nish
nish

Reputation: 1058

Python read next()

next() in python does not work. What is an alternative to reading next line in Python? Here is a sample:

filne = "D:/testtube/testdkanimfilternode.txt"
f = open(filne, 'r+')

while 1:
    lines = f.readlines()
    if not lines:
        break
    for line in lines:
        print line
        if (line[:5] == "anim "):
            print 'next() '
            ne = f.next()
            print ' ne ',ne,'\n'
            break

f.close()

Running this on a file does not show 'ne '.

Upvotes: 24

Views: 224340

Answers (5)

vitaut
vitaut

Reputation: 55524

next() does not work in your case because you first call readlines() which basically sets the file iterator to point to the end of file.

Since you are reading in all the lines anyway you can refer to the next line using an index:

filne = "in"
with open(filne, 'r+') as f:
    lines = f.readlines()
    for i in range(0, len(lines)):
        line = lines[i]
        print line
        if line[:5] == "anim ":
            ne = lines[i + 1] # you may want to check that i < len(lines)
            print ' ne ',ne,'\n'
            break

Upvotes: 26

mouad
mouad

Reputation: 70021

When you do : f.readlines() you already read all the file so f.tell() will show you that you are in the end of the file, and doing f.next() will result in a StopIteration error.

Alternative of what you want to do is:

filne = "D:/testtube/testdkanimfilternode.txt"

with open(filne, 'r+') as f:
    for line in f:
        if line.startswith("anim "):
            print f.next() 
            # Or use next(f, '') to return <empty string> instead of raising a  
            # StopIteration if the last line is also a match.
            break

Upvotes: 40

tzot
tzot

Reputation: 95901

A small change to your algorithm:

filne = "D:/testtube/testdkanimfilternode.txt"
f = open(filne, 'r+')

while 1:
    lines = f.readlines()
    if not lines:
        break
    line_iter= iter(lines) # here
    for line in line_iter: # and here
        print line
        if (line[:5] == "anim "):
            print 'next() '
            ne = line_iter.next() # and here
            print ' ne ',ne,'\n'
            break

f.close()

However, using the pairwise function from itertools recipes:

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = itertools.tee(iterable)
    next(b, None)
    return itertools.izip(a, b)

you can change your loop into:

for line, next_line in pairwise(f): # iterate over the file directly
    print line
    if line.startswith("anim "):
        print 'next() '
        print ' ne ', next_line, '\n'
        break

Upvotes: 4

Cormac Mulhall
Cormac Mulhall

Reputation: 1217

You don't need to read the next line, you are iterating through the lines. lines is a list (an array), and for line in lines is iterating over it. Every time you are finished with one you move onto the next line. If you want to skip to the next line just continue out of the current loop.

filne = "D:/testtube/testdkanimfilternode.txt"
f = open(filne, 'r+')

lines = f.readlines() # get all lines as a list (array)

# Iterate over each line, printing each line and then move to the next
for line in lines:
    print line

f.close()

Upvotes: 2

patapizza
patapizza

Reputation: 2398

lines = f.readlines()

reads all the lines of the file f. So it makes sense that there aren't any more line to read in the file f. If you want to read the file line by line, use readline().

Upvotes: 4

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