fir filter

[dave2k]

i have code for fir filter below. The way it's coded means it is really slow during debugging (vs 8). Can anyone identify where the slow bits are or even better, suggest quicker code?

Thanks

vector<double> filter( const vector<double>& b, const vector<double>& x )
{
vector<double> y;
vector<double> prev_x;


// Cycle for x
for( size_t i = 0; i < x.size(); i++ ) {
double y_n;
prev_x.push_back( x[i] );

if( prev_x.size() > b.size() )
prev_x.erase( prev_x.begin() );

// Cycle for prev_x
size_t nt = prev_x.size();
y_n = 0;
for( size_t j = 0; j < nt; j++ ) {
y_n += b[j] * prev_x[nt-j-1];
}
y.push_back( y_n );
}
return y;
}

by the way vectors are by know means necessary - arrays are ok...