Erlang Programs (L20)

Question 1

Are there void functions in Erlang?

Answer 1

No, all functions return a value.

Question 2

For multi-expression function bodies, which expression is returned by the function?

Answer 2

The last expression.

Question 3

What is wrong the following syntax?

func_name [parm1 parm2 parm3]:
expression_1,
expression_2,
expression_3;

Answer 3

func_name (parm1, parm2, parm3) ->
expression_1,
expression_2,
expression_3.

Question 4

Invoke the following function:
hello_world () ->
io:fwrite(“Hello World\n”).

Answer 4

hello_world().

Question 5

Which format specifier is used to print values in natural form (such as ints, floats, etc.) when using io:fwrite ?

Answer 5

~w

Question 6

Which format specifier is used to print strings when using io:fwrite ?

Answer 6

~s

Question 7

What happens when a string is printed using the ~w format specifier?

Answer 7

The string’s ASCII values are printed.

Question 8

In what two ways can newline characters be printed using io:fwrite ?

Answer 8

\n or ~n.

Question 9

Is the following valid?
io:fwrite(“~w, ~w, ~s~n”, 3, 4.5, “cat”).

Answer 9

False: io:fwrite(“~w, ~w, ~s~n”, [3, 4.5, “cat”]).

Question 10

Multi arity functions are defined within the same function definition in Erlang.

Answer 10

False, each definition is terminated by a period - these are completely separate definitions.

Question 11

Can different arities of the same function call one another in Erlang?

Answer 11

Yes.

Question 12

Can we have multiple variations of a
function, all with the same arity in Erlang?

Answer 12

Yes.

Question 13

When multiple versions of a
function exists, all with the same arity, how does Erlang decide which to use?

Answer 13

Using pattern matching.

Question 14

Are multiple variations of a function, all with the same arity, defined separately or within the same Erlang statement?

Answer 14

Within the same statement, as different variations are separated by a semi colon and only the last definition ends in a period.

Question 15

Define an anonymous function (in Erlang) which returns its single parameter multiplied by 2.

Answer 15

fun (X) -> X * 2 end

Question 16

Erlang also has a map, reduce, and filter function defined in the lists module, which behave the same way as they do in Clojure.

Answer 16

False - Erlang’s reduce function is called fold.

Question 17

What is the Erlang equivalent of Clojure’s doseq function?

Answer 17

lists:foreach

Question 18

Which attribute is used to define an Erlang module? Where is it placed?

Answer 18

-module(mod_name).
Placed atop the module source code file.

Question 19

What is the default code path in Erlang?

Answer 19

The current working directory and Erlang’s system library location.

Question 20

In what environment variable should directories be defined in order to add them to the code path? (Erlang)

Answer 20

ERL_LIBS

Question 21

Functions must be imported from another Erlang module (using -import(…)) in order to use them.

Answer 21

False - importing is optional, but functions must be exported to be callable.

Question 22

The following successfully exports 3 arities of the baz function.
-export(baz/1, baz/2, baz/3).

Answer 22

False, they must be placed in a list:
-export( [baz/1, baz/2, baz/3] ).

Question 23

When is an Erlang function considered private?

Answer 23

When it is not specified in the export list.

Question 24

Where is the -export() attribute placed?

Answer 24

At the top of the source code file.

Question 25

What does the -import() attribute do?

Answer 25

The import attribute eliminates the need to fully qualify the function with the module name.

Question 26

Use the import attribute to validate non fully qualified use of the append (arity 1) and map (arity 2) functions from the lists module.

Answer 26

-import(lists, [append/1, map/2]).

Question 27

Can multiple files be compiled at once using erlc?

Answer 27

Yes.

Question 28

What command is used to execute compiled Erlang code?

Answer 28

erl

Question 29

What erl execute option prevents dropping into the interactive Eshell?

Answer 29

-noshell

Question 30

What commands indicate a module and function combination to load an run?

Answer 30

-s or -run

Question 31

Can only a module be specified following the -s/-run options?

Answer 31

Yes, in which case the start() function is assumed.

Question 32

What is the primary difference between -s and -run?

Answer 32

If command line arguments are provided, s will pass them as atoms (constants), while run will pass them as strings.

Question 33

A second -s option is typically passed with what arguments?

Answer 33

-s init stop

Question 34

What does the second “-s init stop” do?

Answer 34

Tells erl to shutdown the Erlang environment once the program is finished.