From 302055db861c9adc1a96604f59b990dd4ea9ff2d Mon Sep 17 00:00:00 2001
From: Jiri Pittner <jiri@pittnerovi.com>
Date: Mon, 9 Mar 2026 17:10:58 +0100
Subject: [PATCH] tensor: subtensor1()

---
 t.cc      | 22 +++++++++++++++++++++-
 tensor.cc | 20 ++++++++++++++++++++
 tensor.h  |  3 +++
 3 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/t.cc b/t.cc
index 87ac680..81212c0 100644
--- a/t.cc
+++ b/t.cc
@@ -1084,7 +1084,7 @@ NRMat<complex<double> > b=exp(a);
 cout <<b;
 }
 
-if(1)
+if(0)
 {
 int n;
 double d;
@@ -4746,5 +4746,25 @@ cout <<aa;
 cout <<"Error = "<<(ax-aa).norm()<<endl;
 }
 
+if(1)
+{
+int n=3;
+NRVec<INDEXGROUP> s(4);
+for(int i=0; i<4; ++i) 
+	{
+	s[i].number=1;
+	s[i].symmetry=0;
+	s[i].offset=0;
+	s[i].range=n;
+	}
+Tensor<double> t(s);
+t.randomize(1.);
+INDEXNAME list[4]={"i","j","k","l"}; 
+t.names=list;
+cout <<t;
+Tensor<double> t1 = t.subtensor1(2);
+cout <<t1;
+}
+
 
 }//main
diff --git a/tensor.cc b/tensor.cc
index 6c9b118..66a7b4c 100644
--- a/tensor.cc
+++ b/tensor.cc
@@ -2496,6 +2496,26 @@ return false;
 }
 
 
+template<typename T>
+Tensor<T> Tensor<T>::subtensor1(int i) const
+{
+int ind=shape.size();
+if(ind==0) laerror("subtensor of a scalar");
+--ind;
+if(shape[ind].number>1) laerror("last index must be standalone in subtensor1");
+i -= shape[ind].offset;
+if(i<0||i>=shape[ind].range) laerror("index out of range in subtensor1");
+if(ind==0) return Tensor(data[i]); //results is a scalar
+NRVec<INDEXGROUP> newshape = shape.subvector(0,ind-1);
+Tensor<T> r(newshape);
+memcpy(&r.data[0],&data[i*cumsizes[ind]],cumsizes[ind]*sizeof(T));
+if(is_named()) r.names = names.subvector(0,ind-1);
+return r;
+}
+
+
+
+
 template class Tensor<double>;
 template class Tensor<std::complex<double> >;
 template std::ostream & operator<<(std::ostream &s, const Tensor<double> &x);
diff --git a/tensor.h b/tensor.h
index 52cb35f..2a2fc9c 100644
--- a/tensor.h
+++ b/tensor.h
@@ -276,6 +276,9 @@ public:
 	Tensor(int xrank, const NRVec<INDEXGROUP> &xshape, const NRVec<LA_largeindex> &xgroupsizes,  const  NRVec<LA_largeindex> xcumsizes, const  NRVec<T> &xdata) :  myrank(xrank), shape(xshape), groupsizes(xgroupsizes), cumsizes(xcumsizes), data(xdata) {};
 	Tensor(int xrank, const NRVec<INDEXGROUP> &xshape, const NRVec<LA_largeindex> &xgroupsizes,  const  NRVec<LA_largeindex> xcumsizes, const  NRVec<T> &xdata, const NRVec<INDEXNAME>  &xnames) :  myrank(xrank), shape(xshape), groupsizes(xgroupsizes), cumsizes(xcumsizes), data(xdata), names(xnames) {};
 
+	//subtensor - todo: more general versions
+	Tensor subtensor1(int i) const; //for one value of rightmost index
+
 	//conversions from/to matrix and vector
 	explicit Tensor(const NRVec<T> &x);
 	explicit Tensor(const NRMat<T> &x, bool flat=false);