fixed signed/unsigned comparison

Environment/ShowerAxis.cc

@@ -18,18 +18,21 @@ GrammageType ShowerAxis::X(LengthType l) const {
   auto const fractionalBin = l / steplength_;
   int const lower = fractionalBin; // indices of nearest X support points
   auto const lambda = fractionalBin - lower;
-  int const upper = lower + 1;
+  decltype(X_.size()) const upper = lower + 1;
+
+  if (lower < 0) {
+    throw std::runtime_error("cannot extrapolate to points behind point of injection");
+  }
 
   if (upper >= X_.size()) {
     std::stringstream errormsg;
     errormsg << "shower axis too short, cannot extrapolate (l / max_length_ = "
              << l / max_length_ << ")";
     throw std::runtime_error(errormsg.str().c_str());
-  } else if (lower < 0) {
-    throw std::runtime_error("cannot extrapolate to points behind point of injection");
   }
 
   assert(0 <= lambda && lambda <= 1.);
+
   // linear interpolation between X[lower] and X[upper]
   return X_[lower] * lambda + X_[upper] * (1 - lambda);
 }