1 files changed, 4 insertions, 3 deletions

diff --git a/fits.cpp b/fits.cpp
index 370b63d..62d75c9 100644
--- a/fits.cpp
+++ b/fits.cpp
@@ -57,11 +57,12 @@ Real cost(const std::list<std::tuple<Real, Real>>& data, const Vector& coeffs) {
 Vector dCost(const std::list<std::tuple<Real, Real>>& data, const Vector& coeffs) {
   Vector dC = Vector::Zero(coeffs.size());
 
-  for (unsigned j = 0; j < coeffs.size(); j++) {
-    for (const std::tuple<Real, Real>& xy : data) {
+  for (const std::tuple<Real, Real>& xy : data) {
+    for (unsigned j = 0; j < coeffs.size(); j++) {
       Real x = std::get<0>(xy);
       Real y = std::get<1>(xy);
-      dC[j] += (value(coeffs, x) - y) * basisFunctions(coeffs.size(), j, x);
+      Real Δc = value(coeffs, x) - y;
+      dC[j] += Δc * basisFunctions(coeffs.size(), j, x);
     }
   }