"""
Sparse instances.
Each instance is represented as dict with key dimension.
Dimensions not present in the dict have value 0.
"""

from numpy import zeros

def to_vector(instances, dimensions):
    """
    Convert sparse instances to vectors.
    @type instances: list of sparse instances
    @param dimensions: The number of dimensions in each instance.
    @rtype: numpy matrix (instances x dimensions)
    """
    assert isinstance(instances, list)
    v = zeros((len(instances), dimensions))
    l = len(instances)
    for i in range(l):
        for idx in instances[i].keys():
            v[i][idx] = instances[i][idx]
    return v