Hello I try to make null safety migration, but I have an error with a forEach loop who return return true. I don't know how to write correctly in null stafety. Thank you
String sanitize(
String input, List<String> possibleStart, List<String> possibleEnd) {
final String start = possibleStart.join("|");
final String end = possibleEnd.join("|");
final RegExp exp = RegExp("(?<=$start)(.*?)(?=$end)");
final Iterable<Match> matches = exp.allMatches(input);
matches.forEach((match) {
input =
input.replaceFirst(match.group(0)!, match.group(0)!.replaceAll(",", "§").replaceAll(":", "ø").replaceAll("/", "å"));
return true; // error The return type 'bool' isn't a 'void', as required by the closure's context dart flutter
});
return input;
}
function forEach isn't supposed to return anyting, you can see it from iterable.dart
void forEach(void action(E element)) {
for (E element in this) action(element);
}
Related
I need to extract squiggly bracketed template tags from a string. For example:
String str="Hello {{user}}, your reference is {{ref}}"
I would like a to extract the tags in-between the {{..}} into an List. For example:
["user","ref"]
How can I do this, for example with a Regx - I would need to ignore any whitespace in-side the brackets for example {{ user}} would need to return "user".
This question is exactly same as this que.. Want code for flutter dart.
You can use this regex
void main() {
RegExp re = RegExp(r'{{([^]*?)}}');
String data = "Hello {{user}}, your reference is {{ref}}";
var match = re.firstMatch(data);
if (match != null) print(match.group(1));
List something = re.allMatches(data).map((m)=>m[1]).toList();
print(something);
}
OUtput
user
[user, ref]
void main() {
String str="Hello {{user}}, your reference is {{ref}}";
List<String> lstr = getStringBetweenBracket(str);
print(lstr);
}
List<String> getStringBetweenBracket(String str) {
List<String> rstr = [];
var j = str.splitMapJoin(new RegExp(r'\{\{(.*?)\}\}'), onMatch: (e) {
if( e.group(0) != null)
return e.group(0)!.replaceAll("{{","").replaceAll("}}","")+",";
else
return "";
}, onNonMatch: (e) { return ""; });
if(j != "") {
rstr = j.split(",");
rstr.removeAt(rstr.length-1);
}
return rstr;
}
you can do this way get array of data
void main() {
String str="Hello {{user}}, your reference is {{ref}}";
var parts = str.split(' ');
print(parts);
print(parts[1]);
}
void main(){
String str = 'HelloTutorialKart.';
int startIndex = 5;
int endIndex = 13;
//find substring
String result = str.substring(startIndex, endIndex);
print(result);
}
output
Tutorial
I am still coming up to speed with dart and wanted to know if there was an easier way to not execute a statement if the value is null. See example below:
I can always do the if statements below for setting field3 and field4, but felt like something like field5 should work. But when I try to do that, it complains that a null check operator is used on a null value.
Also I don't want to change the Map to have a dynamic value.
Is there a single one liner to do what I am trying to do, or do I just need to check for null before setting the value.
Map<String, Object> myMap = {};
print('running now');
try {
myMap['field1'] = DummyClass.getString('hello');
myMap['field2'] = DummyClass.getString('good');
//Is there a more concise way to do this than the 2 options below?
if (DummyClass.getOptionalString('goodbye') != null) {
myMap['field3'] = DummyClass.getOptionalString('goodbye')!;
}
String? temp = DummyClass.getOptionalString('go');
if (temp != null) {
myMap['field4'] = temp;
}
// This gives an error 'null check operator used on a null value'
// myMap['field5'] ??= DummyClass.getOptionalString('to')!;
} catch (e) {
print('error condition, $e');
}
print(myMap);
}
class DummyClass {
static String getString(String? strParam) {
String? retString = getOptionalString(strParam);
if (retString == null) {
throw ('nulls are not allowed');
}
return retString;
}
static String? getOptionalString(String? strParam) {
if (strParam == null || strParam.length < 3) {
return null;
}
return strParam;
}
}
There's no built-in way to do what you want, but you could write a function (or extension method) to do it. For example:
extension MapTrySet<K, V> on Map<K, V> {
void trySet(K key, V? value) {
if (value != null) {
this[key] = value;
}
}
}
and then you could do:
myMap.trySet('field3', DummyClass.getOptionalString('goodbye'));
myMap.trySet('field4', DummyClass.getOptionalString('go'));
Alternatively, if you really want to use normal Map syntax, you could create your own Map class that has a void operator []=(K key, V? value) override and does nothing when the value is null, but that probably would not be worth the effort.
The issue is that the ??= operator assigns to the left if it is null. Expanded, it would look something like this:
a ??= b;
// Equivalent to:
if (a == null) {
a = b;
}
Which is not something that you're trying to achieve. AFAIK, there is no such operator yet in Dart. However, you can try this:
final possiblyNullValue = '';
final myMap = <String, String>{};
myMap['key'] = possiblyNullValue ?? myMap['key'];
// Equivalent to:
if (possiblyNullValue != null) {
myMap['key'] = possiblyNullValue;
}
// or:
myMap['key'] = possiblyNullValue != null? possiblyNullValue : myMap['key'];
Which would work in your case as a one-liner.
You could create your map with all entries, even null, and then filter the null values out:
void main() {
try {
final myMap = <String, dynamic>{
'field1': DummyClass.getString('hello'),
'field2': DummyClass.getString('good'),
'field3': DummyClass.getOptionalString('goodbye'),
'field4': DummyClass.getOptionalString('go'),
}..removeWhere((k, v) => v == null);
print(myMap);
} catch (e) {
print('error condition, $e');
}
}
I have Created a Map<String,bool>(in which the key are of type String and the values are of type Boolean) in flutter and When I want to use the bool values in if condition it give me error saying "A nullable expression can't be used as a condition.
Try checking that the value isn't 'null' before using it as a condition."
When I use "as bool" then the error is gone but the program is not executed properly and give me the error in the pic
//this is the code
Map<String, bool> _userFilters = {
"gluten": false,
"lactose": false,
"vegan": false,
"vegetarian": false,
};
List<Meal> filteredMeal = DUMMY_MEALS;
void saveFilters(Map<String, bool> filteredData) {
setState(() {
_userFilters = filteredData;
filteredMeal = DUMMY_MEALS.where(
(meal) {
if (_userFilters['gluten']as bool) { // _userFilter['gluten'] is giving error
return false;
}
if (_userFilters['lactose']as bool) {
return false;
}
if (_userFilters['vegan']as bool) {
return false;
}
if (_userFilters['vegetarian'] as bool) {
return false;
}
return true;
},
).toList();
});
}
No need to cast your map entries to booleans. use an exclamation mark at the end of your variable (e.g, _usedFilters['gluten']!) to treat it as non-nullable.
Rewrite all your conditions like this (if you're sure that the value won't be null):
if (_userFilters['gluten']!) {
return false;
}
if (_userFilters['lactose']!) {
return false;
}
if (_userFilters['vegan']!) {
return false;
}
if (_userFilters['vegetarian']!) {
return false;
}
From Dart.dev:
“Casting away nullability” comes up often enough that we have a new
shorthand syntax. A postfix exclamation mark (!) takes the expression
on the left and casts it to its underlying non-nullable type.
static Location fetchById(int id) {
List<Location> locations = Location.fetchAll();
for (var i = 0; i < locations.length; i++) {
if (locations[i].id == id) {
return locations[i];
}
}
return null;
}
// if the condition is not true then return null when I try to return null or false it gives the error 'A value of type 'Null' can't be returned from the method 'fetchById' because it has a return type of 'Location'.
With null-safety feature in the dart language, you have to explicitly tell if you want to make a value nullable.
Define the return type with a ?, so dart knows that return value can be null.
static Location? fetchById(int id)
{
/// function body
}
I am getting the below error when using the forEach loop for the items when used in the function when returning the values.
bool validatedValues(List<String> values){
values.forEach((i){
if (i.length > 3){
return true;
}
});
return false;
}
Im using dart null safety sdk version: ">=2.12.0 <3.0.0"
Complete error:
The return type 'bool' isn't a 'void', as required by the closure's context.dartreturn_of_invalid_type_from_closure
The problem is caused by the return true inside your forEach. forEach is expecting a void Function(T) not a bool Function(T).
I think that what you try to achieve is:
bool validatedValues(List<String> values){
bool result = false;
values.forEach((i){
if (i.length > 3){
result = true;
}
});
return result;
}
Or, probably more elegant:
bool validatedValues(List<String> values) => values.any((i) => i.length > 3);
bool any(bool test(E element));
This returns true if at least one item of the List is validated by test. ref
bool every(bool test(E element));
This return true if all the items of the List are validated by func. ref