Python basics for newbies

We can use *-operator (star operator) to unpack the arguments out of a list or tuple. Here is an example with python built in range() function :

>>> mylist = [3, 10]
>>> mylist
[3, 10]
>>> range(mylist[0], mylist[1])
[3, 4, 5, 6, 7, 8, 9]
>>> range(*mylist)
[3, 4, 5, 6, 7, 8, 9]
>>> 

Input file:

$ cat file.txt
500:120:100:X
:100:120
200:900:125
120:120
:900
120:345
12:900:1234:34
:90

Required: Join the lines which startswith : with the previous line.
i.e. required output is:

500:120:100:X:100:120
200:900:125
120:120:900
120:345
12:900:1234:34:90

The python script to achieve this:

data=open("file.txt").read().split("\n")
for i,line in enumerate(data):
    if line.startswith(":"):
        data[i-1]= data[i-1]+line
        data.pop(i)
print '\n'.join(data),

Executing it:

$ python merge-lines.py
500:120:100:X:100:120
200:900:125
120:120:900
120:345
12:900:1234:34:90

The solution using UNIX Awk can be found here

More about python enumerate function can be found here. Mentioned below is a small example on python enumerate function

>>> for i, student in enumerate(['Alex', 'Ryan', 'Deb']):
...     print i, student
...
0 Alex
1 Ryan
2 Deb
>>>

Related Posts:
- Print section of file using line number - Python
- Print line next to pattern using Python
- Print line above pattern using Python

New learning:
Python list pop method:
list.pop([i])
Remove the item at the given position in the list, and return it. If no index is specified, a.pop() removes and returns the last item in the list.

Input file:

$ cat file.txt
h1|u|1
h2|5|1|1
rec1|1239400800|Sat|fan1|AX|2|10035|-|2|50
rec2|1239400800|Sat|fan1|AX|2|-|-|2|17
rec5|1239400801|Sat|fan3|AY|5|10035|-|2|217
rec8|1239400804|Sat|fan5|AX|2|5|-|2|970

Required Output:
- Lines starting with "h1" or "h2", no action required, just print.
- Lines starting with "rec", divide the values starting from 6th field by 2.

Required output is:

h1|u|1
h2|5|1|1
rec1|1239400800|Sat|fan1|AX|1|5017|-|1|25
rec2|1239400800|Sat|fan1|AX|1|-|-|1|8
rec5|1239400801|Sat|fan3|AY|2|5017|-|1|108
rec8|1239400804|Sat|fan5|AX|1|2|-|1|485

The python script:

fp = open("file.txt", "rU")
lines = fp.readlines()
fp.close()

for line in lines:
       if line.startswith("h1"):
               print line,
       if line.startswith("h2"):
               print line,
       if line.startswith("rec"):
               f=line.split("|")
               r = f[5:]
               l = []
               for each in r:
                       try:
                               l.append(str(int(each)/2))
                       except ValueError:
                               l.append(each)

               t = "|".join(f[0:5]) + "|" + "|".join(l)
               print t.rstrip()