Question

我正在尝试扫描csv文件并逐行进行调整。最后，我想删除最后一行。如何删除同一扫描循环中的最后一行？

我的代码从原始文件中读取，进行调整，最后写入新文件。

import csv

raw_data = csv.reader(open("original_data.csv", "r"), delimiter=",")
output_data = csv.writer(open("final_data.csv", "w"), delimiter=",")
lastline = # integer index of last line

for i, row in enumerate(raw_data):
    if i == 10:
        # some operations
        output_data.writerow(row)
    elif i > 10 and i < lastline:
        # some operations
        output_data.writerow(row)
    elif i == lastline:
        output_data.writerow([])
    else:
        continue

Answer 1

您可以使生成器生成除最后一个元素之外的所有元素：

def remove_last_element(iterable):
    iterator = iter(iterable)
    try:
        prev = next(iterator)
        while True:
            cur = next(iterator)
            yield prev
            prev = cur
    except StopIteration:
        return

然后你只需将raw_data包裹在其中：

for i, row in enumerate(remove_last_element(raw_data)):
    # your code

最后一行将被自动忽略。

这种方法的好处是只能读取一次文件。

Answer 2

@Kolmar's idea的变体：

def all_but_last(it):
    buf = next(it)
    for item in it:
        yield buf
        buf = item

for line in all_but_last(...):

这是针对负面索引扩展islice（双args版本）的更通用的代码：

import itertools, collections

def islice2(it, stop):
    if stop >= 0:
        for x in itertools.islice(it, stop):
            yield x
    else:
        d = collections.deque(itertools.islice(it, -stop))
        for item in it:
            yield d.popleft()
            d.append(item)


for x in islice2(xrange(20), -5):
    print x,

# 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Answer 3

您可以使用大小为2的窗口进行迭代，并仅在窗口中打印第一个值。这将导致跳过最后一个元素：

from itertools import izip, tee

def pairwise(iterable):
    a, b = itertools.tee(iterable)
    next(b, None)
    return izip(a, b)

for row, _ in pairwise(raw_data):
    output_data.writerow(row)

output_data.writerow([])

Answer 4

一个想法是计算你迭代的每一行的长度，然后当到达最后一行时截断文件，从而缩短文件＆＃34;。不确定这是不是很好的做法......

例如 Python: truncate a file to 100 lines or less

Answer 5

不要在每次循环迭代中写入当前行，而是尝试编写先前读取的行：

import csv

raw_data = csv.reader(open("original_data.csv", "r"), delimiter=",")
output_data = csv.writer(open("final_data.csv", "w"), delimiter=",")
last_iter = (None, None)

try:
    last_iter = (0, raw_data.next())
except StopIteration:
    # The file is empty
    pass
else:
    for new_row in raw_data:
        i, row = last_iter
        last_iter = (i + 1, new_row)

        if i == 10:
            # some operations
            output_data.writerow(row)
        elif i > 10:
            # some operations
            output_data.writerow(row)

    # Here, the last row of the file is in the `last_iter` variable.
    # It won't get written into the output file.
    output_data.writerow([])

Python逐行扫描文件并删除同一循环中的最后一行

5 个答案: