Something about forecast()

Hi all:
I want to know something about the forecast().
I have already known that forcast() can tell scheduler how
many input items are required for each output item.
1.But now i have read two example:
The first one:
void your_block::forecast(int
noutput_items,gr_vector_int &ninput_items_required){
ninput_items_required[0]=100 *

                                ninput_items_required[1]=100 * 

noutput_items; }
I have already understand it.
But the second one:
void forecast (int noutput_items,
gr_vector_int &ninput_items_required)
unsigned ninputs =
ninput_items_required.size ();
for (unsigned i = 0; i < ninputs; i++)
ninput_items_required[i] = 1;}
I can’t understand since we can’t know how many
input items we required,why use ninput_items_required.size ().Can
someone help me?
2.I want to know if we use the general_work().Is it means
that we must use the forcast()?Thanks.
Best regards

Hi Xianda,

Easiest answer first:

  1. You need to write a forecast if, and only if, you’re using
    general_work. I generally try to avoid doing that.

ninput_items_required is, as you can see in the function signature, a
reference to a vector.
The size of the vector is the number of input ports.
Compare to


On Wed, Jun 4, 2014 at 2:22 PM, xianda [email protected] wrote:

                                ninput_items_required[1]=100 * 

noutput_items; }

In your first example above, you have exactly two input ports.
That’s why you need to specify “ninput_items_required[0]” and
“ninput_items_required[1]”, there are in total 2 ports.
In fact in this example the value of “ninput_items_required.size()” is
In alternative, you could also replace above two lines with:

for (unsigned i=0; i < 2; i++)
   ninput_items_required[i] = 100 * noutput_items;

Or, to be more generic, you could also use this:

for (unsigned i=0; i < ninput_items_required.size(); i++)
ninput_items_required[i] = 100 * noutput_items;

         2.I want to know if we use the general_work().Is it means that we 

must use the forcast()?Thanks.

Best regards

If you use “general” block, you need to specify “forecast()”.
“General” block has “general_work()” function but not “general()”

If you use sync block, decim, interpolation block etc, you have “work()”
but not “general_work()”, and you don’t need to write “forecast()”.
This is very straight-forward.

Hi Xianda,
the io_signature specifies a minimum and a maximum number of inputs, so
this might be 1,2 or 3.


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