Comparing sets
In R, a vector may contain duplicated elements:
v = "A"
w = c("A", "A")
However, a set contains only one copy of each element. R treats a vector like a set by taking only its distinct elements, so the two vectors above are regarded as the same:
set...
The %in% operator compares a vector with a set.
v = "A"
w = c("A", "A")
w %in% v
# TRUE TRUE
v %in% w
# TRUE
Each element on the left is treated individually and tested for membership in the set associated with the vector on the right (consisting of all its...
To find every vector of the form (x, y) where x is drawn from vector X and y from Y, we use expand.grid:
X = c(1, 1, 2)
Y = c(4, 5)
expand.grid(X, Y)
# Var1 Var2
# 1 1 4
# 2 1 4
# 3 2 4
# 4 1 5
# 5 1 5
# 6 2 5
The result is a data.frame with one...
unique drops duplicates so that each element in the result is unique (only appears once):
x = c(2, 1, 1, 2, 1)
unique(x)
# 2 1
Values are returned in the order they first appeared.
duplicated tags each duplicated element:
duplicated(x)
# FALSE FALSE TRUE TRUE TRUE
anyDuplicated(x) >...
To count how many elements of two sets overlap, one could write a custom function:
xtab_set <- function(A, B){
both <- union(A, B)
inA <- both %in% A
inB <- both %in% B
return(table(inA, inB))
}
A = 1:20
B = 10:30
xtab_set(A, B)
# inB
...
Imagine you have the following HTML:
<div>
<label>Name:</label>
John Smith
</div>
And you need to locate the text "John Smith" after the label element.
In this case, you can locate the label element by text and then use .next_sibling property:
from ...
BeautifulSoup has a limited support for CSS selectors, but covers most commonly used ones. Use select() method to find multiple elements and select_one() to find a single element.
Basic example:
from bs4 import BeautifulSoup
data = """
<ul>
<li class="item&quo...
Sets are unordered collections of distinct elements. But sometimes we want to work with unordered collections of elements that are not necessarily distinct and keep track of the elements' multiplicities.
Consider this example:
>>> setA = {'a','b','b','c'}
>>> setA
set(['a', 'c'...
You can restrict the valid types used in a generic class by bounding that type in the class definition. Given the following simple type hierarchy:
public abstract class Animal {
public abstract String getSound();
}
public class Cat extends Animal {
public String getSound() {
...
Sometimes it's necessary to be able to extract values from an object using only references (ie. without transferring ownership).
struct Token {
pub id: u32
}
struct User {
pub token: Option<Token>
}
fn main() {
// Create a user with an arbitrary token
let user = User...
Have a table
NameAgeCityBob10ParisMat20BerlinMary24Prague
select Name from table where Age>10 AND City='Prague'
Gives
NameMary
select Name from table where Age=10 OR City='Prague'
Gives
NameBobMary
Light Table is a good editor to learn, experiment and run Clojure projects.
You can also run lein/boot projects by opening project.clj file. It will load all project dependencies.
It supports inline evalution, plugins and much more, so there's no need to add print statements and check the output i...
To setup Emacs for working with Clojure, install clojure-mode and cider package from melpa:
M-x package-install [RET] clojure-mode [RET]
M-x package-install [RET] cider [RET]
Now when you open a .clj file, run M-x cider-jack-in to connect to a REPL. Alternatively, you can use C-u M-x (cider-jac...
All built-in Clojure collections are immutable and heterogeneous, have literal syntax, and support the conj, count, and seq functions.
conj returns a new collection that is equivalent to an existing collection with an item "added", in either "constant" or logarithmic time. Wha...
A sequence is very much like a list: it is an immutable object that can give you its first element or the rest of its elements in constant time. You can also construct a new sequence from an existing sequence and an item to stick at the beginning.
You can test whether something is a sequence using ...
A vector is denoted by square brackets:
[]
;;=> []
[:foo]
;;=> [:foo]
[:foo :bar]
;;=> [:foo :bar]
[1 (+ 1 1) 3]
;;=> [1 2 3]
In addition using to the literal syntax, you can also use the vector function to construct a vector:
(vector)
;;=> []
(vector :foo)
;;=&...
Like maps, sets are associative and unordered. Unlike maps, which contain mappings from keys to values, sets essentially map from keys to themselves.
A set is denoted by curly braces preceded by an octothorpe:
#{}
;;=> #{}
#{:foo}
;;=> #{:foo}
#{:foo :bar}
;;=> #{:bar :foo}
As...
For if you have to fine-tune what is published.
import { Mongo } from 'meteor/mongo';
import { Meteor } from 'meteor/meteor';
import { Random } from 'meteor/random';
if (Meteor.isClient) {
// established this collection on the client only.
// a name is required (first parameter) and this...
On the server, you can create a publication like this. this.userId is the id of the user who is currently logged in. If no user is logged in, you might want to throw an error and respond to it.
import Secrets from '/imports/collections/Secrets';
Meteor.publish('protected_data', function () {
...
