Re: Index Error: output_index < noutputs

[Vasil Velichkov]

Hi Elmore,

On 27/09/2023 03.17, Elmore Family wrote:
> I am using a gnu radio flowgraph to attempt to select the appropriate 
> Selector Block output as explained below. The following error occurs when I 
> run the flowgraph: Index Error: output_index must be < noutputs.

> I forgot to include the version: 3.9.4.0. 

Lets try analyzing the source code where this error is thrown. 

$ git checkout v3.9.4.0

$ grep "output_index must be < noutputs" . -R
./gr-blocks/lib/selector_impl.cc:        throw std::out_of_range("output_index 
must be < noutputs");

https://github.com/gnuradio/gnuradio/blob/v3.9.4.0/gr-blocks/lib/selector_impl.cc#L66-L73

 66 void selector_impl::set_output_index(unsigned int output_index)
 67 {
 68     gr::thread::scoped_lock l(d_mutex);
 69     if (output_index < d_num_outputs)
 70         d_output_index = output_index;
 71     else
 72         throw std::out_of_range("output_index must be < noutputs");
 73 }

The code is pretty simple, the only possible case is the value of output_index 
variable to be greater or equal to d_num_outputs variable. You are calling 
set_output_index with 1 so d_num_outputs must be either 0 or 1.

Now let's check where d_num_outputs gets its value.

$ grep -n d_num_outputs ./gr-blocks/lib/selector_impl.cc
40:      d_num_outputs(0)
69:    if (output_index < d_num_outputs)
100:        if (new_port < d_num_outputs)
138:        d_num_outputs = (unsigned int)noutputs;

It is initialized to 0 in the constructor on line 40 and get assigned in 
selector_impl::check_topology function on line 138.

https://github.com/gnuradio/gnuradio/blob/v3.9.4.0/gr-blocks/lib/selector_impl.cc#L134-L145

So it looks that you are calling set set_output_index(1) before 
check_topology() has been called, before the flowgraph is started. 

Looks like you need to start the flowgraph first and then call 
set_output_index(1)

Hope this helps.

Regards,
Vasil