How to convert a list from one type to another without a for loop?
List <String> lstring = <String>["1", "2"];
List <int> lint = lstring.map(int.parse);
I get the error:
type 'MappedListIterable<String, int>' is not a subtype of type 'List<int>'
You need to add a toList() to the end of the second line.
List <String> lstring = <String>["1", "2"];
List <int> lint = lstring.map(int.parse).toList();
This will do it.
final List<String> lstring = ["1", "2"];
final List<int> newList = lstring.map((e)=>int.parse(e)).toList();
Related
How can i outputs result between 2 specific characters in dart String
example
String myVlue = 'helloWorld';
wanted result is : anything between 'hel' and 'ld'
so the result is 'loWor'
Note : in my case the two specific characters are fixed and Unique
How can i tell dart to do that in best way .
thanks
You could define a regular expression to catch a group from your input:
void main() {
String myValue = 'helloWorld';
RegExp regExp = RegExp(r'hel(.*)ld');
String extract = regExp.firstMatch(myValue)![1]!;
print(extract); // loWor
}
I'm a beginner in dart.
void main() {
var abf = '+37.4054-122.0999/';
var abf2;
abf2 = abf.replaceAll("+"," ");
var abf1 = abf2.split(RegExp('(?=[+-])'));
print (abf1[0]);
print (abf1[1]);
}
The above code splits abf into two values for me
I want to remove the ending '/'. I tried many split methods using other variables but it's not removing the '/' even though its removing the '+'.
It's not really clear what you're trying to do with the split.
But if you're looking the remove the / this should work:
String number = '+37.4054-122.0999/';
number = number.replaceAll("/"," ");
You can create substring from this while you like to remove last element.
String abf = '+37.4054-122.0999/';
final result = abf.substring(0, abf.length - 1);
print(result);
Dart's List class has a built-in removeLast method. Maybe you can try to split the string and then removing the last element:
String str = "str";
String newStr = str.split(''). removeLast().join('');
The end goal is to assign a list of Futures to a list of variables.
This is how I would accomplish this goal in node;
let name;
let number;
[name, number] = await Promise.all([promiseName(), promiseNumber()]);
I know that with Dart there is await Future.wait([]);, but I'm unsure how to go about assign the results to different variables instead of having the List<Future> assigned to a single variable and then I go through that variable via index.
Thanks!
There is currently no way in Dart to assign to more than one variable at a time.
You'll have to do:
String name;
int number;
var list = await Future.wait([asyncName(), asyncNumber()]);
name = list[0] as String;
number = list[1] as int;
if you use existing variables, or
var list = await Future.wait([asyncName(), asyncNumber()]);
var name = list[0] as String;
var number = list[1] as int;
if you can declare them after you have the results.
I wrote this code to convert dynamic list to Word list but linter says:
Omit type annotations for local variables. on 2nd line.
However if I omit type annotations, I get an error A value of type 'List<dynamic>' can't be returned from method 'convert' because it has a return type of 'List<Word>'.
It there any smarter way to convert?
static List<Word> convert(List<dynamic> words) {
final List<Word> wordsList = [];
words.forEach((v) {
final map = Map<String, dynamic>.from(v as Map<dynamic, dynamic>);
wordsList.add(Word.fromMap(map));
});
return wordsList;
}
Word.fromMap is:
Word.fromMap(Map<String, dynamic> map)
: text = map['text'] as String,
count = map['count'] as int;
To avoid the warning, and put the type on the right-hand side as the lint wants, just write:
final wordsList = <Word>[];
I assume words is JSON data, so the maps are already Map<String, dynamic>. Then you can also do everything in one line:
static List<Word> convert(List<dynamic> words) =>
[for (var v in words) Word.fromMap(v)];
Use the cast() method like this:
class Word {
final String text;
final int count;
static List<Word> convert(List<dynamic> words) {
final List<Word> wordsList = [];
words.cast<Map<dynamic, dynamic>>().forEach((v) { // <-- look here
final map = Map<String, dynamic>.from(v);
wordsList.add(Word.fromMap(map));
});
return wordsList;
}
Word.fromMap(Map<String, dynamic> map)
: text = map['text'] as String,
count = map['count'] as int;
}
It will ensure the casting are done on each element. Make sure the type are correct since it will else result in a type-cast error.
None of the examples I have seen thus far appear to address the problem of marshaling a structure containing a union of structures that contain recursive references. I am attempting to write a marshaler for a structure which contains these and have so far failed.
For example:
typedef enum {
My_StructA = 0x7878,
My_StructB
} MyStructTag;
typedef struct _MyStruct MyStruct;
struct _MyStruct {
MyStructTag discriminator;
union {
struct {
int a;
int b;
} StructA;
struct {
int c;
MyStruct* d;
} StructB;
} MyUnion;
};
I attempted to define the structures as follows:
type MyStructTag =
| My_StructA = 0x7878
| My_StructB = 0x7879
[<Struct; StructLayout(LayoutKind.Sequential)>]
type StructA =
val mutable a : int
val mutable b : int
[<Struct; StructLayout(LayoutKind.Sequential)>]
type StructB =
val mutable c : int
val mutable d : MyStruct
[<Struct; StructLayout(LayoutKind.Explicit)>]
type MyStruct =
[<FieldOffset(0)>] val discriminator : MyStructTag
[<FieldOffset(4)>] val structA : StructA
[<FieldOffset(4)>] val structB : StructB
Note that the reason I bothered to define MyStruct explicitly is to allow myself to make use of Marshal.OffsetOf() and Marshal.SizeOf() when writing the custom marshaler for this structure. From what I have seen, writing a custom marshaler is the only way to handle unions. If I am wrong about that, references would be greatly appreciated!
The error I receive when writing the above code is:
error FS0039: The type 'MyStruct' is not defined
I assume this is because only discriminated union types can be defined recursively. However, I am not aware of any other way to represent these structures in F#.
Thank you in advance for your time.
You've got two problems. First of all, any mutually recursive types (whether discriminated unions, classes, or structs) need to be defined using
type A = ...
and B = ...
rather than
type A = ...
type B = ...
(and note that attributes can come before or after the word type, but only after the word and...). However, if you try this, you'll see that you just get a different error, because structs can't be directly recursive as fields of each other. If struct A had a field which was a struct B and struct B had a field which was a struct A (and either of them had any other fields), then the size would be infinite. Note that this is true of your C code as well - StructB contains a pointer to a MyStruct, not a MyStruct itself. In .NET, you can use an IntPtr for this - in F# you can use the nativeint alias or nativeptr<MyStruct>. Try this:
open System.Runtime.InteropServices
type MyStructTag =
| My_StructA = 0x7878
| My_StructB = 0x7879
[<Struct; StructLayout(LayoutKind.Sequential)>]
type StructA =
val mutable a : int
val mutable b : int
[<Struct; StructLayout(LayoutKind.Sequential)>]
type StructB =
val mutable c : int
val mutable d : nativeptr<MyStruct>
and [<Struct; StructLayout(LayoutKind.Explicit)>]MyStruct =
[<FieldOffset(0)>] val discriminator : MyStructTag
[<FieldOffset(4)>] val structA : StructA
[<FieldOffset(4)>] val structB : StructB