| Author |
Message |
< Erlang ~ function_clause error when invoking a function with a float? |
| billrobertson42 |
 Posted: Mon Jan 07, 2008 11:41 pm |
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Joined: 07 Jan 2008
Posts: 13
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I was going through some starter exercises and experimenting with the factorial function and I ran into this error which I don't understand.
Here is the module
Code:
-module(foo).
-export([fact/1]).
fact(N) when N > 0 -> N * fact(N-1);
fact(0) -> 1;
fact(N) when N < 0 -> {invalid_object, N}.
And here is an example of calling it with the error at the end. Will somebody please explain why this is incorrect?
Code:
1> c(foo).
{ok,foo}
2> foo:fact(0).
1
3> foo:fact(1).
1
4> foo:fact(2).
2
5> foo:fact(20).
2432902008176640000
6> foo:fact(-1).
{invalid_object,-1}
7> foo:fact(5.0).
=ERROR REPORT==== 7-Jan-2008::15:23:25 ===
Error in process <0.98.0> with exit value: {function_clause,[{foo,fact,[0.000000
e+000]},{foo,fact,1},{erl_eval,do_apply,5},{shell,exprs,6},{shell,eval_loop,3}]}
** exited: {function_clause,[{foo,fact,[0.00000e+0]},
{foo,fact,1},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_loop,3}]} **
Thanks! |
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| Mazen |
| Posted: Tue Jan 08, 2008 10:44 am |
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Joined: 20 Jul 2006
Posts: 164
Location: London
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Hi Bill,
Here is a hint:
Code:
1> 0.000000000e+0 == 0.
true
2> 0.000000000e+0 =:= 0.
false
3>
I guess function headers (with all right) use =:= to match.  |
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| billrobertson42 |
| Posted: Tue Jan 08, 2008 6:48 pm |
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Joined: 07 Jan 2008
Posts: 13
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Thanks. I appreciate the help.
Yes, I see now. So I made that error go away, and experimented more to challenge my low level of understanding.
Now, when passing in a deliberately broken argument. I get a badarith error. It was o.k. with 5.0, but bombs in unexpected way (to me). The shell lets me mix ints and floats in arithmetic.
Any hints on what I don't understand here please? Also, is there a description of how to read the error message? Is it telling me that it failed in line one on the second recursive call of the fact() function?
Thanks!
Code:
-module(foo).
-export([fact/1]).
fact(N) when N > 0 -> N * fact(N-1);
fact(0) -> 1;
fact(0.0) -> 1;
fact(N) when N < 0 -> {invalid_object, N}.
Code:
2> c(foo).
{ok,foo}
3> foo:fact(5.0).
120.000
4> foo:fact(5.2).
=ERROR REPORT==== 8-Jan-2008::13:36:18 ===
Error in process <0.30.0> with exit value: {badarith,[{foo,fact,1},{foo,fact,1},
{erl_eval,do_apply,5},{shell,exprs,6},{shell,eval_loop,3}]}
** exited: {badarith,[{foo,fact,1},
{foo,fact,1},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_loop,3}]} **
5> 5.0 - 1.
4.00000
6> 5.2 - 1.
4.20000
7>
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| billrobertson42 |
| Posted: Tue Jan 08, 2008 7:57 pm |
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Joined: 07 Jan 2008
Posts: 13
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I think I know why it is failing now. When it gets to -0.2 it returns the tuple, and then attempts to multiply that by the value it has so far, thus causing the badarith error.
Still would appreciate advice or a link on reading the stack traces though.
Thanks.
*edit*
Yes, that's it.
Code:
-module(foo).
-export([fact/1]).
fact(N) when N > 0 -> N * fact(N-1);
fact(0) -> 1;
fact(0.0) -> 1;
fact(N) when N < 0 -> erlang:error(no_negatives, N).
Code:
15> foo:fact(5.2).
=ERROR REPORT==== 8-Jan-2008::15:00:21 ===
Error in process <0.47.0> with exit value: {no_negatives,[{foo,fact,-8.000000e-0
01},{foo,fact,1},{erl_eval,do_apply,5},{shell,exprs,6},{shell,eval_loop,3}]}
** exited: {no_negatives,[{foo,fact,-0.800000},
{foo,fact,1},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_loop,3}]} **
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| Mazen |
| Posted: Wed Jan 09, 2008 2:00 pm |
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Joined: 20 Jul 2006
Posts: 164
Location: London
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When it comes to the error descriptions Erlang has had its fair share of critisism for being a little bit unclear. This is however far from the truth. The error descriptions are pretty easy to understand as long as you know what to look for.
One thing that is important to keep in mind is that even though you get an error, the hard part is to understand how you end up in the faulty state. Lets say that you expected a number but instead got the atom undefined, it is sometimes hard to know where or what the cause is for the undefined to appear (in large systems atleast).
Anyway... enough with my babble the best resource so far is probably this page:
http://www.erlang.org/doc/reference_manual/errors.html
R,
/M |
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| billrobertson42 |
| Posted: Fri Jan 11, 2008 5:05 am |
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Joined: 07 Jan 2008
Posts: 13
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Erlang's stack trace is nice, although I would have to say Java's stack trace with line numbers is what I am more used to.
I think most of just boils down to familiarity and experience with whatever language and then problem solving in general.
Writing software is the easy part, making it work is another story.
Thanks again for the help. |
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| timrila |
| Posted: Tue Jun 12, 2012 9:23 am |
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| goods |
| Posted: Fri Jun 29, 2012 2:03 am |
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| Posted: Thu Sep 13, 2012 6:50 am |
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| Posted: Wed Sep 19, 2012 7:59 am |
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