':param features: list of feature string which is needed to convert to binary features
:type features: list(str)
:return : string of binary features in libsvm format  which is \'featureID:value\' pairs'

def _convert_to_binary_features(self, features):

    unsorted_result = []
    for feature in features:
        self._dictionary.setdefault(feature, len(self._dictionary))
        unsorted_result.append(self._dictionary[feature])
    return '    '.join(((str(featureID) + ':1.0') for featureID in sorted(unsorted_result)))
