Dart suggest me to use if null operator . Why ? and how to use it?
This is my code:
var name;
name != null ? name : 'nobody';
In other languages we can use the logical-or shortcut. If maybeSomeNumber() returns null, assign a default value of 2:
value = maybeSomeNumber() || 2
In Dart we can't do this because the expression needs to be a boolean (“the operands of the || operator must be assignable to bool”).
That's why the ?? operator exists:
var value = maybeSomeNumber() ?? 2;
Similarly, if we wanted to ensure a value argument was not-null we'd do:
value = value ?? 2;
But there's an even simpler way.
Fallback assignment operator: ??=
value ??= 2;
check the original artical: https://flutterigniter.com/checking-null-aware-operators-dart/
I don't think we need to use conditions or ternary operator here. It will increase program complexity. Try below
name = name || 'nobody';
Related
This question already has answers here:
What are the ?? double question marks in Dart?
(5 answers)
Closed 2 years ago.
I've seen this code and need an explanation for "??".
I know ternary operators like "?" and then the true-condition and after ":" the false/else condition.
But what means the double "??" ?
Thanks in advance
widget.secondaryImageTop ??
(widget.height / 2) - (widget.secondaryImageHeight / 2); ```
List of all dart operators
it's the coalesce operator.
a ?? b
means: if a is not null, it resolves to a. if a is null, it resolves to b.
SQL and a few other languages have this operator.
You example:
widget.secondaryImageTop ??
(widget.height / 2) - (widget.secondaryImageHeight / 2);
This will use widget.secondaryImageTop unless it is null, in which case it will use (widget.height / 2) - (widget.secondaryImageHeight / 2).
Source and detail, including dartpad where you can try things out with pre-populated examples:
https://dart.dev/codelabs/dart-cheatsheet
An example from that documentation, using the = sign as well.
the ??= assignment operator, which assigns a value to a variable only
if that variable is currently null:
int a; // The initial value of a is null.
a ??= 3;
print(a); // <-- Prints 3.
a ??= 5;
print(a); // <-- Still prints 3.
It means => if exists and if not null...
I am importing price information and adding + or -.
I put the title code in print () and it works but I do not know what it means.
print("\(IntValue ?? 0 == -1 ? "-" : "+")")
Please explain it briefly to me.
Kevin's answer is very good.
Some background that helps explain further:
The code you posted uses two rather cryptic operators together.
?? is the nil-coalescing operator.
It takes an optional value, which can contain nil, and provides a new value to use when it does contain nil.
Edit:
(Note that you can skip the nil-coalescing operator and use IntValue == -1 instead. That works because only a non-nil value of -1 is equal to -1. An optional that contains nil is not equal to -1.
You could rewrite the line as
print("\(IntValue == -1 ? "-" : "+")")
And get the same result.)
The next tricky bit is the "ternary operator". This comes from C. It's quite cryptic, but also quite useful.
It takes the form boolean ? value_for_true : value_for_false
Where boolean is a boolean expression that evaluates to true or false.
If boolean is true, then the result of the whole ternary expression is the value_for_true sub-expression.
If boolean is false the result of the whole ternary expression is the value_for_false sub-expression.
IntValue ?? 0 == -1 is the boolean part of your ternary expression. It evaluates as true if IntValue is -1. It evaluates as false if IntValue contains any other value, or if it contains nil.
(Note that variables and let constants should start with lower-case letters, so IntValue should be intValue.)
The variable IntValue is an optional, which means its either an Integer or nil. IntValue ?? 0 means that if IntValue exists, then use the value of IntValue. If IntValue is nil, then use the value 0. Next, compare that value with -1. If that value is equal to -1, then print -. If that value does not equal -1, then print +.
Here's equivalent code with only if statements:
var defaultInt = 0
if IntValue != nil {
defaultInt = IntValue! // force unwrap the optional value
}
if defaultInt == -1 {
print("-")
}
else {
print("+")
}
It is necessary to summarize the field $F{energy} wit conditions $F{type_energy} like "Active Energy".
I try with this(define Variable Sum) , with conditions in Variable Expression:
$F{type_energy} == ("Active Energy")? $F{energy}: new Double(0)
Boolean.valueOf($F{vrsta_energije}=="Active energy")?$F{iznos_naknade_km}.doubleValue():new Double(0)
new Double($F{vrsta_energije}!= null && $F{vrsta_energije}.equals("Active energy")?$F{kolicina_vrsna_snaga}.doubleValue():0)
IF($F{type_energy} == ("Active Energy"), $F{energy}, null)
where type of fild is:
$F{type_energy} java.lang.String
and $F{energy} java.math.BigDecimal
Keep getting highscore 0.0 or null
change data type of $F{energy} to double in feild expression.
Define a variable sum ($V{Sum}).
keep calculationFunction to none.
Initial value expression as 0.0.
In variable expression have
("Active Energy").equalEgnoreCase( $F{type_energy} ) ? $V{Sum}+$F{energy}: $V{Sum}
I am doing the following, but it is not working properly:
my $enabled = $hash &&
$hash->{'key'} &&
$hash->{'key'}->{'enabled'} &&
$hash->{'key'}->{'active'};
Is this an acceptable way to assign a boolean value to a scalar variable?
My code is misbehaving in strange ways, as it is, and I believe it is because of this assignment. I have validated that the individual values exist for all of these keys and are set to a value.
P.S. Sorry for being a noob! I googled for ~10 minutes and couldn't find the answer anywhere.
The Perl boolean operators like &&, ||, and, or don't return a boolean value, they return the value of one of their arguments:
say 2 && 3;
outputs 3.
You can force it to a boolean with the double negation trick:
say !!(2 && 3);
# or
say not not 2 && 3;
outputs 1.
I have two variables in my pre block, and I need a third (a boolean) that identifies whether certain properties hold of those two:
str = "some string from a datasource";
qty = 15; //Also from a datasource
The third variable, valid, needs to be true when str is not empty and qty is greater than 0. How do I do that?
Oh! I just figured it out:
valid = not (str eq "") && (qty > 0);
I had some syntax errors in the rest of my ruleset; that's where the trouble was coming from.