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SQL keys, MUL vs PRI vs UNI
...s example has neither PRI, MUL, nor UNI:
mysql> create table penguins (foo INT);
Query OK, 0 rows affected (0.01 sec)
mysql> desc penguins;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+...
class
...
First, the class << foo syntax opens up foo's singleton class (eigenclass). This allows you to specialise the behaviour of methods called on that specific object.
a = 'foo'
class << a
def inspect
'"bar"'
end
end
a.inspect # =>...
p vs puts in Ruby
...
p foo prints foo.inspect followed by a newline, i.e. it prints the value of inspect instead of to_s, which is more suitable for debugging (because you can e.g. tell the difference between 1, "1" and "2\b1", which you can't when...
Automatically expanding an R factor into a collection of 1/0 indicator variables for every factor le
...the ade4 package :
R> library(ade4)
R> df <-data.frame(eggs = c("foo", "foo", "bar", "bar"), ham = c("red","blue","green","red"))
R> acm.disjonctif(df)
eggs.bar eggs.foo ham.blue ham.green ham.red
1 0 1 0 0 1
2 0 1 1 0 ...
How to check if a variable is set in Bash?
...-------------+-----------------+-----------------+
| Expression | FOO="world" | FOO="" | unset FOO |
| in script: | (Set and Not Null) | (Set But Null) | (Unset) |
+--------------------+----------------------+-----------------+-----------------+
| $...
Regex: match everything but specific pattern
...t, but I think you could use a negative lookahead from the start, e.g. ^(?!foo).*$ shouldn't match anything starting with foo.
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How does the const constructor actually work?
... meaningful sorting order.
This means that const expressions like const Foo(1, 1) can represent any usable form that is useful for comparison in virtual machine.
The VM only needs to take into account the value type and arguments in the order in which they occur in this const expression. And, of...
From an array of objects, extract value of a property as array
...ere is a shorter way of achieving it:
let result = objArray.map(a => a.foo);
OR
let result = objArray.map(({ foo }) => foo)
You can also check Array.prototype.map().
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PowerShell: Setting an environment variable for a single command only
...ariable. But if that is what you need to do you can do it this way:
$env:FOO = 'BAR'; ./myscript
The environment variable $env:FOO can be deleted later like so:
Remove-Item Env:\FOO
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What are static factory methods?
... a factory method, you would simply call the class's constructor directly: Foo x = new Foo(). With this pattern, you would instead call the factory method: Foo x = Foo.create(). The constructors are marked private, so they cannot be called except from inside the class, and the factory method is m...