// return this + that
public SparseVector plus(SparseVector that) {
if (this.N != that.N) throw new RuntimeException("Vector lengths disagree");
SparseVector c = new SparseVector(N);
for (int i : this.st.keys()) c.put(i, this.get(i)); // c = this
for (int i : that.st.keys()) c.put(i, that.get(i) + c.get(i)); // c = c + that
return c;
}
// return the dot product of this vector with that vector
public double dot(SparseVector that) {
if (this.N != that.N) throw new RuntimeException("Vector lengths disagree");
double sum = 0.0;
// iterate over the vector with the fewest nonzeros
if (this.st.size() <= that.st.size()) {
for (int i : this.st.keys()) if (that.st.contains(i)) sum += this.get(i) * that.get(i);
} else {
for (int i : that.st.keys()) if (this.st.contains(i)) sum += this.get(i) * that.get(i);
}
return sum;
}
