To access functions and properties of nullable types, you have to use special operators.
The first one, ?., gives you the property or function you're trying to access, or it gives you null if the object is null:
val string: String? = "Hello World!"
print(string.length) // Compile erro...
If the compiler can infer that an object can't be null at a certain point, you don't have to use the special operators anymore:
var string: String? = "Hello!"
print(string.length) // Compile error
if(string != null) {
// The compiler now knows that string can't be null
print(...
In order to install PostgreSQL on OSX, you need to know which versions are currently supported.
Use this command to see what versions you have available.
sudo port list | grep "^postgresql[[:digit:]]\{2\}[[:space:]]"
You should get a list that looks something like the following:
post...
Use drop_duplicates:
In [216]: df = pd.DataFrame({'A':[1,2,3,3,2],
...: 'B':[1,7,3,0,8]})
In [217]: df
Out[217]:
A B
0 1 1
1 2 7
2 3 3
3 3 0
4 2 8
# keep only the last value
In [218]: df.drop_duplicates(subset=['A'], keep='last')
Out[218]:
...
class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
class Pet
{
public string Name { get; set; }
public Person Owner { get; set; }
}
public static void Main(string[] args)
{
var magnus = new Person { FirstName = "Magnus...
The unary plus (+) precedes its operand and evaluates to its operand. It attempts to convert the operand to a number, if it isn't already.
Syntax:
+expression
Returns:
a Number.
Description
The unary plus (+) operator is the fastest (and preferred) method of converting something into a n...
The delete operator deletes a property from an object.
Syntax:
delete object.property
delete object['property']
Returns:
If deletion is successful, or the property did not exist:
true
If the property to be deleted is an own non-configurable property (can't be deleted):
false in non...
The typeof operator returns the data type of the unevaluated operand as a string.
Syntax:
typeof operand
Returns:
These are the possible return values from typeof:
TypeReturn valueUndefined"undefined"Null"object"Boolean"boolean"Number"number"String"...
The void operator evaluates the given expression and then returns undefined.
Syntax:
void expression
Returns:
undefined
Description
The void operator is often used to obtain the undefined primitive value, by means of writing void 0 or void(0). Note that void is an operator, not a functio...
The unary negation (-) precedes its operand and negates it, after trying to convert it to number.
Syntax:
-expression
Returns:
a Number.
Description
The unary negation (-) can convert the same types / values as the unary plus (+) operator can.
Values that can't be converted will evaluat...
The bitwise NOT (~) performs a NOT operation on each bit in a value.
Syntax:
~expression
Returns:
a Number.
Description
The truth table for the NOT operation is:
aNOT a01101337 (base 10) = 0000010100111001 (base 2)
~1337 (base 10) = 1111101011000110 (base 2) = -1338 (base 10)
A bit...
The logical NOT (!) operator performs logical negation on an expression.
Syntax:
!expression
Returns:
a Boolean.
Description
The logical NOT (!) operator performs logical negation on an expression.
Boolean values simply get inverted: !true === false and !false === true.
Non-boolean val...
DynamoDB is a fully managed service provided by AWS. It does not need to be installed or configured. AWS is responsible for all administrative burdens of operating, scalling and backup/restore of the distributed database.
All DATEs have a time component; however, it is customary to store dates which do not need to include time information with the hours/minutes/seconds set to zero (i.e. midnight).
Use an ANSI DATE literal (using ISO 8601 Date format):
SELECT DATE '2000-01-01' FROM DUAL;
Convert it from a string ...
Convert it from a string literal using TO_DATE():
SELECT TO_DATE( '2000-01-01 12:00:00', 'YYYY-MM-DD HH24:MI:SS' ) FROM DUAL;
Or use a TIMESTAMP literal:
CREATE TABLE date_table(
date_value DATE
);
INSERT INTO date_table ( date_value ) VALUES ( TIMESTAMP '2000-01-01 12:00:00' );
Oracl...
In Oracle a DATE data type does not have a format; when Oracle sends a DATE to the client program (SQL/Plus, SQL/Developer, Toad, Java, Python, etc) it will send 7- or 8- bytes which represent the date.
A DATE which is not stored in a table (i.e. generated by SYSDATE and having "type 13" ...
Use TO_CHAR( date [, format_model [, nls_params]] ):
(Note: if a format model is not provided then the NLS_DATE_FORMAT session parameter will be used as the default format model; this can be different for every session so should not be relied on. It is good practice to always specify the format mod...
When Oracle implicitly converts from a DATE to a string or vice-versa (or when TO_CHAR() or TO_DATE() are explicitly called without a format model) the NLS_DATE_FORMAT session parameter will be used as the format model in the conversion. If the literal does not match the format model then an excepti...
When SQL/Plus or SQL Developer display dates they will perform an implicit conversion to a string using the default date format model (see the Setting the Default Date Format Model example).
You can change how a date is displayed by changing the NLS_DATE_FORMAT parameter.
CREATE CONTEXT my_ctx USING my_pkg;
This creates a context that can only be set by routines in the database package my_pkg, e.g.:
CREATE PACKAGE my_pkg AS
PROCEDURE set_ctx;
END my_pkg;
CREATE PACKAGE BODY my_pkg AS
PROCEDURE set_ctx IS
BEGIN
DBMS_SESSION.set_context('MY_CTX','...
