Erlang learning 01

Download and configure Erlang Environment moves to other articles , Here we only talk about basic operations and concepts
start-up Erlang command , open CMD Input erl

>erl
Eshell V8.3  (abort with ^G)
1> 

stop it Erlang Shell The order is Ctrl+C, Immediately stop the system executable expression halt()
stay Erlang All variables in the must begin with Capital start , also Erlang Unlike other languages, its variables are one-time assignment variables , You can no longer try to change its value after it is set, otherwise an error will be reported , stay Erlang in , A variable is simply a reference to a value ：Erlang The implementation of the method uses pointers to represent bound variables , Point to a store containing values . This value cannot be modified .

>erl
Eshell V8.3  (abort with ^G)
1> X = 20.
20
2> X =30.
** exception error: no match of right hand side value 30

Different from other languages, there are also Java The middle end is generally used ; stay Erlang Use in . As the end （.+ Space /Tab/ enter ） 

Floating point numbers

stay Erlang No matter whether it is divisible or not, as long as the operator is used / The results will be floating point numbers , for example ：

>erl
Eshell V8.3  (abort with ^G)
1> 4/2.
2.0

 div Is to take an integer ,rem It's the remainder .

1> 5 div 3.
1
2> 5 rem 3.
2

atom

stay Erlang in , Atoms are global , And it can be realized without macro definition or include file , Atoms are represented by Lowercase letters start , Followed by a string of letters 、 Numbers 、 Underline （_） or at（@） Symbol , Atoms can also be placed in single quotation marks （'） Inside , therefore 'a' and a Means exactly the same . You can use this quotation mark form to create a quotation mark that begins with a capital letter （ Otherwise it will be interpreted as a variable ） Or atoms containing characters other than alphanumeric , In some languages Yan Li , Single quotation marks and double quotation marks can be used interchangeably .Erlang That's not the case in the library . The usage of single quotation marks is shown above , Use double quotation marks To give a string literal （string literal） Delimit .
The value of an atom is itself . therefore , If you enter an atom as a command ,Erlang shell Will print out the value of this atom .

>erl
Eshell V8.3  (abort with ^G)
1> hello.
hello

If you want to group some fixed number of items into a single entity , Will use tuples .Erlang No type declaration , So create a “ Coordinates ”, Just write it like this .

1> P = {10, 45}.
{10,45}
2> P. 
{10,45}

To make it easier to remember the purpose of tuples , A kind of The common practice is to take the atom as the first element of the tuple , Use it to express what a tuple is .

1> F = {firstName, jok}.
{first,jok}
2> L = {lastName,armstrong}.
{last,armstrong}
3> P = {preson, F, L}.
{preson,{firstName,jok},{lastName,armstrong}}

If there is a complex tuple , You can write a shape with the tuple （ structure ） Same pattern , And the value to be extracted Add unbound variables to the position of to extract the value of the tuple .

 1> Point1={point,25,25}.
{point,25,25}
2> {point,C,C}=Point1.
{point,25,25}
3> C.
25

take _ As placeholder , Used to represent those variables that are not of interest . Symbol _ By Called anonymous variables . Different from normal variables , Multiple in the same mode _ You don't have to bind the same value .

1> Person={person,{name,joe,armstrong},{footsize,42}}.
{person,{name,joe,armstrong},{footsize,42}}
2> {_,{_,Who,_},_}=Person.
{person,{name,joe,armstrong},{footsize,42}}
3> Who.
joe

list

list （list） Used to store any number of things . The way to create a list is to enclose the list elements with square brackets , And separate them with commas .
The first element of the list is called the list header . Suppose you remove the list header , The rest is called the end of the list , The list header can be anything , But the end of the list is usually still a list .
If T It's a list , that [H|T] It's also a list , Its head is H, The tail is T. A vertical bar （|） Match the header of the list with The tail is separated .[] It's an empty list .

1> ThingsToBuy=[{orange,4},{pears,6},{milk,3}].
[{orange,4},{pears,6},{milk,3}]
2> ThingsToBuy1=[{orange,4},{newspaper,1}|ThingsToBuy].
[{orange,4},{newspaper,1},{orange,4},{pears,6},{milk,3}]
3> [Buy1|ThingsToBuy2]=ThingsToBuy1.
[{orange,4},{newspaper,1},{orange,4},{pears,6},{milk,3}]

4> [Buy2,Buy3|ThingsToBuy3]=ThingsToBuy2.
[{newspaper,1},{orange,4},{pears,6},{milk,3}]

If there is a non empty list L, So the expression [X|Y] = L（X and Y Are unbound variables ） The list header will be extracted as X, The end of the list is Y. 

character string

Strictly speaking ,Erlang There is no string in . To be in Erlang Inside represents a string , You can choose a list of integers or a binary . In some programming languages , Strings can be delimited with single or double quotation marks , And in the Erlang in , You must use double quotes .

1> Name="hello".
"hello"

When shell When printing the value of a list , If all integers in the list represent printable characters , It will print it as Literal of a string . otherwise , Print as a list notation .

1> X=[97,98,99].
"abc"

2> I=$a.

97

f() Order to let shell Forget any existing bindings . After this command , All variables will become unbound

modular

The module is Erlang Basic code unit .

-module(geometry).
-export([area/1]).

area({rectangle,Width,Height}) -> Width * Height;
area({square,Side}) ->Side * Side.

The first line of the file is the module declaration . The module name in the declaration must be the same as the main file name where the module is stored .
The second line is the export declaration .Name/N This notation refers to a with N A function of parameters Name,N It is called the element of a function .export The parameters of are determined by Name/N A list of items . therefore ,-export([area/1]) It means Is a function with one parameter area Can be called outside this module . Functions not exported from the module can only be called within the module . Exported functions are equivalent to object-oriented programming languages （OOPL） Public methods in , An unexported function is equivalent to OOPL Private methods in

1> c(geometry).
{ok,geometry}
2> geometry:area({rectangle,10,15}).
150
3> geometry:area({square,3}).
9

Add tests to the code

-module(geometry1).
-export([test/0, area/1]).

test() ->
	12 = area({rectangle,3,4}),
	144 = area({square,12}),
	tests_worked.
	
area({rectangle,Width,Height}) -> Width * Height;
area({square,Side}) ->Side * Side.

1> c(geometry1).
{ok,geometry1}
2> geometry1:test().
tests_worked

12 = area({rectangle, 3, 4}) This line of code is a test . If area({rectangle, 3, 4}) No return 12, Pattern matching will fail , We will get an error message . perform geometry1:test() And see the knot Fruit is tests_worked when , You can be sure test/0 All tests in the main body have passed successfully .