How to jump to a particular line in a huge text file?


Question

Are there any alternatives to the code below:

startFromLine = 141978 # or whatever line I need to jump to

urlsfile = open(filename, "rb", 0)

linesCounter = 1

for line in urlsfile:
    if linesCounter > startFromLine:
        DoSomethingWithThisLine(line)

    linesCounter += 1

If I'm processing a huge text file (~15MB) with lines of unknown but different length, and need to jump to a particular line which number I know in advance? I feel bad by processing them one by one when I know I could ignore at least first half of the file. Looking for more elegant solution if there is any.

1
99
9/12/2017 6:05:49 AM

Accepted Answer

linecache:

The linecache module allows one to get any line from a Python source file, while attempting to optimize internally, using a cache, the common case where many lines are read from a single file. This is used by the traceback module to retrieve source lines for inclusion in the formatted traceback...

27
9/30/2016 7:28:52 PM

You can't jump ahead without reading in the file at least once, since you don't know where the line breaks are. You could do something like:

# Read in the file once and build a list of line offsets
line_offset = []
offset = 0
for line in file:
    line_offset.append(offset)
    offset += len(line)
file.seek(0)

# Now, to skip to line n (with the first line being line 0), just do
file.seek(line_offset[n])

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