void LinearFit2D::evaluate_error(const algebra::Vector2Ds& data,
const Floats &errors) {
error_ = 0.0;
for(unsigned int i=0; i<data.size(); i++) {
double diff = (a_*data[i][0] + b_ - data[i][1]);
if (!errors.empty()) {
diff/=errors[i];
}
error_ += diff*diff;
}
//std::cerr << error_ << std::endl;
}
IMPALGEBRA_BEGIN_NAMESPACE
LinearFit2D::LinearFit2D(const algebra::Vector2Ds& data,
const Floats &errors) {
// check that there are at least 3 points
IMP_USAGE_CHECK(data.size() > 1,
"At least 2 points are required for LinearFit2D "
<< data.size() << " given");
IMP_USAGE_CHECK(errors.empty() || errors.size()==data.size(),
"Either there must be no error bars given or one per"
<< " point.");
find_regression(data, errors);
evaluate_error(data, errors);
}
void LinearFit2D::find_regression(const algebra::Vector2Ds& data,
const Floats &errors) {
double x(0.0), y(0.0), x2(0.0), xy(0.0), w(0.0);
for(unsigned int i=0; i<data.size(); i++) {
double wi;
if (!errors.empty()) {
wi=1.0/IMP::base::square(errors[i]);
} else {
wi=1.0;
}
w+=wi;
x+= wi*data[i][0];
y+= wi*data[i][1];
xy+= wi*data[i][0]*data[i][1];
x2 += wi*data[i][0]*data[i][0];
}
a_ = (w*xy-x*y)/(w*x2 - x*x);
b_ = y/w - a_*x/w;
}
