View Source Sequential Programming
The Erlang Shell
Most operating systems have a command interpreter or shell, UNIX and Linux have
many, Windows has the command prompt. Erlang has its own shell where bits of
Erlang code can be written directly, and be evaluated to see what happens (see
the shell
manual page in STDLIB).
Start the Erlang shell (in Linux or UNIX) by starting a shell or command
interpreter in your operating system and typing erl
. You will see something
like this.
$ erl
Erlang R15B (erts-5.9.1) [source] [smp:8:8] [rq:8] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.9.1 (abort with ^G)
1>
Type "2 + 5." in the shell and then press Enter (carriage return). Notice that you tell the shell you are done entering code by finishing with a full stop "." and a carriage return.
1> 2 + 5.
7
2>
As shown, the Erlang shell numbers the lines that can be entered, (as 1> 2>) and that it correctly says that 2 + 5 is 7. If you make writing mistakes in the shell, you can delete with the backspace key, as in most shells. There are many more editing commands in the shell (see tty - A command line interface in ERTS User's Guide).
(Notice that many line numbers given by the shell in the following examples are out of sequence. This is because this tutorial was written and code-tested in separate sessions).
Here is a bit more complex calculation:
2> (42 + 77) * 66 / 3.
2618.0
Notice the use of brackets, the multiplication operator "*", and the division operator "/", as in normal arithmetic (see Expressions).
Press Control-C to shut down the Erlang system and the Erlang shell.
The following output is shown:
BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded
(v)ersion (k)ill (D)b-tables (d)istribution
a
$
Type "a" to leave the Erlang system.
Another way to shut down the Erlang system is by entering halt/0
:
3> halt().
$
Modules and Functions
A programming language is not much use if you only can run code from the shell.
So here is a small Erlang program. Enter it into a file named tut.erl
using a
suitable text editor. The file name tut.erl
is important, and also that it is
in the same directory as the one where you started erl
). If you are lucky your
editor has an Erlang mode that makes it easier for you to enter and format your
code nicely (see The Erlang mode for Emacs
in Tools User's Guide), but you can manage perfectly well without. Here is the
code to enter:
-module(tut).
-export([double/1]).
double(X) ->
2 * X.
It is not hard to guess that this program doubles the value of numbers. The
first two lines of the code are described later. Let us compile the program.
This can be done in an Erlang shell as follows, where c
means compile:
3> c(tut).
{ok,tut}
The {ok,tut}
means that the compilation is OK. If it says "error" it means
that there is some mistake in the text that you entered. Additional error
messages gives an idea to what is wrong so you can modify the text and then try
to compile the program again.
Now run the program:
4> tut:double(10).
20
As expected, double of 10 is 20.
Now let us get back to the first two lines of the code. Erlang programs are written in files. Each file contains an Erlang module. The first line of code in the module is the module name (see Modules):
-module(tut).
Thus, the module is called tut. Notice the full stop "." at the end of the
line. The files which are used to store the module must have the same name as
the module but with the extension ".erl". In this case the file name is
tut.erl
. When using a function in another module, the syntax
module_name:function_name(arguments)
is used. So the following means call
function double
in module tut
with argument "10".
4> tut:double(10).
The second line says that the module tut
contains a function called double
,
which takes one argument (X
in our example):
-export([double/1]).
The second line also says that this function can be called from outside the
module tut
. More about this later. Again, notice the "." at the end of the
line.
Now for a more complicated example, the factorial of a number. For example, the factorial of 4 is 4 3 2 * 1, which equals 24.
Enter the following code in a file named tut1.erl
:
-module(tut1).
-export([fac/1]).
fac(1) ->
1;
fac(N) ->
N * fac(N - 1).
So this is a module, called tut1
that contains a function called fac>
, which
takes one argument, N
.
The first part says that the factorial of 1 is 1.:
fac(1) ->
1;
Notice that this part ends with a semicolon ";" that indicates that there is
more of the function fac>
to come.
The second part says that the factorial of N is N multiplied by the factorial of N - 1:
fac(N) ->
N * fac(N - 1).
Notice that this part ends with a "." saying that there are no more parts of this function.
Compile the file:
5> c(tut1).
{ok,tut1}
And now calculate the factorial of 4.
6> tut1:fac(4).
24
Here the function fac>
in module tut1
is called with argument 4
.
A function can have many arguments. Let us expand the module tut1
with the
function to multiply two numbers:
-module(tut1).
-export([fac/1, mult/2]).
fac(1) ->
1;
fac(N) ->
N * fac(N - 1).
mult(X, Y) ->
X * Y.
Notice that it is also required to expand the -export
line with the
information that there is another function mult
with two arguments.
Compile:
7> c(tut1).
{ok,tut1}
Try out the new function mult
:
8> tut1:mult(3,4).
12
In this example the numbers are integers and the arguments in the functions in
the code N
, X
, and Y
are called variables. Variables must start with a
capital letter (see Variables). Examples of
variables are Number
, ShoeSize
, and Age
.
Atoms
Atom is another data type in Erlang. Atoms start with a small letter (see
Atom), for example, charles
, centimeter
, and
inch
. Atoms are simply names, nothing else. They are not like variables, which
can have a value.
Enter the next program in a file named tut2.erl
). It can be useful for
converting from inches to centimeters and conversely:
-module(tut2).
-export([convert/2]).
convert(M, inch) ->
M / 2.54;
convert(N, centimeter) ->
N * 2.54.
Compile:
9> c(tut2).
{ok,tut2}
Test:
10> tut2:convert(3, inch).
1.1811023622047243
11> tut2:convert(7, centimeter).
17.78
Notice the introduction of decimals (floating point numbers) without any explanation. Hopefully you can cope with that.
Let us see what happens if something other than centimeter
or inch
is
entered in the convert
function:
12> tut2:convert(3, miles).
** exception error: no function clause matching tut2:convert(3,miles) (tut2.erl, line 4)
The two parts of the convert
function are called its clauses. As shown,
miles
is not part of either of the clauses. The Erlang system cannot match
either of the clauses so an error message function_clause
is returned. The
shell formats the error message nicely, but the error tuple is saved in the
shell's history list and can be output by the shell command v/1
:
13> v(12).
{'EXIT',{function_clause,[{tut2,convert,
[3,miles],
[{file,"tut2.erl"},{line,4}]},
{erl_eval,do_apply,6,
[{file,"erl_eval.erl"},{line,677}]},
{shell,exprs,7,[{file,"shell.erl"},{line,687}]},
{shell,eval_exprs,7,[{file,"shell.erl"},{line,642}]},
{shell,eval_loop,3,
[{file,"shell.erl"},{line,627}]}]}}
Tuples
Now the tut2
program is hardly good programming style. Consider:
tut2:convert(3, inch).
Does this mean that 3 is in inches? Or does it mean that 3 is in centimeters and is to be converted to inches? Erlang has a way to group things together to make things more understandable. These are called tuples and are surrounded by curly brackets, "{" and "}".
So, {inch,3}
denotes 3 inches and {centimeter,5}
denotes 5 centimeters. Now
let us write a new program that converts centimeters to inches and conversely.
Enter the following code in a file called tut3.erl
):
-module(tut3).
-export([convert_length/1]).
convert_length({centimeter, X}) ->
{inch, X / 2.54};
convert_length({inch, Y}) ->
{centimeter, Y * 2.54}.
Compile and test:
14> c(tut3).
{ok,tut3}
15> tut3:convert_length({inch, 5}).
{centimeter,12.7}
16> tut3:convert_length(tut3:convert_length({inch, 5})).
{inch,5.0}
Notice on line 16 that 5 inches is converted to centimeters and back again and
reassuringly get back to the original value. That is, the argument to a function
can be the result of another function. Consider how line 16 (above) works. The
argument given to the function {inch,5}
is first matched against the first
head clause of convert_length
, that is, convert_length({centimeter,X})
. It
can be seen that {centimeter,X}
does not match {inch,5}
(the head is the bit
before the "->"). This having failed, let us try the head of the next clause
that is, convert_length({inch,Y})
. This matches, and Y
gets the value 5.
Tuples can have more than two parts, in fact as many parts as you want, and contain any valid Erlang term. For example, to represent the temperature of various cities of the world:
{moscow, {c, -10}}
{cape_town, {f, 70}}
{paris, {f, 28}}
Tuples have a fixed number of items in them. Each item in a tuple is called an
element. In the tuple {moscow,{c,-10}}
, element 1 is moscow
and element 2
is {c,-10}
. Here c
represents Celsius and f
Fahrenheit.
Lists
Whereas tuples group things together, it is also needed to represent lists of things. Lists in Erlang are surrounded by square brackets, "[" and "]". For example, a list of the temperatures of various cities in the world can be:
[{moscow, {c, -10}}, {cape_town, {f, 70}}, {stockholm, {c, -4}},
{paris, {f, 28}}, {london, {f, 36}}]
Notice that this list was so long that it did not fit on one line. This does not matter, Erlang allows line breaks at all "sensible places" but not, for example, in the middle of atoms, integers, and others.
A useful way of looking at parts of lists, is by using "|". This is best explained by an example using the shell:
17> [First |TheRest] = [1,2,3,4,5].
[1,2,3,4,5]
18> First.
1
19> TheRest.
[2,3,4,5]
To separate the first elements of the list from the rest of the list, |
is
used. First
has got value 1 and TheRest
has got the value [2,3,4,5].
Another example:
20> [E1, E2 | R] = [1,2,3,4,5,6,7].
[1,2,3,4,5,6,7]
21> E1.
1
22> E2.
2
23> R.
[3,4,5,6,7]
Here you see the use of |
to get the first two elements from the list. If you
try to get more elements from the list than there are elements in the list, an
error is returned. Notice also the special case of the list with no elements,
[]:
24> [A, B | C] = [1, 2].
[1,2]
25> A.
1
26> B.
2
27> C.
[]
In the previous examples, new variable names are used, instead of reusing the
old ones: First
, TheRest
, E1
, E2
, R
, A
, B
, and C
. The reason for
this is that a variable can only be given a value once in its context (scope).
More about this later.
The following example shows how to find the length of a list. Enter the
following code in a file named tut4.erl
):
-module(tut4).
-export([list_length/1]).
list_length([]) ->
0;
list_length([First | Rest]) ->
1 + list_length(Rest).
Compile and test:
28> c(tut4).
{ok,tut4}
29> tut4:list_length([1,2,3,4,5,6,7]).
7
Explanation:
list_length([]) ->
0;
The length of an empty list is obviously 0.
list_length([First | Rest]) ->
1 + list_length(Rest).
The length of a list with the first element First
and the remaining elements
Rest
is 1 + the length of Rest
.
(Advanced readers only: This is not tail recursive, there is a better way to write this function.)
In general, tuples are used where "records" or "structs" are used in other languages. Also, lists are used when representing things with varying sizes, that is, where linked lists are used in other languages.
Erlang does not have a string data type. Instead, strings can be represented by
lists of Unicode characters. This implies for example that the list [97,98,99]
is equivalent to "abc". The Erlang shell is "clever" and guesses what list you
mean and outputs it in what it thinks is the most appropriate form, for example:
30> [97,98,99].
"abc"
Maps
Maps are a set of key to value associations. These associations are encapsulated with "#{" and "}". To create an association from "key" to value 42:
> #{ "key" => 42 }.
#{"key" => 42}
Let us jump straight into the deep end with an example using some interesting features.
The following example shows how to calculate alpha blending using maps to
reference color and alpha channels. Enter the code in a file named color.erl
):
-module(color).
-export([new/4, blend/2]).
-define(is_channel(V), (is_float(V) andalso V >= 0.0 andalso V =< 1.0)).
new(R,G,B,A) when ?is_channel(R), ?is_channel(G),
?is_channel(B), ?is_channel(A) ->
#{red => R, green => G, blue => B, alpha => A}.
blend(Src,Dst) ->
blend(Src,Dst,alpha(Src,Dst)).
blend(Src,Dst,Alpha) when Alpha > 0.0 ->
Dst#{
red := red(Src,Dst) / Alpha,
green := green(Src,Dst) / Alpha,
blue := blue(Src,Dst) / Alpha,
alpha := Alpha
};
blend(_,Dst,_) ->
Dst#{
red := 0.0,
green := 0.0,
blue := 0.0,
alpha := 0.0
}.
alpha(#{alpha := SA}, #{alpha := DA}) ->
SA + DA*(1.0 - SA).
red(#{red := SV, alpha := SA}, #{red := DV, alpha := DA}) ->
SV*SA + DV*DA*(1.0 - SA).
green(#{green := SV, alpha := SA}, #{green := DV, alpha := DA}) ->
SV*SA + DV*DA*(1.0 - SA).
blue(#{blue := SV, alpha := SA}, #{blue := DV, alpha := DA}) ->
SV*SA + DV*DA*(1.0 - SA).
Compile and test:
> c(color).
{ok,color}
> C1 = color:new(0.3,0.4,0.5,1.0).
#{alpha => 1.0,blue => 0.5,green => 0.4,red => 0.3}
> C2 = color:new(1.0,0.8,0.1,0.3).
#{alpha => 0.3,blue => 0.1,green => 0.8,red => 1.0}
> color:blend(C1,C2).
#{alpha => 1.0,blue => 0.5,green => 0.4,red => 0.3}
> color:blend(C2,C1).
#{alpha => 1.0,blue => 0.38,green => 0.52,red => 0.51}
This example warrants some explanation:
-define(is_channel(V), (is_float(V) andalso V >= 0.0 andalso V =< 1.0)).
First a macro is_channel
is defined to help with the guard tests. This is only
here for convenience and to reduce syntax cluttering. For more information about
macros, see The Preprocessor.
new(R,G,B,A) when ?is_channel(R), ?is_channel(G),
?is_channel(B), ?is_channel(A) ->
#{red => R, green => G, blue => B, alpha => A}.
The function new/4
creates a new map term and lets the keys red
, green
,
blue
, and alpha
be associated with an initial value. In this case, only
float values between and including 0.0 and 1.0 are allowed, as ensured by the
?is_channel/1
macro for each argument. Only the =>
operator is allowed when
creating a new map.
By calling blend/2
on any color term created by new/4
, the resulting color
can be calculated as determined by the two map terms.
The first thing blend/2
does is to calculate the resulting alpha channel:
alpha(#{alpha := SA}, #{alpha := DA}) ->
SA + DA*(1.0 - SA).
The value associated with key alpha
is fetched for both arguments using the
:=
operator. The other keys in the map are ignored, only the key alpha
is
required and checked for.
This is also the case for functions red/2
, blue/2
, and green/2
.
red(#{red := SV, alpha := SA}, #{red := DV, alpha := DA}) ->
SV*SA + DV*DA*(1.0 - SA).
The difference here is that a check is made for two keys in each map argument. The other keys are ignored.
Finally, let us return the resulting color in blend/3
:
blend(Src,Dst,Alpha) when Alpha > 0.0 ->
Dst#{
red := red(Src,Dst) / Alpha,
green := green(Src,Dst) / Alpha,
blue := blue(Src,Dst) / Alpha,
alpha := Alpha
};
The Dst
map is updated with new channel values. The syntax for updating an
existing key with a new value is with the :=
operator.
Standard Modules and Manual Pages
Erlang has many standard modules to help you do things. For example, the module
io
contains many functions that help in doing formatted input/output. To look
up information about standard modules, the command erl -man
can be used at the
operating shell or command prompt (the same place as you started erl
). Try the
operating system shell command:
$ erl -man io
ERLANG MODULE DEFINITION io(3)
MODULE
io - Standard I/O Server Interface Functions
DESCRIPTION
This module provides an interface to standard Erlang IO
servers. The output functions all return ok if they are suc-
...
If this does not work on your system, the documentation is included as HTML in the Erlang/OTP release. You can also read the documentation as HTML or download it as PDF from either of the sites www.erlang.se (commercial Erlang) or www.erlang.org (open source). For example, for Erlang/OTP release R9B:
http://www.erlang.org/doc/r9b/doc/index.html
Writing Output to a Terminal
It is nice to be able to do formatted output in examples, so the next example
shows a simple way to use the io:format
function. Like all other exported
functions, you can test the io:format
function in the shell:
31> io:format("hello world~n", []).
hello world
ok
32> io:format("this outputs one Erlang term: ~w~n", [hello]).
this outputs one Erlang term: hello
ok
33> io:format("this outputs two Erlang terms: ~w~w~n", [hello, world]).
this outputs two Erlang terms: helloworld
ok
34> io:format("this outputs two Erlang terms: ~w ~w~n", [hello, world]).
this outputs two Erlang terms: hello world
ok
The function format/2
(that is, format
with two arguments) takes two lists.
The first one is nearly always a list written between " ". This list is printed
out as it is, except that each ~w is replaced by a term taken in order from the
second list. Each ~n is replaced by a new line. The io:format/2
function
itself returns the atom ok
if everything goes as planned. Like other functions
in Erlang, it crashes if an error occurs. This is not a fault in Erlang, it is a
deliberate policy. Erlang has sophisticated mechanisms to handle errors which
are shown later. As an exercise, try to make io:format
crash, it should not be
difficult. But notice that although io:format
crashes, the Erlang shell itself
does not crash.
A Larger Example
Now for a larger example to consolidate what you have learnt so far. Assume that you have a list of temperature readings from a number of cities in the world. Some of them are in Celsius and some in Fahrenheit (as in the previous list). First let us convert them all to Celsius, then let us print the data neatly.
%% This module is in file tut5.erl
-module(tut5).
-export([format_temps/1]).
%% Only this function is exported
format_temps([])-> % No output for an empty list
ok;
format_temps([City | Rest]) ->
print_temp(convert_to_celsius(City)),
format_temps(Rest).
convert_to_celsius({Name, {c, Temp}}) -> % No conversion needed
{Name, {c, Temp}};
convert_to_celsius({Name, {f, Temp}}) -> % Do the conversion
{Name, {c, (Temp - 32) * 5 / 9}}.
print_temp({Name, {c, Temp}}) ->
io:format("~-15w ~w c~n", [Name, Temp]).
35> c(tut5).
{ok,tut5}
36> tut5:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
moscow -10 c
cape_town 21.11111111111111 c
stockholm -4 c
paris -2.2222222222222223 c
london 2.2222222222222223 c
ok
Before looking at how this program works, notice that a few comments are added
to the code. A comment starts with a %-character and goes on to the end of the
line. Notice also that the -export([format_temps/1]).
line only includes the
function format_temps/1
. The other functions are local functions, that is,
they are not visible from outside the module tut5
.
Notice also that when testing the program from the shell, the input is spread over two lines as the line was too long.
When format_temps
is called the first time, City
gets the value
{moscow,{c,-10}}
and Rest
is the rest of the list. So the function
print_temp(convert_to_celsius({moscow,{c,-10}}))
is called.
Here is a function call as convert_to_celsius({moscow,{c,-10}})
as the
argument to the function print_temp
. When function calls are nested like
this, they execute (evaluate) from the inside out. That is, first
convert_to_celsius({moscow,{c,-10}})
is evaluated, which gives the value
{moscow,{c,-10}}
as the temperature is already in Celsius. Then
print_temp({moscow,{c,-10}})
is evaluated. The function convert_to_celsius
works in a similar way to the convert_length
function in the previous example.
print_temp
simply calls io:format
in a similar way to what has been
described above. Notice that ~-15w says to print the "term" with a field length
(width) of 15 and left justify it. (see the io(3)) manual page
in STDLIB.
Now format_temps(Rest)
is called with the rest of the list as an argument.
This way of doing things is similar to the loop constructs in other languages.
(Yes, this is recursion, but do not let that worry you.) So the same
format_temps
function is called again, this time City
gets the value
{cape_town,{f,70}}
and the same procedure is repeated as before. This is done
until the list becomes empty, that is [], which causes the first clause
format_temps([])
to match. This simply returns (results in) the atom ok
, so
the program ends.
Matching, Guards, and Scope of Variables
It can be useful to find the maximum and minimum temperature in lists like this. Before extending the program to do this, let us look at functions for finding the maximum value of the elements in a list:
-module(tut6).
-export([list_max/1]).
list_max([Head|Rest]) ->
list_max(Rest, Head).
list_max([], Res) ->
Res;
list_max([Head|Rest], Result_so_far) when Head > Result_so_far ->
list_max(Rest, Head);
list_max([Head|Rest], Result_so_far) ->
list_max(Rest, Result_so_far).
37> c(tut6).
{ok,tut6}
38> tut6:list_max([1,2,3,4,5,7,4,3,2,1]).
7
First notice that two functions have the same name, list_max
. However, each of
these takes a different number of arguments (parameters). In Erlang these are
regarded as completely different functions. Where you need to distinguish
between these functions, you write Name/Arity, where Name is the function name
and Arity is the number of arguments, in this case list_max/1
and
list_max/2
.
In this example you walk through a list "carrying" a value, in this case
Result_so_far
. list_max/1
simply assumes that the max value of the list is
the head of the list and calls list_max/2
with the rest of the list and the
value of the head of the list. In the above this would be
list_max([2,3,4,5,7,4,3,2,1],1)
. If you tried to use list_max/1
with an
empty list or tried to use it with something that is not a list at all, you
would cause an error. Notice that the Erlang philosophy is not to handle errors
of this type in the function they occur, but to do so elsewhere. More about this
later.
In list_max/2
, you walk down the list and use Head
instead of
Result_so_far
when Head
> Result_so_far
. when
is a special word used
before the -> in the function to say that you only use this part of the function
if the test that follows is true. A test of this type is called guard. If the
guard is false (that is, the guard fails), the next part of the function is
tried. In this case, if Head
is not greater than Result_so_far
, then it must
be smaller or equal to it. This means that a guard on the next part of the
function is not needed.
Some useful operators in guards are:
<
less than>
greater than==
equal>=
greater or equal=<
less or equal/=
not equal
(see Guard Sequences).
To change the above program to one that works out the minimum value of the
element in a list, you only need to write < instead of >. (But it would be wise
to change the name of the function to list_min
.)
Earlier it was mentioned that a variable can only be given a value once in its
scope. In the above you see that Result_so_far
is given several values. This
is OK since every time you call list_max/2
you create a new scope and one can
regard Result_so_far
as a different variable in each scope.
Another way of creating and giving a variable a value is by using the match
operator = . So if you write M = 5
, a variable called M
is created with the
value 5. If, in the same scope, you then write M = 6
, an error is returned.
Try this out in the shell:
39> M = 5.
5
40> M = 6.
** exception error: no match of right hand side value 6
41> M = M + 1.
** exception error: no match of right hand side value 6
42> N = M + 1.
6
The use of the match operator is particularly useful for pulling apart Erlang terms and creating new ones.
43> {X, Y} = {paris, {f, 28}}.
{paris,{f,28}}
44> X.
paris
45> Y.
{f,28}
Here X
gets the value paris
and Y
the value {f,28}
.
If you try to do the same again with another city, an error is returned:
46> {X, Y} = {london, {f, 36}}.
** exception error: no match of right hand side value {london,{f,36}}
Variables can also be used to improve the readability of programs. For example,
in function list_max/2
above, you can write:
list_max([Head|Rest], Result_so_far) when Head > Result_so_far ->
New_result_far = Head,
list_max(Rest, New_result_far);
This is possibly a little clearer.
More About Lists
Remember that the | operator can be used to get the head of a list:
47> [M1|T1] = [paris, london, rome].
[paris,london,rome]
48> M1.
paris
49> T1.
[london,rome]
The | operator can also be used to add a head to a list:
50> L1 = [madrid | T1].
[madrid,london,rome]
51> L1.
[madrid,london,rome]
Now an example of this when working with lists - reversing the order of a list:
-module(tut8).
-export([reverse/1]).
reverse(List) ->
reverse(List, []).
reverse([Head | Rest], Reversed_List) ->
reverse(Rest, [Head | Reversed_List]);
reverse([], Reversed_List) ->
Reversed_List.
52> c(tut8).
{ok,tut8}
53> tut8:reverse([1,2,3]).
[3,2,1]
Consider how Reversed_List
is built. It starts as [], then successively the
heads are taken off of the list to be reversed and added to the the
Reversed_List
, as shown in the following:
reverse([1|2,3], []) =>
reverse([2,3], [1|[]])
reverse([2|3], [1]) =>
reverse([3], [2|[1])
reverse([3|[]], [2,1]) =>
reverse([], [3|[2,1]])
reverse([], [3,2,1]) =>
[3,2,1]
The module lists
contains many functions for manipulating lists, for example,
for reversing them. So before writing a list-manipulating function it is a good
idea to check if one not already is written for you (see the lists
manual
page in STDLIB).
Now let us get back to the cities and temperatures, but take a more structured approach this time. First let us convert the whole list to Celsius as follows:
-module(tut7).
-export([format_temps/1]).
format_temps(List_of_cities) ->
convert_list_to_c(List_of_cities).
convert_list_to_c([{Name, {f, F}} | Rest]) ->
Converted_City = {Name, {c, (F -32)* 5 / 9}},
[Converted_City | convert_list_to_c(Rest)];
convert_list_to_c([City | Rest]) ->
[City | convert_list_to_c(Rest)];
convert_list_to_c([]) ->
[].
Test the function:
54> c(tut7).
{ok, tut7}.
55> tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
[{moscow,{c,-10}},
{cape_town,{c,21.11111111111111}},
{stockholm,{c,-4}},
{paris,{c,-2.2222222222222223}},
{london,{c,2.2222222222222223}}]
Explanation:
format_temps(List_of_cities) ->
convert_list_to_c(List_of_cities).
Here format_temps/1
calls convert_list_to_c/1
. convert_list_to_c/1
takes
off the head of the List_of_cities
, converts it to Celsius if needed. The |
operator is used to add the (maybe) converted to the converted rest of the list:
[Converted_City | convert_list_to_c(Rest)];
or:
[City | convert_list_to_c(Rest)];
This is done until the end of the list is reached, that is, the list is empty:
convert_list_to_c([]) ->
[].
Now when the list is converted, a function to print it is added:
-module(tut7).
-export([format_temps/1]).
format_temps(List_of_cities) ->
Converted_List = convert_list_to_c(List_of_cities),
print_temp(Converted_List).
convert_list_to_c([{Name, {f, F}} | Rest]) ->
Converted_City = {Name, {c, (F -32)* 5 / 9}},
[Converted_City | convert_list_to_c(Rest)];
convert_list_to_c([City | Rest]) ->
[City | convert_list_to_c(Rest)];
convert_list_to_c([]) ->
[].
print_temp([{Name, {c, Temp}} | Rest]) ->
io:format("~-15w ~w c~n", [Name, Temp]),
print_temp(Rest);
print_temp([]) ->
ok.
56> c(tut7).
{ok,tut7}
57> tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
moscow -10 c
cape_town 21.11111111111111 c
stockholm -4 c
paris -2.2222222222222223 c
london 2.2222222222222223 c
ok
Now a function has to be added to find the cities with the maximum and minimum temperatures. The following program is not the most efficient way of doing this as you walk through the list of cities four times. But it is better to first strive for clarity and correctness and to make programs efficient only if needed.
-module(tut7).
-export([format_temps/1]).
format_temps(List_of_cities) ->
Converted_List = convert_list_to_c(List_of_cities),
print_temp(Converted_List),
{Max_city, Min_city} = find_max_and_min(Converted_List),
print_max_and_min(Max_city, Min_city).
convert_list_to_c([{Name, {f, Temp}} | Rest]) ->
Converted_City = {Name, {c, (Temp -32)* 5 / 9}},
[Converted_City | convert_list_to_c(Rest)];
convert_list_to_c([City | Rest]) ->
[City | convert_list_to_c(Rest)];
convert_list_to_c([]) ->
[].
print_temp([{Name, {c, Temp}} | Rest]) ->
io:format("~-15w ~w c~n", [Name, Temp]),
print_temp(Rest);
print_temp([]) ->
ok.
find_max_and_min([City | Rest]) ->
find_max_and_min(Rest, City, City).
find_max_and_min([{Name, {c, Temp}} | Rest],
{Max_Name, {c, Max_Temp}},
{Min_Name, {c, Min_Temp}}) ->
if
Temp > Max_Temp ->
Max_City = {Name, {c, Temp}}; % Change
true ->
Max_City = {Max_Name, {c, Max_Temp}} % Unchanged
end,
if
Temp < Min_Temp ->
Min_City = {Name, {c, Temp}}; % Change
true ->
Min_City = {Min_Name, {c, Min_Temp}} % Unchanged
end,
find_max_and_min(Rest, Max_City, Min_City);
find_max_and_min([], Max_City, Min_City) ->
{Max_City, Min_City}.
print_max_and_min({Max_name, {c, Max_temp}}, {Min_name, {c, Min_temp}}) ->
io:format("Max temperature was ~w c in ~w~n", [Max_temp, Max_name]),
io:format("Min temperature was ~w c in ~w~n", [Min_temp, Min_name]).
58> c(tut7).
{ok, tut7}
59> tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
moscow -10 c
cape_town 21.11111111111111 c
stockholm -4 c
paris -2.2222222222222223 c
london 2.2222222222222223 c
Max temperature was 21.11111111111111 c in cape_town
Min temperature was -10 c in moscow
ok
If and Case
The function find_max_and_min
works out the maximum and minimum temperature. A
new construct, if
, is introduced here. If works as follows:
if
Condition 1 ->
Action 1;
Condition 2 ->
Action 2;
Condition 3 ->
Action 3;
Condition 4 ->
Action 4
end
Notice that there is no ";" before end
. Conditions do the same as guards, that
is, tests that succeed or fail. Erlang starts at the top and tests until it
finds a condition that succeeds. Then it evaluates (performs) the action
following the condition and ignores all other conditions and actions before the
end
. If no condition matches, a run-time failure occurs. A condition that
always succeeds is the atom true
. This is often used last in an if
, meaning,
do the action following the true
if all other conditions have failed.
The following is a short program to show the workings of if
.
-module(tut9).
-export([test_if/2]).
test_if(A, B) ->
if
A == 5 ->
io:format("A == 5~n", []),
a_equals_5;
B == 6 ->
io:format("B == 6~n", []),
b_equals_6;
A == 2, B == 3 -> %That is A equals 2 and B equals 3
io:format("A == 2, B == 3~n", []),
a_equals_2_b_equals_3;
A == 1 ; B == 7 -> %That is A equals 1 or B equals 7
io:format("A == 1 ; B == 7~n", []),
a_equals_1_or_b_equals_7
end.
Testing this program gives:
60> c(tut9).
{ok,tut9}
61> tut9:test_if(5,33).
A == 5
a_equals_5
62> tut9:test_if(33,6).
B == 6
b_equals_6
63> tut9:test_if(2, 3).
A == 2, B == 3
a_equals_2_b_equals_3
64> tut9:test_if(1, 33).
A == 1 ; B == 7
a_equals_1_or_b_equals_7
65> tut9:test_if(33, 7).
A == 1 ; B == 7
a_equals_1_or_b_equals_7
66> tut9:test_if(33, 33).
** exception error: no true branch found when evaluating an if expression
in function tut9:test_if/2 (tut9.erl, line 5)
Notice that tut9:test_if(33,33)
does not cause any condition to succeed. This
leads to the run time error if_clause
, here nicely formatted by the shell. See
Guard Sequences for details of the many guard tests
available.
case
is another construct in Erlang. Recall that the convert_length
function
was written as:
convert_length({centimeter, X}) ->
{inch, X / 2.54};
convert_length({inch, Y}) ->
{centimeter, Y * 2.54}.
The same program can also be written as:
-module(tut10).
-export([convert_length/1]).
convert_length(Length) ->
case Length of
{centimeter, X} ->
{inch, X / 2.54};
{inch, Y} ->
{centimeter, Y * 2.54}
end.
67> c(tut10).
{ok,tut10}
68> tut10:convert_length({inch, 6}).
{centimeter,15.24}
69> tut10:convert_length({centimeter, 2.5}).
{inch,0.984251968503937}
Both case
and if
have return values, that is, in the above example case
returned either {inch,X/2.54}
or {centimeter,Y*2.54}
. The behaviour of
case
can also be modified by using guards. The following example clarifies
this. It tells us the length of a month, given the year. The year must be known,
since February has 29 days in a leap year.
-module(tut11).
-export([month_length/2]).
month_length(Year, Month) ->
%% All years divisible by 400 are leap
%% Years divisible by 100 are not leap (except the 400 rule above)
%% Years divisible by 4 are leap (except the 100 rule above)
Leap = if
trunc(Year / 400) * 400 == Year ->
leap;
trunc(Year / 100) * 100 == Year ->
not_leap;
trunc(Year / 4) * 4 == Year ->
leap;
true ->
not_leap
end,
case Month of
sep -> 30;
apr -> 30;
jun -> 30;
nov -> 30;
feb when Leap == leap -> 29;
feb -> 28;
jan -> 31;
mar -> 31;
may -> 31;
jul -> 31;
aug -> 31;
oct -> 31;
dec -> 31
end.
70> c(tut11).
{ok,tut11}
71> tut11:month_length(2004, feb).
29
72> tut11:month_length(2003, feb).
28
73> tut11:month_length(1947, aug).
31
Built-In Functions (BIFs)
BIFs are functions that for some reason are built-in to the Erlang virtual
machine. BIFs often implement functionality that is impossible or is too
inefficient to implement in Erlang. Some BIFs can be called using the function
name only but they are by default belonging to the erlang
module. For example,
the call to the BIF trunc
below is equivalent to a call to erlang:trunc
.
As shown, first it is checked if a year is leap. If a year is divisible by 400,
it is a leap year. To determine this, first divide the year by 400 and use the
BIF trunc
(more about this later) to cut off any decimals. Then multiply by
400 again and see if the same value is returned again. For example, year 2004:
2004 / 400 = 5.01
trunc(5.01) = 5
5 * 400 = 2000
2000 is not the same as 2004, so 2004 is not divisible by 400. Year 2000:
2000 / 400 = 5.0
trunc(5.0) = 5
5 * 400 = 2000
That is, a leap year. The next two trunc
-tests evaluate if the year is
divisible by 100 or 4 in the same way. The first if
returns leap
or
not_leap
, which lands up in the variable Leap
. This variable is used in the
guard for feb
in the following case
that tells us how long the month is.
This example showed the use of trunc
. It is easier to use the Erlang operator
rem
that gives the remainder after division, for example:
74> 2004 rem 400.
4
So instead of writing:
trunc(Year / 400) * 400 == Year ->
leap;
it can be written:
Year rem 400 == 0 ->
leap;
There are many other BIFs such as trunc
. Only a few BIFs can be used in
guards, and you cannot use functions you have defined yourself in guards. (see
Guard Sequences) (For advanced readers: This is to
ensure that guards do not have side effects.) Let us play with a few of these
functions in the shell:
75> trunc(5.6).
5
76> round(5.6).
6
77> length([a,b,c,d]).
4
78> float(5).
5.0
79> is_atom(hello).
true
80> is_atom("hello").
false
81> is_tuple({paris, {c, 30}}).
true
82> is_tuple([paris, {c, 30}]).
false
All of these can be used in guards. Now for some BIFs that cannot be used in guards:
83> atom_to_list(hello).
"hello"
84> list_to_atom("goodbye").
goodbye
85> integer_to_list(22).
"22"
These three BIFs do conversions that would be difficult (or impossible) to do in Erlang.
Higher-Order Functions (Funs)
Erlang, like most modern functional programming languages, has higher-order functions. Here is an example using the shell:
86> Xf = fun(X) -> X * 2 end.
#Fun<erl_eval.5.123085357>
87> Xf(5).
10
Here is defined a function that doubles the value of a number and assigned this
function to a variable. Thus Xf(5)
returns value 10. Two useful functions when
working with lists are foreach
and map
, which are defined as follows:
foreach(Fun, [First|Rest]) ->
Fun(First),
foreach(Fun, Rest);
foreach(Fun, []) ->
ok.
map(Fun, [First|Rest]) ->
[Fun(First)|map(Fun,Rest)];
map(Fun, []) ->
[].
These two functions are provided in the standard module lists
. foreach
takes
a list and applies a fun to every element in the list. map
creates a new list
by applying a fun to every element in a list. Going back to the shell, map
is
used and a fun to add 3 to every element of a list:
88> Add_3 = fun(X) -> X + 3 end.
#Fun<erl_eval.5.123085357>
89> lists:map(Add_3, [1,2,3]).
[4,5,6]
Let us (again) print the temperatures in a list of cities:
90> Print_City = fun({City, {X, Temp}}) -> io:format("~-15w ~w ~w~n",
[City, X, Temp]) end.
#Fun<erl_eval.5.123085357>
91> lists:foreach(Print_City, [{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
moscow c -10
cape_town f 70
stockholm c -4
paris f 28
london f 36
ok
Let us now define a fun that can be used to go through a list of cities and temperatures and transform them all to Celsius.
-module(tut13).
-export([convert_list_to_c/1]).
convert_to_c({Name, {f, Temp}}) ->
{Name, {c, trunc((Temp - 32) * 5 / 9)}};
convert_to_c({Name, {c, Temp}}) ->
{Name, {c, Temp}}.
convert_list_to_c(List) ->
lists:map(fun convert_to_c/1, List).
92> tut13:convert_list_to_c([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
[{moscow,{c,-10}},
{cape_town,{c,21}},
{stockholm,{c,-4}},
{paris,{c,-2}},
{london,{c,2}}]
The convert_to_c
function is the same as before, but here it is used as a fun:
lists:map(fun convert_to_c/1, List)
When a function defined elsewhere is used as a fun, it can be referred to as
Function/Arity
(remember that Arity
= number of arguments). So in the
map
-call lists:map(fun convert_to_c/1, List)
is written. As shown,
convert_list_to_c
becomes much shorter and easier to understand.
The standard module lists
also contains a function sort(Fun, List)
where
Fun
is a fun with two arguments. This fun returns true
if the first argument
is less than the second argument, or else false
. Sorting is added to the
convert_list_to_c
:
-module(tut13).
-export([convert_list_to_c/1]).
convert_to_c({Name, {f, Temp}}) ->
{Name, {c, trunc((Temp - 32) * 5 / 9)}};
convert_to_c({Name, {c, Temp}}) ->
{Name, {c, Temp}}.
convert_list_to_c(List) ->
New_list = lists:map(fun convert_to_c/1, List),
lists:sort(fun({_, {c, Temp1}}, {_, {c, Temp2}}) ->
Temp1 < Temp2 end, New_list).
93> c(tut13).
{ok,tut13}
94> tut13:convert_list_to_c([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
[{moscow,{c,-10}},
{stockholm,{c,-4}},
{paris,{c,-2}},
{london,{c,2}},
{cape_town,{c,21}}]
In sort
the fun is used:
fun({_, {c, Temp1}}, {_, {c, Temp2}}) -> Temp1 < Temp2 end,
Here the concept of an anonymous variable "_" is introduced. This is simply
shorthand for a variable that gets a value, but the value is ignored. This can
be used anywhere suitable, not just in funs. Temp1 < Temp2
returns true
if
Temp1
is less than Temp2
.