(',') + '}';
gap = mind;
return v;
}
}
// If the JSON object does not yet have a stringify method, give it one.
if (typeof JSON.stringify !== 'function') {
JSON.stringify = function (value, replacer, space) {
// The stringify method takes a value and an optional replacer, and an optional
// space parameter, and returns a JSON text. The replacer can be a function
// that can replace values, or an array of strings that will select the keys.
// A default replacer method can be provided. Use of the space parameter can
// produce text that is more easily readable.
var i;
gap = '';
indent = '';
// If the space parameter is a number, make an indent string containing that
// many spaces.
if (typeof space === 'number') {
for (i = 0; i < space; i += 1) {
indent += ' ';
}
// If the space parameter is a string, it will be used as the indent string.
} else if (typeof space === 'string') {
indent = space;
}
// If there is a replacer, it must be a function or an array.
// Otherwise, throw an error.
rep = replacer;
if (replacer && typeof replacer !== 'function' &&
(typeof replacer !== 'object' ||
typeof replacer.length !== 'number')) {
throw new Error('JSON.stringify');
}
// Make a fake root object containing our value under the key of ''.
// Return the result of stringifying the value.
return str('', {'': value});
};
}
// If the JSON object does not yet have a parse method, give it one.
if (typeof JSON.parse !== 'function') {
JSON.parse = function (text, reviver) {
// The parse method takes a text and an optional reviver function, and returns
// a JavaScript value if the text is a valid JSON text.
var j;
function walk(holder, key) {
// The walk method is used to recursively walk the resulting structure so
// that modifications can be made.
var k, v, value = holder[key];
if (value && typeof value === 'object') {
for (k in value) {
if (Object.prototype.hasOwnProperty.call(value, k)) {
v = walk(value, k);
if (v !== undefined) {
value[k] = v;
} else {
delete value[k];
}
}
}
}
return reviver.call(holder, key, value);
}
// Parsing happens in four stages. In the first stage, we replace certain
// Unicode characters with escape sequences. JavaScript handles many characters
// incorrectly, either silently deleting them, or treating them as line endings.
text = String(text);
cx.lastIndex = 0;
if (cx.test(text)) {
text = text.replace(cx, function (a) {
return '\\u' +
('0000' + a.charCodeAt(0).toString(16)).slice(-4);
});
}
// In the second stage, we run the text against regular expressions that look
// for non-JSON patterns. We are especially concerned with '()' and 'new'
// because they can cause invocation, and '=' because it can cause mutation.
// But just to be safe, we want to reject all unexpected forms.
// We split the second stage into 4 regexp operations in order to work around
// crippling inefficiencies in IE's and Safari's regexp engines. First we
// replace the JSON backslash pairs with '#' (a non-JSON character). Second, we
// replace all simple value tokens with ']' characters. Third, we delete all
// open brackets that follow a colon or comma or that begin the text. Finally,
// we look to see that the remaining characters are only whitespace or ']' or
// ',' or ':' or '{' or '}'. If that is so, then the text is safe for eval.
if (/^[\],:{}\s]*$/
.test(text.replace(/\\(?:["\\\/bfnrt]|u[0-9a-fA-F]{4})/g, '#')
.replace(/"[^"\\\n\r]*"|true|false|null|-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?/g, ']')
.replace(/(?:^|:|,)(?:\s*\[)+/g, ''))) {
// In the third stage we use the eval function to compile the text into a
// JavaScript structure. The '{' operator is subject to a syntactic ambiguity
// in JavaScript: it can begin a block or an object literal. We wrap the text
// in parens to eliminate the ambiguity.
j = eval('(' + text + ')');
// In the optional fourth stage, we recursively walk the new structure, passing
// each name/value pair to a reviver function for possible transformation.
return typeof reviver === 'function'
? walk({'': j}, '')
: j;
}
// If the text is not JSON parseable, then a SyntaxError is thrown.
throw new SyntaxError('JSON.parse');
};
}
}());
Note: The above code is not my implementation. It is from the source https://github.com/douglascrockford/JSON-js I have just modified it a little to avoid any conflicts with Telerik or otherwise.
I have had exactly the same problem.
I couldn't find any other solution than to edit the json2.js file like you suggested, thanks for that.
However, I found that this would fix the issue for IE7 and still work in IE8/9 as well as firefox, but it now stopped working in Chrome ("this" for [this.valueOf === 'function'] is undefined).
Have you run into that issue, too, or did yours work in Chrome? I'm trying to figure out if this is related to my data or telerik-internal.
Thanks for your post!
Edit:
For now I have just returned null if "this" is undefined/null (in all three functions). Seems to work in all browsers and allows the Telerik grid to rebind without problems.
I don't know how correct this is in the global context of json2.js .toJSON method, though.