This question already has answers here:
"The argument type 'String?' can't be assigned to the parameter type 'String'" when using stdin.readLineSync()
(3 answers)
Closed 1 year ago.
I'm new at dart. I think that it's about the new dart version that I can't assign String to stdin.readLineSync(). Can Someone Pls tell me the alternatives?
This is just a basic calculator where I'm trying to add two numbers.
import 'dart:io';
import 'dart:math';
void main() {
print("Enter first number: ");
String? num1 = stdin.readLineSync();
print("Enter second number: ");
String? num2 = stdin.readLineSync();
print(num1 + num2);
}
This is the alternative way how you do:
In Dart programming language, you can take standard input from the user through the console via the use of .readLineSync() function.
import 'dart:io';
void main()
{
print("Enter first number");
int? n1 = int.parse(stdin.readLineSync()!);
print("Enter second number");
int? n2 = int.parse(stdin.readLineSync()!);
int sum = n1 + n2;
print("Sum is $sum");
}
and here it is clearly explained about your problem "The argument type 'String?' can't be assigned to the parameter type 'String'" when using stdin.readLineSync()
Related
This question already has answers here:
"The argument type 'String?' can't be assigned to the parameter type 'String'" when using stdin.readLineSync()
(3 answers)
Closed 6 months ago.
I am trying to parse an input with the type 'String?' into a double, but it is saying that -
The argument type 'String?' can't be assigned to the parameter type 'String'.
stdout.write("Enter the first number - ");
String? vari = stdin.readLineSync();
var first = int.parse(vari);
Change it to:
int? first = int.tryParse(vari.toString());
This is because String? is nullable, vs String.
.parse functions require non-nullable Strings.
tryParse returns an int if the string is parsable, and returns null if it can't be parsed. This is why I changed var first to int? first.
You can check afterwards if first is null or not. You can also perform this check before parsing it, and your code would look like this:
String? vari = stdin.readLineSync();
if (vari != null) var first = int.parse(vari);
This would work.
This question already has answers here:
What is the difference between 'is' and '==' in Dart?
(2 answers)
Closed 11 months ago.
I'm new to dart and reading about dart operators. In the book fluter in Action by Eric Windmill (Chapter 2, 2.24, page 34) the auther says:
is and is! verify that two objects are of the same type. They are equivalent to == and !=.
Trying to implement this as shown in the code below
void main() {
int a = 7;
int b = 2;
bool z = a == b; // It works when I use the equals symbol
print('Result: $z');
}
But when I use the ```is`` keyword, I get an error
void main() {
int a = 7;
int b = 2;
bool z = a is b; // It doesn't work here
print('Result: $z');
}
Error
The name 'b' isn't a type and can't be used in an 'is' expression.
Try correcting the name to match an existing type.
Not sure what the context of that statement is but is and is! is not the same as == and != in the sense that they does the same. I guess what the want to explain, is that the opposite of is is is! like the opposite of == is !=.
== is used to check if two objects are equal (based on what this object at left side defines to be equal). So for int we return true when using == if both numbers have the same numeric value.
The is operator is used to test if a given object does implement a given interface. An example:
void main() {
Object myList = <int>[];
if (myList is List<int>) {
print('We have a list of numbers.');
}
}
The error you are getting are telling you that for using is you need to provide an actual type at the right side of the is and not an object instance.
I recently started dart programing, I encountered a issue while doing it,
I was doing it by referring this website https://hackmd.io/#kuzmapetrovich/S1x90jWGP
Error: A value of type 'String?' can't be assigned to a variable of type 'num'.
My code :
import 'dart:io';
void main() {
print('Enter a number');
var num1 = stdin.readLineSync();
var num2= 100-num1;
}
can anyone tell whats the issue and how to fix.
you can parse String into
int
var num2 = 100 - int.parse(num1!);
or double
var num2 = 100 - double.parse(num1!);
So stdin.readLineSync() reads your console input and will store it to num1 of type var. As you can see in the documentation, stdin.readLineSync() returns a String?.
Because the data type of var variables is resolved during runtime, num1 will be of type String? after assigning the console input to it.
So the problem of your code is, that you try to subtract 100 from a string, which is not possible.
To solve this issue you should better convert your input into a numerical type, like int or double. But be careful! Your input could also contain character that can't be converted to a number. One solution could look like following:
void main() {
print('Enter a number');
var input = stdin.readLineSync();
var num1 = double.parse(input);
if (num1 != null){
var num2 = 100 - num1;
}else{
// num1 could not be converted to a double
// handle exception here
}
}
For the beginning, I recommend you to use data types in your code instead of var and val. Maybe reading something about dart's type system may also be helpful.
I keep getting this error => "The operator '+' isn't defined for the type 'Object'. (view docs). Try defining the operator '+'."
How could I define it?
import 'dart:math';
bool isArmstrongNumber(int number) {
var numberString = number.toString();
return number ==
numberString.split("").fold(0,
(prev, curr) => prev! + pow(int.parse(curr), numberString.length));
}
main() {
var result = isArmstrongNumber(153);
print(result);
}
fold in Dart can have some problems when it comes to automatically determine what type it should return and handle. In these cases, we need to manually enter the type like this (fold<int>()):
import 'dart:math';
bool isArmstrongNumber(int number) {
final numberString = number.toString();
return number ==
numberString.split("").fold<int>(
0,
(prev, curr) =>
prev + pow(int.parse(curr), numberString.length).toInt(),
);
}
void main() {
final result = isArmstrongNumber(153);
print(result); // true
}
I also fixed a problem where pow returns num which is a problem. In this case, we can safely just cast it to int without issues.
Details about this problem with fold
The problem here is that Dart tries to guess the generic of the fold based on the expected returned type of the method. Since the == operator expects an Object to compare against, fold will also expect to just return Object and the generic ends up being fold<Object>.
This is not a problem for the first parameter since int is an Object. But it becomes a problem with your second argument where you expect an int object and not Object since Object does not have the + operator.
This question already has answers here:
"The argument type 'String?' can't be assigned to the parameter type 'String'" when using stdin.readLineSync()
(3 answers)
Closed 1 year ago.
I recently got into flutter and dart and I've been writing basic programs. Just this morning I ran into an error when trying to convert user input from a string into an integer so I could perform a mathematical operation. Here's the code.
import "dart:io";
import "dart:math";
import "dart:convert";
void main() {
print("Enter a number: ");
String num1 = stdin.readLineSync();
print("Enter a second number");
String num2 = stdin.readLineSync();
print(int.parse(num1) + int.parse(num2));
}
Surprisingly this runs well on the online dart compiler and interpreter. (replit)
but when I run it on vscode, I get this error
"Error: A value of type 'String?' can't be assigned to a variable of type 'String' because 'String?' is nullable and 'String' isn't.
String num2 = stdin.readLineSync();"
Whichever data type I use, I get the same error. be it var, or string, or int or double.
I tried another method of conversion but its the same thing. works on the online compiler, doesn't work on my machine. here's the code.
import "dart:io";
import "dart:math";
import "dart:convert";
void main() {
print("Enter a number: ");
var num1 = stdin.readLineSync();
var num2 = int.parse(num1);
print("Enter a second number");
var num3 = stdin.readLineSync();
var num4 = int.parse(num3);
print(num2 + num4);
}
stdin.readLineSync() returns a String?. Types ending in a ? are nullable. String? means either String or null.
Also int.parse takes a parameter of type String not String? so you cannot pass a value to it which might be null. There are a few different approaches you can take to work around this. One thing you can do is check if they are not null in an if statement, and in doing so the values of num1 and num2 will be promoted to String within the body of the if statement. Another approach is to use the ?? operator on num1 and num2, which is called the "if null" operator and allows you to provide a default value in the event that num1 or num2 is null.
import "dart:io";
import "dart:math";
import "dart:convert";
void main() {
print("Enter a number: ");
// set type to String? (you can also use var or final)
String? num1 = stdin.readLineSync();
print("Enter a second number");
String? num2 = stdin.readLineSync();
// check for null values before using num1 and num2
if (num1 != null && num2 != null) {
print(int.parse(num1) + int.parse(num2));
}
// alternatively you can convert the values to String
// using the ?? operator. if the value of num1 is null
// then you will get '' (empty string) instead.
print(int.parse(num1 ?? '') + int.parse(num2 ?? ''));
}
I am not familiar with replit, but it may be using an older version of dart language. dart version 2.12 introduced features related to null safety.
See also:
https://medium.com/dartlang/announcing-dart-2-12-499a6e689c87
https://dart.dev/null-safety/understanding-null-safety
https://dart.dev/codelabs/null-safety