mat: add NonZeroDoer interfaces and implementations

Also clean up some documentation and missing type checks related to
tests for NonZeroDoers.

mat/symband.go

@@ -16,6 +16,10 @@ var (
 	_            Banded           = symBandDense
 	_            RawSymBander     = symBandDense
 	_            MutableSymBanded = symBandDense
+
+	_ NonZeroDoer    = symBandDense
+	_ RowNonZeroDoer = symBandDense
+	_ ColNonZeroDoer = symBandDense
 )
 
 // SymBandDense represents a symmetric band matrix in dense storage format.
@@ -41,7 +45,7 @@ type RawSymBander interface {
 // NewSymBandDense creates a new SymBand matrix with n rows and columns. If data == nil,
 // a new slice is allocated for the backing slice. If len(data) == n*(k+1),
 // data is used as the backing slice, and changes to the elements of the returned
-// BandDense will be reflected in data. If neither of these is true, NewSymBandDense
+// SymBandDense will be reflected in data. If neither of these is true, NewSymBandDense
 // will panic. k must be at least zero and less than n, otherwise NewBandDense will panic.
 //
 // The data must be arranged in row-major order constructed by removing the zeros
@@ -126,3 +130,46 @@ func (s *SymBandDense) TBand() Banded {
 func (s *SymBandDense) RawSymBand() blas64.SymmetricBand {
 	return s.mat
 }
+
+// DoNonZero calls the function fn for each of the non-zero elements of s. The function fn
+// takes a row/column index and the element value of s at (i, j).
+func (s *SymBandDense) DoNonZero(fn func(i, j int, v float64)) {
+	for i := 0; i < s.mat.N; i++ {
+		for j := max(0, i-s.mat.K); j < min(s.mat.N, i+s.mat.K+1); j++ {
+			v := s.at(i, j)
+			if v != 0 {
+				fn(i, j, v)
+			}
+		}
+	}
+}
+
+// DoRowNonZero calls the function fn for each of the non-zero elements of row i of s. The function fn
+// takes a row/column index and the element value of s at (i, j).
+func (s *SymBandDense) DoRowNonZero(i int, fn func(i, j int, v float64)) {
+	if i < 0 || s.mat.N <= i {
+		panic(ErrRowAccess)
+	}
+	for j := max(0, i-s.mat.K); j < min(s.mat.N, i+s.mat.K+1); j++ {
+		v := s.at(i, j)
+		if v != 0 {
+			fn(i, j, v)
+		}
+	}
+}
+
+// DoColNonZero calls the function fn for each of the non-zero elements of column j of s. The function fn
+// takes a row/column index and the element value of s at (i, j).
+func (s *SymBandDense) DoColNonZero(j int, fn func(i, j int, v float64)) {
+	if j < 0 || s.mat.N <= j {
+		panic(ErrColAccess)
+	}
+	for i := 0; i < s.mat.N; i++ {
+		if i-s.mat.K <= j && j < i+s.mat.K+1 {
+			v := s.at(i, j)
+			if v != 0 {
+				fn(i, j, v)
+			}
+		}
+	}
+}