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calculate the length of a sequence after adding the length of previous sequences

Hi all,

I want to determine length of individual sequences in a multifasta file. I got this biopython code from the bio manual as: 

from Bio import SeqIO
import sys
cmdargs = str(sys.argv)
for seq_record in SeqIO.parse(str(sys.argv[1]), "fasta"):
 output_line = '%s\t%i' % \
seq_record.id, len(seq_record))
 print(output_line)

My input file is like:

>Protein1
MNT
>Protein2
TSMN
>Protein3
TTQRT

And the code yields:

Protein1        3
Protein2        4
Protein3        5

But I want to calculate the length of a sequence after adding the length of previous sequences. It would be like:

Protein1        1-3
Protein2        4-7
Protein3        8-12

I don't know in which of the above line in the code I need to change to get that output. I'd appreciate any help on this issue, thanks!!!!

fasta python sequence
• 1,814 views • link
updated by WouterDeCoster WouterDeCoster • written by User 6777 • User 6777

1 answer

You probably want to store the length and sequentially add each iteration in the loop the length.

I don't get the 1,4 and 8. Where do these numbers come from?

In addition, you don't really need cmdargs = str(sys.argv) (you also don't use it downstream I see)

I would rewrite and simplify your code to:

savedlength = 0 #Initiate the variable we are going to use to incrementally store the length
for seq_record in SeqIO.parse(str(sys.argv[1]), "fasta"):
    savedlength += len(seq_record)    
    print("{}\t{}".formatseq_record.id, savedlength))

Is this getting closer to what you need?

• 1 views • link
written by WouterDeCoster WouterDeCoster

I think he is adding the previous length 3+1 7+1 but this is also not clear to me where is this 1 came from

• 1 views • link
written by Medhat Medhat

It could also be that he is creating a begin and end 'position' for each sequence, but since it's unclear I prefer to ask rather than assuming something :p

• 1 views • link
written by WouterDeCoster WouterDeCoster

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