I've started a little challenge on a website, and the first one was about counting different DNA letters. I've done it, but I found my method very brutal. I have a little experience, and I know that hard-coded information is not good. Any way to improve this code?
chaine = "AAAACCCGGT"
nA = 0
nT = 0
nG = 0
nC = 0
for i in range(len(chaine)):
if chaine[i] == "A":
nA = nA + 1
if chaine[i] == "C":
nC = nC + 1
print nA, nC
for i in range(len(...)): is almost never the right way to go in Python. Instead, in this case, you should iterate directly over the characters:
for char in chaine:
if char == 'A':
...
Rather than separate names nA, nC, etc., use a dictionary to map each character to its count:
counts = {'A': 0, 'C': 0, ...}
Now you don't need the ifs at all:
for char in chaine:
counts[char] += 1
print counts['A'], counts['C']
In fact, Python already implements collections.Counter, a dictionary subclass that makes this task trivial:
from collections import Counter
counts = Counter(chaine)
print counts['A'], counts['C']
The complete version is
from collections import Counter
chaine = "AAAACCCGGT"
counts = Counter(chaine)
for letter, count in counts.items():
print letter, count
This will print all the counts, not just the two in your code. More details on iterating over associative arrays in Python. You've already seen Jon's answer introducing the Counter.
