From 160ca481e85f4b6764ebe7252de2d13c517b20d3 Mon Sep 17 00:00:00 2001 From: Louie Dinh Date: Wed, 26 Jun 2013 16:49:50 -0700 Subject: Add python. Just copied clojure.html.markdown --- python.html.markdown | 305 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 305 insertions(+) create mode 100644 python.html.markdown (limited to 'python.html.markdown') diff --git a/python.html.markdown b/python.html.markdown new file mode 100644 index 00000000..baec27f7 --- /dev/null +++ b/python.html.markdown @@ -0,0 +1,305 @@ +--- +language: Python +author: Louie Dinh +author_url: http://ldinh.ca +--- + +Python was created by Guido Van Rossum in the early 90's. It is not one of the most popular languages in existence. + +```clojure +; Comments start with semicolons. + +; Clojure is written in "forms", which are just +; lists of things inside parentheses, separated by whitespace. +; +; The clojure reader assumes that the first thing is a +; function or macro to call, and the rest are arguments. +; +; Here's a function that sets the current namespace: +(ns test) + +; More basic examples: + +; str will create a string out of all its arguments +(str "Hello" " " "World") ; => "Hello World" + +; Math is straightforward +(+ 1 1) ; => 2 +(- 2 1) ; => 1 +(* 1 2) ; => 2 +(/ 2 1) ; => 2 + +; Equality is = +(= 1 1) ; => true +(= 2 1) ; => false + +; You need not for logic, too +(not true) ; => false + +; Nesting forms works as you expect +(+ 1 (- 3 2)) ; = 1 + (3 - 2) => 2 + +; Types +;;;;;;;;;;;;; + +; Clojure uses Java's object types for booleans, strings and numbers. +; Use `class` to inspect them. +(class 1) ; Integer literals are java.lang.Long by default +(class 1.); Float literals are java.lang.Double +(class ""); Strings always double-quoted, and are java.lang.String +(class false) ; Booleans are java.lang.Boolean +(class nil); The "null" value is called nil + +; If you want to create a literal list of data, use ' to make a "symbol" +'(+ 1 2) ; => (+ 1 2) + +; You can eval symbols. +(eval '(+ 1 2)) ; => 3 + +; Collections & Sequences +;;;;;;;;;;;;;;;;;;; + +; Vectors and Lists are java classes too! +(class [1 2 3]); => clojure.lang.PersistentVector +(class '(1 2 3)); => clojure.lang.PersistentList + +; A list would be written as just (1 2 3), but we have to quote +; it to stop the reader thinking it's a function. +; Also, (list 1 2 3) is the same as '(1 2 3) + +; Both lists and vectors are collections: +(coll? '(1 2 3)) ; => true +(coll? [1 2 3]) ; => true + +; Only lists are seqs. +(seq? '(1 2 3)) ; => true +(seq? [1 2 3]) ; => false + +; Seqs are an interface for logical lists, which can be lazy. +; "Lazy" means that a seq can define an infinite series, like so: +(range 4) ; => (0 1 2 3) +(range) ; => (0 1 2 3 4 ...) (an infinite series) +(take 4 (range)) ; (0 1 2 3) + +; Use cons to add an item to the beginning of a list or vector +(cons 4 [1 2 3]) ; => (4 1 2 3) +(cons 4 '(1 2 3)) ; => (4 1 2 3) + +; Use conj to add an item to the beginning of a list, +; or the end of a vector +(conj [1 2 3] 4) ; => [1 2 3 4] +(conj '(1 2 3) 4) ; => (4 1 2 3) + +; Use concat to add lists or vectors together +(concat [1 2] '(3 4)) ; => (1 2 3 4) + +; Use filter, map to interact with collections +(map inc [1 2 3]) ; => (2 3 4) +(filter even? [1 2 3]) ; => (2) + +; Use reduce to reduce them +(reduce + [1 2 3 4]) +; = (+ (+ (+ 1 2) 3) 4) +; => 10 + +; Reduce can take an initial-value argument too +(reduce conj [] '(3 2 1)) +; = (conj (conj (conj [] 3) 2) 1) +; => [3 2 1] + +; Functions +;;;;;;;;;;;;;;;;;;;;; + +; Use fn to create new functions. A function always returns +; its last statement. +(fn [] "Hello World") ; => fn + +; (You need extra parens to call it) +((fn [] "Hello World")) ; => "Hello World" + +; You can create a var using def +(def x 1) +x ; => 1 + +; Assign a function to a var +(def hello-world (fn [] "Hello World")) +(hello-world) ; => "Hello World" + +; You can shorten this process by using defn +(defn hello-world [] "Hello World") + +; The [] is the list of arguments for the function. +(defn hello [name] + (str "Hello " name)) +(hello "Steve") ; => "Hello Steve" + +; You can also use this shorthand to create functions: +(def hello2 #(str "Hello " %1)) +(hello2 "Fanny") ; => "Hello Fanny" + +; You can have multi-variadic functions, too +(defn hello3 + ([] "Hello World") + ([name] (str "Hello " name))) +(hello3 "Jake") ; => "Hello Jake" +(hello3) ; => "Hello World" + +; Functions can pack extra arguments up in a seq for you +(defn count-args [& args] + (str "You passed " (count args) " args: " args)) +(count-args 1 2 3) ; => "You passed 3 args: (1 2 3)" + +; You can mix regular and packed arguments +(defn hello-count [name & args] + (str "Hello " name ", you passed " (count args) " extra args")) +(hello-count "Finn" 1 2 3) +; => "Hello Finn, you passed 3 extra args" + + +; Hashmaps +;;;;;;;;;; + +(class {:a 1 :b 2 :c 3}) ; => clojure.lang.PersistentArrayMap + +; Keywords are like strings with some efficiency bonuses +(class :a) ; => clojure.lang.Keyword + +; Maps can use any type as a key, but usually keywords are best +(def stringmap (hash-map "a" 1, "b" 2, "c" 3)) +stringmap ; => {"a" 1, "b" 2, "c" 3} + +(def keymap (hash-map :a 1 :b 2 :c 3)) +keymap ; => {:a 1, :c 3, :b 2} (order is not guaranteed) + +; By the way, commas are always treated as whitespace and do nothing. + +; Retrieve a value from a map by calling it as a function +(stringmap "a") ; => 1 +(keymap :a) ; => 1 + +; Keywords can be used to retrieve their value from a map, too! +(:b keymap) ; => 2 + +; Don't try this with strings. +;("a" stringmap) +; => Exception: java.lang.String cannot be cast to clojure.lang.IFn + +; Retrieving a non-present value returns nil +(stringmap "d") ; => nil + +; Use assoc to add new keys to hash-maps +(assoc keymap :d 4) ; => {:a 1, :b 2, :c 3, :d 4} + +; But remember, clojure types are immutable! +keymap ; => {:a 1, :b 2, :c 3} + +; Use dissoc to remove keys +(dissoc keymap :a :b) ; => {:c 3} + +; Sets +;;;;;; + +(class #{1 2 3}) ; => clojure.lang.PersistentHashSet +(set [1 2 3 1 2 3 3 2 1 3 2 1]) ; => #{1 2 3} + +; Add a member with conj +(conj #{1 2 3} 4) ; => #{1 2 3 4} + +; Remove one with disj +(disj #{1 2 3} 1) ; => #{2 3} + +; Test for existence by using the set as a function: +(#{1 2 3} 1) ; => 1 +(#{1 2 3} 4) ; => nil + +; There are more functions in the clojure.sets namespace. + +; Useful forms +;;;;;;;;;;;;;;;;; + +; Logic constructs in clojure are just macros, and look like +; everything else +(if false "a" "b") ; => "b" +(if false "a") ; => nil + +; Use let to create temporary bindings +(let [a 1 b 2] + (> a b)) ; => false + +; Group statements together with do +(do + (print "Hello") + "World") ; => "World" (prints "Hello") + +; Functions have an implicit do +(defn print-and-say-hello [name] + (print "Saying hello to " name) + (str "Hello " name)) +(print-and-say-hello "Jeff") ;=> "Hello Jeff" (prints "Saying hello to Jeff") + +; So does let +(let [name "Urkel"] + (print "Saying hello to " name) + (str "Hello " name)) ; => "Hello Urkel" (prints "Saying hello to Urkel") + +; Modules +;;;;;;;;;;;;;;; + +; Use "use" to get all functions from the module +(use 'clojure.set) + +; Now we can use set operations +(intersection #{1 2 3} #{2 3 4}) ; => #{2 3} +(difference #{1 2 3} #{2 3 4}) ; => #{1} + +; You can choose a subset of functions to import, too +(use '[clojure.set :only [intersection]]) + +; Use require to import a module +(require 'clojure.string) + +; Use / to call functions from a module +(clojure.string/blank? "") ; => true + +; You can give a module a shorter name on import +(require '[clojure.string :as str]) +(str/replace "This is a test." #"[a-o]" str/upper-case) ; => "THIs Is A tEst." +; (#"" denotes a regular expression literal) + +; You can use require (and use, but don't) from a namespace using :require. +; You don't need to quote your modules if you do it this way. +(ns test + (:require + [clojure.string :as str] + [clojure.set :as set])) + +; Java +;;;;;;;;;;;;;;;;; + +; Java has a huge and useful standard library, so +; you'll want to learn how to get at it. + +; Use import to load a java module +(import java.util.Date) + +; You can import from an ns too. +(ns test + (:import java.util.Date + java.util.Calendar)) + +; Use the class name with a "." at the end to make a new instance +(Date.) ; + +; Use . to call methods. Or, use the ".method" shortcut +(. (Date.) getTime) ; +(.getTime (Date.)) ; exactly the same thing. + +; Use / to call static methods +(System/currentTimeMillis) ; (system is always present) + +; Use doto to make dealing with (mutable) classes more tolerable +(import java.util.Calendar) +(doto (Calendar/getInstance) + (.set 2000 1 1 0 0 0) + .getTime) ; => A Date. set to 2000-01-01 00:00:00 +``` -- cgit v1.2.3