将文件写入元组列表

Question

I am trying to read a file and write its contents to a list of tuples. the length of each tuple is 2 and the length of the list is variable depending on which file we read from. Each tuple should represent a point in the x, y plane with the first value being the x coord and the second the y. The issue I am having is that I believe a for loop is the best way to do this but the first line in the file is a single value representing how many points are in the file and should not be included in the final list at all.

def readDataPts(filename):
    """Reads data from an input file and returns a list of tuples
       [(x0,y0), (x1, y1), ...]
    """
    file = open(filename, 'r')
    lines = file.readlines()
    listPts = []
    for line in lines:
        for ptx, pty in line:
            x = (ptx, pty)
        listPts.append(x)
    return listPts

An example of an input is:

10
96 571
45 734
174 416
357 259
88 97
188 495
348 443
301 503
719 177
182 237

and the output should be:

[(96, 571), (45, 734), (174, 416), (357, 259), (88, 97).....]

Is there a way to start the for loop from the second line? or is there an easier way to solve this?

Thanks

Answer 1

You can call next on the file object to skip the first line and start at the second then either split each line and call tuple or let the csv.reader parse each row and map to tuple:

splitting:

with open("in.txt") as f:
    next(f) # skip first line
    arr = [tuple(line.split()) for line in f]
    print(arr)

csv lib:

import  csv
with open("in.txt") as f:
    next(f) # skip first line
    arr = list(map(tuple, csv.reader(f,delimiter=" ")))
    print(arr)

Both will return:

[('96', '571'), ('45', '734'), ('174', '416'), ('357', '259'), ('88', '97'), ('188', '495'), ('348', '443'), ('301', '503'), ('719', '177'), ('182', '237')]

If you want ints:

with open("in.txt") as f:
    next(f) # skip first line
    arr = [tuple(map(int, line.split())) for line in f]
    print(arr)

And the csv.reader:

import  csv
with open("in.txt") as f:
    next(f) # skip first line
    arr = [tuple(map(int,row) for row in  csv.reader(f,delimiter=" "))]
    print(arr)

Which will give you:

 [(96, 571), (45, 734), (174, 416), (357, 259), (88, 97), (188, 495), (348, 443), (301, 503), (719, 177), (182, 237

Answer 2

You can use .split() to create the tuple out of each line and check the length:

def readDataPts(filename):
    listPts = []
    with open(filename, 'r') as f:
        for line in f:
            numbers = line.split()
            if len(numbers) == 2:
                listPts.append(map(int, numbers))
    return listPts

Answer 3

You can use tell the loop to only iterate over lines[1:] which will skip the first line and use all the rest.

def readDataPts(filename):
"""Reads data from an input file and returns a list of tuples
   [(x0,y0), (x1, y1), ...]
"""

    file = open(filename, 'r')
    lines = file.readlines()
    listPts = []
    for line in lines[1:]:
        for ptx, pty in line:
            x = (ptx, pty)
        listPts.append(x)
    return listPts

Answer 4

Just another idea.

with open('in.txt') as f:
    nums = map(int, f.read().split())
    print zip(nums[1::2], nums[2::2])