test if a string is a number

Question

How to test if a string is composed only of digits ?

YOUR ARGUMENT IS VALID · Answer 1 · 2010-12-29T01:36:14.583

5

One way is to convert it to an integer and if that fails then you'll know it's not an integer.

is_integer(S) ->
    try
        _ = list_to_integer(S),
        true
    catch error:badarg ->
        false
    end.

Or you can just check all of the digits, but you do have to check the edge case of an empty list:

is_integer("") ->
    false;
is_integer(S) ->
    lists:all(fun (D) -> D >= $0 andalso D =< $9 end, S).

edited Dec 29 '10 at 01:36

answered Dec 27 '10 at 00:46

YOUR ARGUMENT IS VALID

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What will this do for the input "9999999999999999999999999999"? – Mark Byers Dec 27 '10 at 00:49
For the input "9999999999999999999999999999" the first solution would create a big integer and then throw it away. – YOUR ARGUMENT IS VALID Dec 27 '10 at 00:50
and with a regular expression ? – Bertaud Dec 27 '10 at 00:54
The catch clause should be : catch error:badarg -> false end. – Tim Dec 28 '10 at 20:54

Roberto Aloi · Accepted Answer · 2010-12-27T18:24:09.707

4

For example, using the re module:

1> re:run("1234a56", "^[0-9]*$").
nomatch
2> re:run("123456", "^[0-9]*$"). 
{match,[{0,6}]}

Or, using list comprehension:

[Char || Char <- String, Char < $0 orelse Char > $9] == [].

Note that both solutions will consider the empty list valid input.

edited Dec 27 '10 at 18:24

answered Dec 27 '10 at 18:09

Roberto Aloi

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score 2 · Answer 3 · answered Dec 27 '10 at 00:41

2

Use a regular expression.

answered Dec 27 '10 at 00:41

duffymo

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re:match(S,"[0-9]) like this ? – Bertaud Dec 27 '10 at 00:46
I haven't tested it, but I am guessing that using a regexp would be slower than using list_to_integer/1. – Tim Dec 28 '10 at 20:49
I'd rather not have logic that depends on throwing an exception. Sounds like a micro-optimization to me. – duffymo Dec 28 '10 at 21:47

score 2 · Answer 4 · answered Dec 27 '10 at 00:54

2

not lists:any(fun(C)->C < $0 or C > $9 end, YourString).

In production, though, I would agree with the list_to_integer as it's probably better optimized. I'm not sure whether there are different traversal characteristics on 'any' vs 'all' either - you could implement each of them in terms of the other, but they're probably both implemented in C.

answered Dec 27 '10 at 00:54

Chris Hagan

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I'm going to go ahead and downvote myself for writing "not lists any fun" about a functional language :D – Chris Hagan Dec 27 '10 at 03:32

score 1 · Answer 5 · answered Sep 14 '18 at 03:11

Erlang doesn't come with a way to say if a string represents a numeric value or not, but does come with the built-in function is_number/1, which will return true if the argument passed is either an integer or a float. Erlang also has two functions to transform a string to either a floating number or an integer, which will be used in conjunction with is_number/1.

is_numeric(L) ->
    Float = (catch erlang:list_to_float(L)),
    Int = (catch erlang:list_to_integer(L)),
    is_number(Float) orelse is_number(Int).

https://rosettacode.org/wiki/Determine_if_a_string_is_numeric#Erlang

test if a string is a number

5 Answers5