It seems I can't initialize a constant float without strange errors.
Float a = 2; //A value of type 'int' can't be assigned to a variable of type 'Float'
Float a = 2.0; //A value of type 'double' can't be assigned to a variable of type 'Float'
How do I initialize a float other than just using a double?
Thanks in advance.
Dart auto-imported the package 'dart:ffi', leading me to think Float is a native type. It isn't and double is the only floating point type.
#Christopher Moore's comment let me find the problem.
Related
This question already has answers here:
What is Null Safety in Dart?
(2 answers)
Closed 1 year ago.
After defining a map (with letters as keys and scrabble tile scores as values)
Map<String, int> letterScore //I'm omitting the rest of the declaration
when I experiment with this function (in DartPad)
int score(String aWord) {
int result = 0;
for (int i = 0; i < aWord.length; ++i) {
result += letterScore[aWord[i]];
}
return result;
}
I consistently get error messages, regardless of whether I experiment by declaring variables as num or int:
Error: A value of type 'int?' can't be assigned to a variable of type
'num' because 'int?' is nullable and 'num' isn't [I got this after declaring all the numerical variables as int]
Error: A value of type 'num' can't be returned from a function with
return type 'int'.
Error: A value of type 'num?' can't be assigned to a variable of type
'num' because 'num?' is nullable and 'num' isn't.
I understand the difference between an integer and a floating point (or double) number, it's the int vs int? and num vs num? I don't understand, as well as which form to use when declaring variables. How should I declare and use int or num variables to avoid these errors?
Take this for example:
int x; // x has value as null
int x = 0; // x is initialized as zero
Both the above code are fine and compilable code. But if you enable Dart's null-safety feature, which you should, it will make the above code work differently.
int x; // compilation error: "The non-nullable variable must be assigned before can be used"
int x = 0; // No Error.
This is an effort made from the compiler to warn you wherever your variable can be null, but during the compile time. Awesome.
But what happens, if you must declare a variable as null because you don't know the value at the compile time.
int? x; // Compiles fine because it's a nullable variable
The ? is a way for you tell the compiler that you want this variable to allow null. However, when you say a variable can be null, then every time you use the variable, the compiler will remind you to check whether the variable is null or not before you can use it.
Hence the other use of the ?:
int? x;
print(x?.toString() ?? "0");
Further readings:
Official Docs: https://dart.dev/null-safety/understanding-null-safety
Null-aware operators: https://dart.dev/codelabs/dart-cheatsheet
According to the Dart docs for Object.runtimeType, the field's type is Type. Which is confusing because I get an error from the compiler complaining about this field not being a type.
See this sample code:
final double first = 1.0;
final int second = 2;
final third = second as double; // works fine, unlike declaration below.
assert(first.runtimeType == double); // true
final fourth = second as first.runtimeType;
The last line throws this compile-time error:
The name 'first.runtimeType' isn't a type, so it can't be used in an 'as' expression.
The sample code shows that first.runtimeType == double, so wouldn't it follow that _ as first.runtimeType is equivalent to _ as double?
I think it is simple actually, runtimeType is only available at RunTime and cannot be statically analyzed by the compiler.
weight is a field (Number in Firestore), set as 100.
int weight = json['weight'];
double weight = json['weight'];
int weight works fine, returns 100 as expected, but double weight crashes (Object.noSuchMethod exception) rather than returning 100.0, which is what I expected.
However, the following works:
num weight = json['weight'];
num.toDouble();
When parsing 100 from Firestore (which actually does not support a "number type", but converts it), it will by standard be parsed to an int.
Dart does not automatically "smartly" cast those types. In fact, you cannot cast an int to a double, which is the problem you are facing. If it were possible, your code would just work fine.
Parsing
Instead, you can parse it yourself:
double weight = json['weight'].toDouble();
Casting
What also works, is parsing the JSON to a num and then assigning it to a double, which will cast num to double.
double weight = json['weight'] as num;
This seems a bit odd at first and in fact the Dart Analysis tool (which is e.g. built in into the Dart plugin for VS Code and IntelliJ) will mark it as an "unnecessary cast", which it is not.
double a = 100; // this will not compile
double b = 100 as num; // this will compile, but is still marked as an "unnecessary cast"
double b = 100 as num compiles because num is the super class of double and Dart casts super to sub types even without explicit casts.
An explicit cast would be the follwing:
double a = 100 as double; // does not compile because int is not the super class of double
double b = (100 as num) as double; // compiles, you can also omit the double cast
Here is a nice read about "Types and casting in Dart".
Explanation
What happened to you is the following:
double weight;
weight = 100; // cannot compile because 100 is considered an int
// is the same as
weight = 100 as double; // which cannot work as I explained above
// Dart adds those casts automatically
You can do it in one line:
double weight = (json['weight'] as num).toDouble();
You can Parse the data Like given below:
Here document is a Map<String,dynamic>
double opening = double.tryParse(document['opening'].toString());
In Dart, int and double are separate types, both subtypes of num.
There is no automatic conversion between number types. If you write:
num n = 100;
double d = n;
you will get a run-time error. Dart's static type system allows unsafe down-casts, so the unsafe assignment of n to d (unsafe because not all num values are double values) is treated implicitly as:
num n = 100;
double d = n as double;
The as double checks that the value is actually a double (or null), and throws if it isn't. If that check succeeds, then it can safely assign the value to d since it is known to match the variable's type.
That's what's happening here. The actual value of json['weight'] (likely with static type Object or dynamic) is the int object with value 100. Assigning that to int works. Assigning it to num works. Assigning it to double throws.
The Dart JSON parser parses numbers as integers if they have no decimal or exponent parts (0.0 is a double, 0e0 is a double, 0 is an integer). That's very convenient in most cases, but occasionally annoying in cases like yours where you want a double, but the code creating the JSON didn't write it as a double.
In cases like that, you just have to write .toDouble() on the values when you extract them. That's a no-op on actual doubles.
As a side note, Dart compiled to JavaScript represents all numbers as the JavaScript Number type, which means that all numbers are doubles. In JS compiled code, all integers can be assigned to double without conversion. That will not work when the code is run on a non-JS implementation, like Flutter, Dart VM/server or ahead-of-time compilation for iOS, so don't depend on it, or your code will not be portable.
Simply convert int to double like this
int a = 10;
double b = a + 0.0;
I'm using Xcode 7.3.1 and Swift, and i'm trying to set a random number between 1 and 50 like that: variableName = random()%50
Then i have to move an ImageView in the Y axe of that random value:
imageviewName.center.y = imageviewName.center.y - variableName
But it gives me the following error: "cannot convert value of type 'int' to expected argument type 'CGFloat'.
So I declared the variable like that:
var RandomSquirrel2 = CGFloat() but it still doesn't wok.
How can I generate a random number in Swift?
You need to covert the Int to CGFloat
imageviewName.center.y = imageviewName.center.y - CGFloat(variableName)
You need to cast variableName to CGFloat. because random() returns an Int and not A CGFloat.
imageviewName.center.y = imageviewName.center.y - CGFloat(variableName)
I'm trying to create a block variable that takes a CGFloat argument and returns a CGFloat.
CGFloat (^debt)(CGFloat) = ^(CGFloat myFloat) {
return myFloat * 444563.4004;
};
What is wrong with this definition? Why am I getting this warning?
Another possible solution is to use a block with an explicit return type:
CGFloat (^debt)(CGFloat) = ^CGFloat (CGFloat myFloat) {
return myFloat * 444563.4004;
};
to avoid that the compiler tries to "guess" the return type from the return statement.
Compare Creating a Block in "Blocks Programming Topics":
If you don’t explicitly declare the return value of a block
expression, it can be automatically inferred from the contents of the
block.
On iOS (and other 32-bit platforms), CGFloat is an alias for float.
Your literal (444563.4004) is a double, which promotes myFloat to a double and makes the return type of your block double (and not the float you said it would be when you declared debt). Either change the literal to a float (append f to the end of it), or cast it to CGFloat.