Tutorial by Examples: var

Interfaces may have variant type parameters. interface IEnumerable<out T> { // ... } interface IComparer<in T> { // ... } but classes and structures may not class BadClass<in T1, out T2> // not allowed { } struct BadStruct<in T1, out T2> // not allo...

Delegates may have variant type parameters. delegate void Action<in T>(T t); // T is an input delegate T Func<out T>(); // T is an output delegate T2 Func<in T1, out T2>(); // T1 is an input, T2 is an output This follows from the Liskov Substitution Principle, w...

If a covariant type appears as an output, the containing type is covariant. Producing a producer of Ts is like producing Ts. interface IReturnCovariant<out T> { IEnumerable<T> GetTs(); } If a contravariant type appears as an output, the containing type is contravariant. Produc...

The semantics for this are similar to that of default value substitution, but instead of substituting a default value, it errors out with the provided error message. The forms are ${VARNAME?ERRMSG} and ${VARNAME:?ERRMSG}. The form with : will error our if the variable is unset or empty, whereas the ...

import pandas as pd df = pd.DataFrame(np.random.randn(5, 5), columns=list('ABCDE')) To generate various summary statistics. For numeric values the number of non-NA/null values (count), the mean (mean), the standard deviation std and values known as the five-number summary : min: minimum (smal...

The process.env property returns an object containing the user environment. It returns an object like this one : { TERM: 'xterm-256color', SHELL: '/usr/local/bin/bash', USER: 'maciej', PATH: '~/.bin/:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin', PWD: '/Users/maciej', EDITOR: 'vi...

Declaration and usage. // a is const int, so it can't be changed const int a = 15; a = 12; // Error: can't assign new value to const variable a += 1; // Error: can't assign new value to const variable Binding of references and pointers int &b = a; // Error: ca...

To obtain a Java's Class object from Kotlin's KClass use the .java extension property: val stringKClass: KClass<String> = String::class val c1: Class<String> = stringKClass.java val c2: Class<MyClass> = MyClass::class.java The latter example will be optimized by the compile...

Python 2.x2.3 x = 'hello world!' vowels = [x for x in 'AEIOU'] print (vowels) # Out: ['A', 'E', 'I', 'O', 'U'] print(x) # Out: 'U' Python 3.x3.0 x = 'hello world!' vowels = [x for x in 'AEIOU'] print (vowels) # Out: ['A', 'E', 'I', 'O', 'U'] print(x) # Out: 'hello world!' ...

int a; std::cout << a; // Undefined behavior! This results in undefined behavior, because a is uninitialised. It is often, incorrectly, claimed that this is because the value is "indeterminate", or "whatever value was in that memory location before". However, it is th...

You can set an environment variable called PYTHONSTARTUP for Python's console. Whenever you enter the Python console, this file will be executed, allowing for you to add extra functionality to the console such as importing commonly-used modules automatically. If the PYTHONSTARTUP variable was set t...

Introduce environment variable VAGRANT_DEFAULT_PROVIDER export VAGRANT_DEFAULT_PROVIDER=vmware_fusion

Customer[] customers = Customers.ToArray(); Purchase[] purchases = Purchases.ToArray(); var groupJoinQuery = from c in customers join p in purchases on c.ID equals p.CustomerID into custPurchases select new { CustName = c.Name, custPurchases }; ...

When you bind a service to your application credentials become available through the VCAP_SERVICES environment variable. This environment variable contains JSON containing the credentials for all bound services. Example VCAP_SERVICES environment variable { "push-reappt": [ { ...

To create a variable: variableType variableName; For example: int a; To create a variable and initialize it: variableType variableName = initialValue; For example: int a = 2;

If you have a variable declared before, you can assign some value to it: For example: int a; // declared previously a = 2; Or change the value: int a = 3; // initalized previously a = 2;

char : signed 1-byte character value byte : unsigned 8-bit integer int : signed 16-bit (on ATMEGA based boards) or 32-bit (on Arduino Due) integer unsigned int : unsigned 16-bit (on ATMEGA based boards) or 32-bit (on Arduino Due) integer long : signed 32-bit integer unsigned long : unsigned 3...

DECLARE @Employees TABLE ( EmployeeID INT NOT NULL PRIMARY KEY, FirstName NVARCHAR(50) NOT NULL, LastName NVARCHAR(50) NOT NULL, ManagerID INT NULL ) When you create a normal table, you use CREATE TABLE Name (Columns) syntax. When creating a table variable, you use DECLARE @...

DECLARE @VariableName INT SET @VariableName = 1 PRINT @VariableName 1 Using SET, you can only update one variable at a time.

Using SELECT, you can update multiple variables at once. DECLARE @Variable1 INT, @Variable2 VARCHAR(10) SELECT @Variable1 = 1, @Variable2 = 'Hello' PRINT @Variable1 PRINT @Variable2 1 Hello When using SELECT to update a variable from a table column, if there are multiple values, it wi...