Question

Python: How would I write a function that takes a string representing a DNA sequence, and return a string representing the complement ('A' and 'T' are complements of each other, as are 'C' and 'G'). The hints given were to use a dictionary to map a symbol A to its complement T, and to use join() rather than string addition.

    def dna_complement(text: str) -> str:

assert(dna_complement('AAAACCCGGT') == 'ACCGGGTTTT')
assert dna_complement('AcgTTTcAgCCC') == 'GGGCTGAAACGT'

Answer 1

Solution:-

Here is the Python code for the given requirement. Comments have been added in the code for better explanation.

def dna_complement(text):
    # Creating a dictionary mapping the symbols with its complement
    dic = {'A':'T','T':'A','C':'G','G':'C'}
    
    # define an empty list
    ls = []
    
    # for each character in the input string
    for i in range(len(text)):
        # if the upper case of the character is a key in a dictionary
        if text[i].upper() in dic.keys():
            # append the value of that key to the list
            ls.append(dic.get(text[i].upper()))
    
    # list ls will contain a list with all characters complement. 
    # we use the join function to createa string from that list.
    # and then use the slicing shortcut [::-1] to reverse that string and return it.         
    return((''.join(ls))[::-1])

if __name__ == '__main__':
    print('The complement of DNA string AAAACCCGGT is ',dna_complement("AAAACCCGGT"))
    print('The complement of DNA string AcgTTTcAgCCC is ',dna_complement("AcgTTTcAgCCC"))

The output is as follows: