The goal is to replace all characters within a string using a for loop and replace.
My code looks like this:
strand_1 = input("type DNA sequence here: ")
for i in ("a", "t"),("t", "a"),("g", "c"),("c", "g"):
comp_strand = strand_1.replace(*i)
print(f' the complementary strand is: {comp_strand.upper()}')
The output for using 'agtcagtcagtc' looks like this:
type DNA sequence here: agtcagtcagtc
the complementary strand is: AGTGAGTGAGTG
For some reason i don't understand, only the last pair ("c", "g") actually gets replaced while the others won't.
What might be the cause for this to happen, and how can i make this work?
The reason is that you're overwriting comp_strand each loop, and not saving the result. However even if you fix that, it still won't work, as 0x5453 explained in a comment:
it still won't do what you want, because you do the swaps one character at a time. For example,
'atta'would become'tttt'after the first iteration, and then'aaaa'after the second.
The better solution for multiple replacements is str.translate() with str.maketrans(). For example:
table = str.maketrans('atgc', 'tacg')
strand = 'agtcagtcagtc'
complementary = strand.translate(table)
print(complementary) # -> tcagtcagtcag