How to find a overlap between 2 sequences, and make the function return the overlap

[AnneTanne]

I need to make a function that determines the overlap between 2 sequences, and then return the overlap.
The overlap is a coherent sequence that is in the left end of the first sequence, and in the right end of the second sequence.

my sequences are: s1= "CGATTCCAGGCTCCCCACGGGGTACCCATAACTTGACAGTAGATC TC"
s2= "GGCTCCCCACGGGGTACCCATAACTTGACAGTAGATCTCGTCCAGACCC CTAGC"

my function should be named def getOverlap(left, right)

and should return ‘GGCTCCCCACGGGGTACCCATAACTTGACAGTAGATCTC’
if s1 is the left sequence and s2 the right one.

[bvdet]

Have you developed an algorithm? Have you tried anything yet?

[AnneTanne]

I have tried this:

left = "CGATTCCAGGCTCCCCACGGGGTACCCATAACTTGACAGTAGATCTC"
right = "GGCTCCCCACGGGGTACCCATAACTTGACAGTAGATCTCGTCCAGACCCCTAGC"
 
 
def getOverlap(left,right):
    if left == right[::-1]:
        return ""
    else:
        for i in left:
            for i in right:
                if left[len(left)-i]==(right[::-1])[len(right)-i]:
                    if False:
                        continue
                    return right[:len(left)-i]

But something is very wrong :)

[AnneTanne]

When i turn the s1, and s2 around, the answer is supposed to be 'c', but i get the same answer.

[bvdet]

It appears to me you have the sequences switched. The overlap you describe is at the left end of s2 and right end of s1. You can use string method find to find the overlap.

A possible algorithm:

1. Initialize an empty string and assign to an identifier, let's say "X"
2. Iterate on the left string, character by character
3. Assign "X"+next character to an identifier, let's sat "temp"
4. If the current value of "temp" is found in the the second string, assign the value of "X" to "temp".
5. If the current value of "temp" is not found in the the second string, return "X"

This assumes the "left" string overlap will always start at the beginning of the string.

[bvdet]

To add to the qualification above - The algorithm I described does not require the overlap to be at the right end of the "right" string.

def getOverlap(left, right):
    overlap = ""
    for c in left:
        temp  = overlap+c
        if right.find(temp) >= 0:
            overlap = temp
        else:
            return overlap
    return    
 
s1 = "GGCTCCCCACGGGGTACCCATAACTTGACAGTAGATCTCGTCCAGACCCCTAGC"
s2 = "CGATTCCAGGCTCCCCACGGGGTACCCATAACTTGACAGTAGATCTC"
 
print getOverlap(s1,s2)
print getOverlap(s2,s1)

Produces:

>>> GGCTCCCCACGGGGTACCCATAACTTGACAGTAGATCTC
CG
>>> 

[bvdet]

You should use the slice operator to determine the overlap, something like this:

s1 = "GGCTCCCCACGGGGTACCCATAACTTGACAGTAGATCTCGTCCAGACCCCTAGC"
s2 = "CGATTCCAGGCTCCCCACGGGGTACCCATAACTTGACAGTAGATCTC"
 
def getOverlap(left, right):
    if left == right[-len(left):]:
        return left
    i = -1
    while True:
        temp = left[0:i]
        if temp == right[-len(temp):]:
            return temp
        elif temp == "":
            return ""
        i -= 1
 
print getOverlap(s1,s2)
print getOverlap(s2,s1)
 

Output:

>>> GGCTCCCCACGGGGTACCCATAACTTGACAGTAGATCTC
C
>>>