I'm currently debugging some code and am confused as to how the following is possible:
void DoSomething(int cell, const std::multimap<int, const Foo*>& map) {
auto range = map.equal_range(cell);
if (range.first != map.end()) {
int iterated = 0;
for (auto iter = range.first; iter != range.second; ++iter) {
iterated++;
}
assert(iterated > 0);
}
}
based on my understanding of std::multimap this assertion should in any case always pass, yet it fails sometimes with iterated = 0.
Under what circumstances can this be possible?
Ok I figured it out.
I was under the wrong assumption that equal_range() would return end() as the first iterator if the multimap does not contain the requested key, but that's not correct.
If the multimap does not contain any elements for a certain key, it does not return map.end() for the first iterator, but instead it returns an iterator to the first element with a key Not Less than the requested key. So, if the multimap doesn't contain the key requested, if (range.first != map.end()) will still pass, since both the first as well as the second iterator will both point to the element with the next larger key, but then there will be no iteration.
Related
T getValue<T>(int i) {
if (T == String) return '$i'; // Error
return i; // Error
}
void main() {
var s = getValue<String>(1);
var i = getValue<int>(1);
}
I want getValue to return string if T is String and int otherwise. How to do that?
You can't restrict the type parameter to just int or String, so it will have to accept more than that (at least their least common supertype, Object, so basically any type).
It's not a particularly helpful way to code. It's possible, but not recommended:
T getValue<T>(int i) {
if (i is T) return i;
return "$i" as T;
}
This will return the int if T allows it (so T being any of int, or a super type of int, which is num, Object, dynamic or void, or any number of Comparable<X> wrappings around any any of those supertypes), and otherwise try to return a string. That will fail with a type error unless T is String (since we've already ruled out all supertypes of String).
You can still call it as getValue<bool>(42) and watch it fail, so the type argument doesn't help with correctness.
It's not particularly effective. I'd rather do:
dynamic getValue(int i, {bool toString = false}) {
if (toString) return "$i";
return i;
}
and call it as:
String x = getValue(42, toString: true); // Add `as String` if you disable downcasts.
int y = getValue(42); // And `as int` here.
The type parameter is really just making things harder. You are going to cast or type-check the result anyway, so might as well do it at the call point, rather than introduce type variables that aren't actually preventing misuse anyway.
(I'd probably just do two different functions, but I assume that there is a reason for wanting one function).
As I mentioned in the comments, I don't see any way that you could use your generic as the return type of your getValue function. Even assuming the return under the if statement worked, there is nothing that can be done about trying to return int i when List is passed as the type. You'll be trying to return an int as a List.
If you change it to dynamic, your code will work fine as it's just using the generic as another parameter.
dynamic getValue<T>(int i) {
if (T == String) return '$i';
return i;
}
void main() {
var s = getValue<String>(1);
var i = getValue<int>(1);
}
Why does this code snippet outputs 5 (as expected due to scope chain) ?
let arr = []
var firstFunc;
for(var i = 0; i < 5; i++) {
var iterFunc = function () {
return function() {
return i
}
}
arr.push(iterFunc())
}
console.log(arr[0]())
but this outputs {a: 0}:
let arr = []
var firstFunc;
for(var i = 0; i < 5; i++) {
var iterFunc = function () {
return {
a: i
}
}
arr.push(iterFunc())
}
console.log(arr[0])
what memory allocation logic occurs under the hood ? Why 'entity' object persists current value in contrast to closure ?
Returning i or {a: i} here doesn't matter.
The important thing is that in first example, iterFunc() returns a function and is inside that (not yet invoked) function where i or {a: i} is evaluated.
Having i is always holding a scalar (immutable) value, that value is what you get in place. (If i would have been an object, a reference to that object would be returned and if, its contents mutated, you could see that mutations).
Being immutable value, you will get that value. But, as you know, i value changes in time, so the key thing here is WHEN that value is read.
If you see at your first example, in console.log(...) statement, you are intentionally invoking it as the function you know it is (the unnamed function returned by iterFunc()) and, that time, i is holding a value of 5.
If, in your first example, you just change following line:
arr.push(iterFunc())
by
arr.push(iterFunc()())
...and, of course:
console.log(arr[0]())
by
console.log(arr[0]) // Same of your second example.
You will realize that output will be the same in both cases (0).
I have a question.
When working with Dart, I can't check to see if 2 arrays are equal.
(in other languages, I can do with ==)
In fact, I just can do that == with String or number.
List arr1 = [1,2,3];
List arr2 = [1,2,3];
if (arr1 == arr2) {
print("equal");
} else {
print("not equal");
}
// Output: not equal.
So I wonder how does that make sense. I mean, How we can do if the == is just work for the cases of String or number (if the values compared are the same).
How do I have to do if I want to check that kind of comparison (equal) for List, Map, ..
It just work for String & number.
arr1 and arr2 are different instances of an object of type List. By default different instances are always different.
When a class implements a custom == operator it can override this behavior. Some classes have a custom implementation by default like int and String.
This can easily be done for immutable objects but not for mutable. One reason is that usually the hashCode is calculated from the values stroed in a class and the hashCode must not change for an instance because this can for example cause that an instance stored in a map can't be retrieved anymore when the hashcode of the key changed.
As a workaround there is a library that provides helper functions to compare lists/iterables.
import 'package:collection/equality.dart';
void main(List<String> args) {
if (const IterableEquality().equals([1,2,3],[1,2,3])) {
// if (const SetEquality().equals([1,2,3].toSet(),[1,2,3].toSet())) {
print("Equal");
} else {
print("Not equal");
}
}
Given a Map, assignment, what is the fastest way to check if it contains any duplicate values in Dart? I am currently using a Set formed from the Map's values and checking its length against the original Map, which works of course, but I'm wondering if there's an especially performant alternative.
Set d = new Set.from(assignment.values);
if (d.length < assignment.length) {
return false; // indicates has duplicates in this context
}
EDIT:
Tried #mezoni's solution modified for my program, but it actually ran a bit slower than my original version. It probably has more to do with constant times than anything else.
List values = new List.from(assignment.values);
Set set = new Set();
for (var i = 0; i < assignment.length; i++) {
if (!set.add(values[i])) {
return false;
}
}
Complexity wise you won't be able to get anything faster. Creating the Set and filling it with the values of the Map is linear in the number of elements. Clearly you have to run through all the values, so you can't do any better than that.
Maybe you could find a solution with a smaller constant factor, but that's not clear. In particular for larger sets I think the Set solution is pretty efficient.
This is more of a algorithms question than a Dart question. In any case, you have to check every value against the others, giving n-1 + n-2 + ... + n-(n-1) checks, or n^2/2. Programmatically, it's easy to create a set, but you could also generate an array, sort the array, and then iterate once to check for duplicates. That finishes in O(n log n).
Fastets way (if you realy need better performance):
void main() {
// Values from map
var values = [1,2,3,2,1,3,2,1];
var length = values.length;
var set = new Set();
var duplicate = false;
// Only for statistics purpose
var statistics = 0;
for(var i = 0; i < length; i++) {
statistics++;
if(!set.add(values[i])) {
duplicate = true;
break;
}
}
print("Duplicate: $duplicate");
print("Performed in ${statistics} iteration(s) from $length possible");
}
Output:
Duplicate: true
Performed in 4 iteration(s) from 8 possible
P.S.
The first example can be recommended to use with List values.
But because Map.values not a List but Iterable then it would be more efficient do not convert them to List but use as is.
Here is modified sample for use with Iterable objects.
It will be work faster because in this algorithm not required convert all values to the List object because it not want using of all elements without exception.
Instead it wants use as less as possible access operation on original source. If the source supports lazy operation of the access to values (as Iterable) this will be even better.
void main() {
// Values from map
var values = [1,2,3,2,1,3,2,1];
var assignment = {};
var length = values.length;
var key = 0;
for(var value in values) {
assignment[key++] = value;
}
var set = new Set();
var duplicate = false;
// Only for statistics purpose
var statistics = 0;
for(var value in assignment.values) {
statistics++;
if(!set.add(value)) {
duplicate = true;
break;
}
}
print("Duplicate: $duplicate");
print("Performed in ${statistics} iteration(s) from $length possible");
}
How to get the length of a string array like
str 30 name[];//dynamic array
I used the following for getting the length,but it showing the error as "the variable is not of the type CLASS."
int len=name.get_length();
It sounds like you might be happier using the Array collection class.
http://msdn.microsoft.com/en-us/library/array.aspx
static void TestArray(Args _args)
{
Array strArray = new Array(Types::String);
;
strArray.value(1, 'abc');
strArray.value(2, 'def');
info(strfmt("%1", strArray.lastIndex()));
}
You need the dimOf function. Take a look to the reference:
http://msdn.microsoft.com/en-us/library/aa597117.aspx
Sorry, there is no build-in function to return the string array size. Since you are in full control what you put in the array, there need not be any!
The built-in function dimof returns the allocated size of the array, which is only of practical value for a fixed size array like str 30 name[20], where dimof(name) returns 20.
A clean way to remain in control, is to use a setter function:
static void TestArray(Args _args)
{
str 30 name[];
int n = 0;
int i;
void nameSet(int _i, str 30 _name)
{
n = max(n,_i);
name[_i] = _name;
}
;
nameSet(2,'abc');
nameSet(4,'def');
for (i = 1; i <= n; i++)
info(name[i]);
}
There is no upper bound index limit, so accessing name[7] is perfectly valid and in this case returns a blank value. This may be used to your advantage, if you always use all holes and never stores a blank:
static void TestArray(Args _args)
{
str 30 name[];
int i;
name[1] = 'abc';
name[2] = 'def';
for (i = 1; name[i]; i++)
info(name[i]);
}
Beware that accessing a higher index (in this case higher than 2) may in fact increase the allocated size of the array.