base) Jianguos-Air:code jianguolu$ python TF_lu.py 
Corpus size:
 unigram	[122988]	[5056]
 bigrams	[67819]	[17667]
Corpus size:
 unigram	[122988, 306057]	[5056, 9719]
 bigrams	[67819, 180682]	[17667, 44607]
('software_engineering', 0.0012013042732109147)
('software_development', 0.0033003300330033004)
('international_conference', 0.005076142131979695)
('software_systems', 0.013888888888888888)
('empirical_study', 0.015384615384615385)
('experience_report', 0.016129032258064516)
('model_driven', 0.017857142857142856)
('software_product', 0.02)
('source_software', 0.021739130434782608)
('product_line', 0.023255813953488372)
('open_source', 0.02564102564102564)
('self_adaptive', 0.02702702702702703)
('neural_network', 0.02702702702702703)
('case_study', 0.028089887640449437)
('data_flow', 0.029411764705882353)
('model_based', 0.031578947368421054)
('formal_methods', 0.03333333333333333)
('system_based', 0.037037037037037035)
('process_modeling', 0.041666666666666664)
('configuration_management', 0.043478260869565216)
('cloud_computing', 0.045454545454545456)
('applications_using', 0.045454545454545456)
('e_using', 0.047619047619047616)
('program_analysis', 0.047619047619047616)
('agent_based', 0.047619047619047616)
('second_international', 0.05)
('cost_estimation', 0.05)
('time_systems', 0.05)
('web_application', 0.05)
('use_case', 0.05263157894736842)
('application_development', 0.05263157894736842)
('f_design', 0.05263157894736842)
('lessons_learned', 0.05555555555555555)
('service_oriented', 0.05555555555555555)
('method_based', 0.05555555555555555)
('driven_approach', 0.058823529411764705)
('panel_session', 0.058823529411764705)
('formal_verification', 0.058823529411764705)