is there a way to replace letters once in a string?

Question

I'm running into a problem where it either replaces all Gs to Cs but doesn't replace the C to Gs, what can I do to fix this problem? the output im getting right now is "GUGAGGGGAG" the output im looking for is "CUCAGCGCAG" This is the code that I have till now:

a_string = "GAGTCGCGTC" 
remove_characters = ["G", "A", "T", "C"]
ch1 = "G"
ch2 = "A"
ch3 = "T"
ch4 = "C"
a_string = a_string.replace (ch1, "C")
a_string = a_string.replace (ch2, "U")
a_string = a_string.replace (ch3, "A")
a_string = a_string.replace (ch4, "G")
print (a_string)

I'm doing a DNA to RNA translation code! so A replaces to U, G to C, T to A and C to G

btw, what is `remove_characters`? It looks like your code doesn't need this. — quamrana, Aug 22 '20 at 17:28
Your first `replace` changes all "G"s to "C"s, but then your last `replace` changes **all** "C"s to "G"s. — Carcigenicate, Aug 22 '20 at 17:28
[How to convert a set of DNA sequences into protein sequences using python programming?](https://stackoverflow.com/questions/40834338) is an associated question. — Trenton McKinney, Aug 22 '20 at 18:02

score 5 · Accepted Answer · answered Aug 22 '20 at 17:33

5

Using str.translate we can change the whole string in one go:

a_string = "GAGTCGCGTC"
string1 = "GATC"
string2 = "CUAG"
print(a_string.translate(str.maketrans(string1, string2)))

Output:

CUCAGCGCAG

answered Aug 22 '20 at 17:33

Hampus Larsson

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score 2 · Answer 2 · answered Aug 22 '20 at 17:39

In Python 3 you can use translate:

a_string = 'GAGTCGCGTCTACACATGCAGTCGAACGGTAGCACAGAGAGCTTGCTCTCGGGTG' 
trans = {
    'G': 'C',
    'A': 'U',
    'T': 'A',
    'C': 'G'
}
print(a_string.translate(str.maketrans(trans)))
# CUCAGCGCAGAUGUGUACGUCAGCUUGCCAUCGUGUCUCUCGAACGAGAGCCCAC

score 1 · Answer 3 · answered Aug 22 '20 at 17:31

1

You can perform the replacement to lowercase letters, and then only when all replacements have been done, turn the string completely to uppercase.

answered Aug 22 '20 at 17:31

trincot

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is there a way to replace letters once in a string?

3 Answers3