Extract certain elements from a list

I have no clue about Python and started to use it on some files. I managed to find out how to do all the things that I need, except for 2 things.

1st

>>>line = ['0', '1', '2', '3', '4', '5', '6']
>>>#prints all elements of line as expected
>>>print string.join(line)
0 1 2 3 4 5 6

>>>#p开发者_开发问答rints the first two elements as expected
>>>print string.join(line[0:2])
0 1

>>>#expected to print the first, second, fourth and sixth element;
>>>#Raises an exception instead
>>>print string.join(line[0:2:4:6])
SyntaxError: invalid syntax

I want this to work similar to awk '{ print $1 $2 $5 $7 }'. How can I accomplish this?

2nd

how can I delete the last character of the line? There is an additional ' that I don't need.

Provided the join here is just to have a nice string to print or store as result (with a coma as separator, in the OP example it would have been whatever was in string).

line = ['A', 'B', 'C', 'D', 'E', 'F', 'G']

print ','.join (line[0:2])

A,B

print ','.join (line[i] for i in [0,1,2,4,5,6])

A,B,C,E,F,G

What you are doing in both cases is extracting a sublist from the initial list. The first one use a slice, the second one use a list comprehension. As others said you could also have accessed to elements one by one, the above syntaxes are merely shorthands for:

print ','.join ([line[0], line[1]])

A,B

print ','.join ([line[0], line[1], line[2], line[4], line[5], line[6]])

A,B,C,E,F,G

I believe some short tutorial on list slices could be helpfull:

• l[x:y] is a 'slice' of list l. It will get all elements between position x (included) and position y (excluded). Positions starts at 0. If y is out of list or missing, it will include all list until the end. If you use negative numbers you count from the end of the list. You can also use a third parameter like in l[x:y:step] if you want to 'jump over' some items (not take them in the slice) with a regular interval.

Some examples:

l = range(1, 100) # create a list of 99 integers from 1 to 99
l[:]    # resulting slice is a copy of the list
l[0:]   # another way to get a copy of the list
l[0:99] # as we know the number of items, we could also do that
l[0:0]  # a new empty list (remember y is excluded]
l[0:1]  # a new list that contains only the first item of the old list
l[0:2]  # a new list that contains only the first two items of the old list
l[0:-1] # a new list that contains all the items of the old list, except the last
l[0:len(l)-1] # same as above but less clear 
l[0:-2] # a new list that contains all the items of the old list, except the last two
l[0:len(l)-2] # same as above but less clear
l[1:-1] # a new list with first and last item of the original list removed
l[-2:] # a list that contains the last two items of the original list
l[0::2] # odd numbers
l[1::2] # even numbers
l[2::3] # multiples of 3

If rules to get items are more complex, you'll use a list comprehension instead of a slice, but it's another subjet. That's what I use in my second join example.

You don't want to use join for that. If you just want to print some bits of a list, then specify the ones you want directly:

print '%s %s %s %s' % (line[0], line[1], line[4], line[6])

Assuming that the line variable should contain a line of cells, separated by commas...

You can use map for that:

line = "1,2,3,4,5,6"
cells = line.split(",")
indices=[0,1,4,6]
selected_elements = map( lambda i: cells[i], indices )
print ",".join(selected_elements)

The map function will do the on-the-fly function for each of the indices in the list argument. (Reorder to your liking)

You could use the following using list comprehension :

indices = [0,1,4,6]
Ipadd = string.join([line[i] for i in xrange(len(line)) if i in indices])

Note : You could also use :

Ipadd = string.join([line[i] for i in indices])

but you will need a sorted list of indices without repetition of course.

Answer to the second question:

If your string is contained in myLine, just do:

myLline = myLine[:-1]

to remove the last character.

Or you could also use rstrip():

myLine = myLine.rstrip("'")

>>> token = ':'
>>> s = '1:2:3:4:5:6:7:8:9:10'
>>> sp = s.split(token)
>>> token.join(filter(bool, map(lambda i: i in [0,2,4,6] and sp[i] or False, range(len(sp)))))
'1:3:5:7'

l = []
l.extend(line[0:2])
l.append(line[5]) # fourth field
l.append(line[7]) # sixth field
string.join(l)

Alternatively

"{l[0]} {l[1]} {l[4]} {l[5]}".format(l=line)

Please see PEP 3101 and stop using the % operator for string formatting.