add numerical recipes library

lib/nr/cpp/recipes/sprsin.cpp

@@ -0,0 +1,24 @@
+#include <cmath>
+#include "nr.h"
+using namespace std;
+
+void NR::sprsin(Mat_I_DP &a, const DP thresh, Vec_O_DP &sa, Vec_O_INT &ija)
+{
+	int i,j,k;
+
+	int n=a.nrows();
+	int nmax=sa.size();
+	for (j=0;j<n;j++) sa[j]=a[j][j];
+	ija[0]=n+1;
+	k=n;
+	for (i=0;i<n;i++) {
+		for (j=0;j<n;j++) {
+			if (fabs(a[i][j]) >= thresh && i != j) {
+				if (++k > nmax) nrerror("sprsin: sa and ija too small");
+				sa[k]=a[i][j];
+				ija[k]=j;
+			}
+		}
+		ija[i+1]=k+1;
+	}
+}