How to read integer in Erlang?

Question

I'm trying to read user input of integer. (like cin >> nInput; in C++)
I found io:fread bif from http://www.erlang.org/doc/man/io.html, so I write code like this.

{ok, X} = io:fread("input : ", "~d"),
io:format("~p~n", [X]).

but when I input 10, the erlang terminal keep giving me "\n" not 10. I assume fread automatically read 10 and conert this into string. How can I read integer value directly? Is there any way to do this? Thank you for reading this.

Answer 1

Erlang represents strings as lists of integers that are within a certain range. Therefore the input will be a number that represents the character "1" you could subtract an offset to get the actual. Number, sorry don't have a VM here to test a solution.

Answer 2

There are various functions in OTP to help you convert a string to an integer. If you just read a string from the user (until newline for example) you can the evaluate it with the function to_integer(String) in the string module:

string:to_integer(String) -> {Int,Rest} | {error,Reason}

There is also the list_to_integer(String) BIF (Built-In Function, just call without a module) but it is not as forgiving as the string:to_integer(String) function:

list_to_integer(String) -> int()

You will get a badarg exception if the string does not contain an integer.

Answer 3

1> {ok, [X]} = io:fread("input : ", "~d").
input : 10
{ok,"\n"}
2> X.
10
3> {ok, [A,B]} = io:fread("input : ", "~d,~d").
input : 456,26
{ok,[456,26]}

That's all.

Answer 4

If you use string:to_integer/1, check that the value of Rest is the empty list []. The function extracts the integer, if any, from the beginning of the string. It does not assure that the full input converts to an integer.

string:to_integer(String) -> {Int,Rest} | {error,Reason}

An example:

{Int, Rest} = string:to_integer("33t").
Int.  % -> 33
Rest. % -> "t"

Why check? If the user's finger slipped and hit 't' instead of 5, then the intended input was 335, not 33.

Answer 5

Try printing the number with ~w instead of ~p:

1> io:format("~w~n", [[10]]).
[10]
ok
2> io:format("~p~n", [[10]]).
"\n"
ok

The ~p format specifier tries to figure out whether the list might be a string, but ~w never guesses; it always prints lists as lists.