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cgr
Posted: Sat Sep 15, 2007 6:17 pm Reply with quote
Joined: 15 Sep 2007 Posts: 5
I've attached a copy of my .erl file to this msg

The problem I'm trying to solve is here: http://acm.uva.es/p/v1/100.html

Basically, everything compiles fine, but whenever I call prob100/2 w/ something like prob100:prob100(1,10), I get a badarith error like this:

Code:
{badarith,[{prob100,cycle_len,2},
           {lists,map,2},
           {prob100,prob100,2},
           {erl_eval,do_apply,5},
           {shell,exprs,6},
           {shell,eval_loop,3}]}


Since the only arithmetic thing I have in cycle_len/2 there is the modulo operation in the case, I'm sure that's what's causing it. Also, if I uncomment that even/1 function and put it in place of (Num rem 2) and change the case guards to true and false, I get the same badarith error w/ the top stacktrace func being even/1. I would appreciate any help on this problem since it's such an easy idiom and I'm new to erlang and can't get it to show me what arg value killed the process.

Thanks

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-cgr
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wolfgke
Posted: Sun Sep 16, 2007 10:31 am Reply with quote
User Joined: 16 Sep 2007 Posts: 16
I'll explain you how I came to the correct solution:

"hen I try

Code:
prob100:cycle_len(5).


the following error occurs:
Quote:

=ERROR REPORT==== 16-Sep-2007::12:23:32 ===
Error in process <0.116.0> with exit value: {badarith,[{prob100,cycle_len,2},{shell,exprs,6},{shell,eval_loop,3}]}

** exited: {badarith,[{prob100,cycle_len,2},
{shell,exprs,6},
{shell,eval_loop,3}]} **


So it seems that the problem lies in cycle_len/2.

Looking at the code there I saw

Code:
0 -> cycle_len(Num / 2, Acc + 1);


Now it was clear to me: 1/2 delivers 0.500000

So it has to be changed to

Code:
0 -> cycle_len(Num div 2, Acc + 1);


then it works.
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cgr
Posted: Sun Sep 16, 2007 2:08 pm Reply with quote
Joined: 15 Sep 2007 Posts: 5
Thanks for figuring that out!

One thing though, explain to me why that particular case clause was doing that when it should never match w/ an odd number. In other words, you said you ran

Code:
prob100:cycle_len(5).


and got that error, but 5 should've made the case go to the second clause

Code:
1 -> cycle_len(3 * Num + 1, Acc + 1)


since 5 rem 2 is 1. Why did the other case clause care if it didn't match the result?

Otherwise, great job, the whole thing works perfectly now!

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-cgr
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wolfgke
Posted: Mon Sep 17, 2007 7:04 pm Reply with quote
User Joined: 16 Sep 2007 Posts: 16
That is very easy.

Type in your Erlang console the following line and see what happens:

Code:

1.5 rem 2.


Quote:

=ERROR REPORT==== 17-Sep-2007::21:03:22 ===
Error in process <0.31.0> with exit value: {badarith,[{erlang,rem,[1.500000e+000
,2]},{erl_eval,do_apply,5},{shell,exprs,6},{shell,eval_loop,3}]}

** exited: {badarith,[{erlang,'rem',[1.50000,2]},
{erl_eval,do_apply,5},
{shell,exprs,6},
{shell,eval_loop,3}]} **


Now have a look at your own code and see the line:

Code:

case (Num rem 2) of


Then you should know what causes the problem.
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cgr
Posted: Mon Sep 17, 2007 7:59 pm Reply with quote
Joined: 15 Sep 2007 Posts: 5
Wow, I'm an idiot...

I knew that particular expression in the case header was the root of the problem, but I was so focused on it that I totally forgot what I was passing to it during each recursive iteration.

Thanks for explaining that.

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wuji
Posted: Wed Aug 15, 2012 8:23 am Reply with quote
User Joined: 10 Aug 2012 Posts: 654
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