Read line once without removing it Python using .readline()

Question

I would like to count the occurences of missings of every line in a txt file.

foo.txt file:

1 1 1 1 1 NA    # so, Missings: 1
1 1 1 NA 1 1    # so, Missings: 1
1 1 NA 1 1 NA   # so, Missings: 2  

But I would also like to obtain the amount of elements for the first line (assuming this is equal for all lines).

miss = []
with open("foo.txt") as f:
    for line in f:
        miss.append(line.count("NA"))

>>> miss
[1, 1, 2]         # correct

The problem is when I try to identify the amount of elements. I did this with the following code:

miss = []
with open("foo.txt") as f:
    first_line = f.readline()
    elements = first_line.count(" ")  # given that values are separated by space
    for line in f:
        miss.append(line.count("NA"))

>>> (elements + 1)
6   # True, this is correct          
>>> miss 
[1,2]  # misses the first item due to readline() removing lines.`

How can I read the first line once without removing it for the further operation?

Answer 1

Try f.seek(0) . This will reset the file handle to the beginning of the file.

Complete example would then be:

miss = []
with open("foo.txt") as f:
    first_line = f.readline()
    elements = first_line.count(" ")  # given that values are separated by space
    f.seek(0)
    for line in f:
        miss.append(line.count("NA"))

Even better would be to read all lines, even the first line, only once, and checking for number of elements only once:

miss = []
elements = None
with open("foo.txt") as f:
    for line in f:
        if elements is None:
            elements = line.count(" ")  # given that values are separated by space
        miss.append(line.count("NA"))

BTW: wouldn't the number of elements be line.count(" ") + 1 ?

I'd recommend using len(line.split()) , as this also handles tabs, double spaces, leading/trailing spaces etc.

Answer 2

Provided all lines have the number of items you can just count items in the last line:

miss = []
with open("foo.txt") as f:
    for line in f:
        miss.append(line.count("NA")
    elements = len(line.split())

A better way to count is probably:

elements = len(line.split())  

because this also counts items separated with multiple spaces or tabs.

Answer 3

You can also just treat the first line separately

with open("foo.txt") as f:
    first_line = next(f1)
    elements = first_line.count(" ")  # given that values are separated by space
    miss = [first_line.count("NA")]
    for line in f:
        miss.append(line.count("NA")