# complement_2.py
# Author: Sami Khuri
# Last updated: January 16, 2016
# Purpose: To construct the complement of a given DNA sequence 
# Program uses dictionary, list comprehension, and join() 

def complement(s): 
    """Return the complementary sequence string of s.""" 
    basecomplement = {'A': 'T', 'C': 'G', 'G': 'C', 'T': 'A'} 
    
    # converting string to list so as to use for loop
    letters = list(s) 
    
    # list comprehension works like a for loop & returns a list, 
    # but in a much more compact and efficient format
    letters = [basecomplement[base] for base in letters] 
     
    # converting list back to string
    return ''.join(letters) 
      

dna_seq = 'CCGGAAGAGCTTACTTAG' 
print "DNA sequence:\t", dna_seq        
print "Its complement:\t", complement(dna_seq)