# [SciPy-User] [SciPy-user] help about lfilter ????

David Cournapeau cournape@gmail....
Thu Jan 28 19:14:45 CST 2010

```On Fri, Jan 29, 2010 at 10:10 AM,  <josef.pktd@gmail.com> wrote:
> On Thu, Jan 28, 2010 at 7:51 PM, persia <m.abdollahi@gmail.com> wrote:
>>
>> lfilter is the linear filtering function in the scipy.signal module. can
>> someone tell me what is wrong with this function please ?
>>
>> I used this function in the following way and it gave me that error !! :
>>
>> In [204]:
>> lfilter(array([1,2,3]),array([-4,5,6]),array([9,8,7,6,5,4,3,2,1]),zi=array([0,0]))
>> ---------------------------------------------------------------------------
>> <type 'exceptions.ValueError'>            Traceback (most recent call last)
>>
>>
>>
>> <type 'exceptions.ValueError'>: linear_filter not available for this type
>>
>>
>> why is it so ??!!!! what is wrong with this "type " ? what is it meant by
>> this type anyway ?! i tried with the same arguments in matlab with the
>> corresponding function and it worked !!
>> and if i multiply the second argument vector say by .4, this will happen :
>>
>> lfilter(array([1,2,3]),.4*array([-4,5,6]),array([9,8,7,6,5,4,3,2,1]),zi=array([0,0]))
>>
>>
>> (array([   -5.625     ,   -23.28125   ,   -68.7890625 ,  -148.40820312,
>>        -312.44384766,  -633.16711426, -1276.37466431, -2557.71900177,
>>       -5120.46074867]),
>>  array([-10242.1544385 ,  -7682.56612301]))
>>
>>
>> see ? no problem then ?!! isnt this wierd ? please help !
>
> it looks like lfilter doesn't like integers

Yes - the error message could be improved at least.

Integers are not supported as is because the difference equation
requires division (for IIR). I wonder whether it would make sense to
automatically convert to floating point if everything is integer.

cheers,

David
```