Vectors

Sparse vectors

Many problems have predictors (multiple $x_m$ ) which are very large but most $x_m$ are zero.

A good example is building a vector for the number of occurences of a word from the dictionary in a text. The vector might have the size of 100k entries, but most are 0. This vector can be described as sparse. On the other hand, it is dense if it contains mostly non-zero values.