Tutorial :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.


Solution:1

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.


Solution: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.


Solution:3

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.


Solution: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.


Solution:5

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.


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