Python - loop through csv file and check next line

Question

I want to loop through a csv document and often check the next row for values, but stick to the same row number. The .next() is skipping a row, which won't work for me, unless I could go back again somehow. Is there a simple way in doing this? I tried the pairwise/cycle but it takes way too long if the file is big. My sample code is:

file1 = open("name.csv", 'rb')
reader = csv.DictReader(file1)
new_rows_list = []

for row in reader:
    if row['IsFixated'] == 'TRUE':
        new_row = [row['Result'], 'Fixation']
        new_rows_list.append(new_row)
    else:
        new_row = [row['Result'], 'Saccade']
        new_rows_list.append(new_row)
        nextRow = reader.next() # check for next row ??
        if nextRow['IsFixated'] == 'TRUE':
            new_rows_list = CheckPreviousPositions(new_rows_list)

file1.close()

Joran Beasley · Accepted Answer · 2016-03-24 21:51:41Z

2

reader1,reader2 = itertools.tee(csv.DictReader(file1))
#this creates two copies of file iterators for the file
next(reader2) #skip first line in second filehandle
for line,next_line in itertools.izip(reader1,reader2):
    #do something?

I guess ... maybe?

answered Mar 24, 2016 at 21:51

Joran Beasley

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Comments

bunji · Accepted Answer · 2016-03-25 00:48:03Z

If you only need to check the next line this generator function should do the trick

def next_reader(file_name):
    with open(file_name) as f:
        reader = csv.reader(f)
        check = 0
        curr_and_next = []
        while True:
            if check == 0:
                first = reader.next()
                second = reader.next()
                curr_and_next = [first, second]
                check = 1
            else:
                curr_and_next = [curr_and_next[1], reader.next()]
            yield curr_and_next

This opens up your file and add the first and second line to the curr_and_next list and returns it. In any subsequent call it will move the second value in curr_and_next into the first spot and then add the next line in the file into the second spot. Now instead of yielding one row at a time, this generator will yield a list containing the next row and the row following it. For example if you have a csv file alpha.csv with the rows:

a,b
c,d
e,f

and you have a loop:

for x in next_reader('alpha.csv'):
    print x

this will print:

[['a','b'],['c','d']]
[['c','d'],['e','f']]

Note that the last line in your csv file will only appear once in the very last iteration. (You won't get something like [['e','f'],None] for the last print).

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