def get_cancer_types(file_paths_by_cancer):
    """
    Maps cancer type to an index value 

    Parameters:
    file_paths_by_cancer: (dict) cancer : list of data files 

    Returns:
    cancer_dict: (dict) cancer : integer identifier
    """
    cancer_dict = {}
    cancer_index = 0
    for cancer in file_paths_by_cancer:
        cancer_dict[cancer] = cancer_index
        cancer_index += 1

    return cancer_dict