python dictionary and deque to print required output based on some condition - python-3.x

Question

I have CSV File which contains some data Produced from Mining
I wanted to print it as shown in required format
Required Format
A -> B -> C -> D -> E -> F
A -> B -> C -> I
X -> Y -> Z
X -> Y -> P -> Q
A -> B -> K -> L
a.csv File
## code
from collections import deque
import pandas as pd
data = pd.read_csv("a.csv")
data['Start'] = data['Start'].str.replace(' ','_')
data['End'] = data['End'].str.replace(' ','_')
fronts = dict()
backs = dict()
sequences = []
position_counter = 0
selector = data.apply(lambda row: row.str.extractall("([\w+\d]+)"), axis=1)
for relation in selector:
front, back = relation[0]
llist = deque((front, back))
finb = front in backs.keys()
if finb:
position = backs[front]
llist2 = sequences[position]
back_llist2 = llist2.pop()
llist = llist2 + llist
sequences[position] = llist
backs[llist[-1]] = position
if front in fronts.keys():
del fronts[front]
if back_llist2 in backs.keys():
del backs[back_llist2]
if not finb:
sequences.append(llist)
fronts[front] = position_counter
backs[back] = position_counter
position_counter += 1
data = []
for s in sequences:
data.append(' -> '.join(str(el) for el in s))
data
What I'm Getting is:
'A -> B -> C -> D -> E -> F'
'C -> I'
'A -> N -> A'
'X -> Y -> Z'
'Y -> P -> Q'
'B -> K -> L'
'X1 -> Y1'

Answer 1

You need to search the existing paths for the starting element of the new row. If found, append to the existing path or copy the path and append the new end element.
Try this code:
ss = '''
A B
B C
C D
D E
E F
C I
A N
N A
X Y
Y Z
Y P
P Q
B K
K L
X1 Y1
'''.strip()
lst = []
for r in ss.split('\n'):
lst.append(r.split())
################
paths = []
for e in lst: # each row in source data
pnew = [] # new path
for p in paths:
if e[0] in p: # if start in existing path
if p.index(e[0]) == len(p)-1: # if end of path
p.append(e[1]) # add to path
else:
pnew.append(p[:p.index(e[0])+1]+[e[1]]) # copy path then add
break
else: # loop completed, not found
paths.append(list(e)) # create new path
if len(pnew): # copied path
paths.extend(pnew) # add copied path
print('\n'.join([' => '.join(e) for e in paths]))
Output
A => B => C => D => E => F
A => B => C => I
A => N => A
X => Y => Z
X => Y => P => Q
A => B => K => L
X1 => Y1
The A->N->A and X1->Y1 are correct based on the source data. I don't know why they would be excluded in the desired output.