函数内的 Python readline() 跳过文件中的第二行

Question

I'm an absolute beginner in the programming field. I'm doing some exercises from a book and there's something I don't understand. I'm using a readline() command inside a function and then starting a for loop in main() to find the largest number in a file. The file has a list of numbers in each line: the largest number is in the second line, but for some reason the program skips the second line (which contains the largest number). This is the code:

def reader(filename):
    nums = filename.readline()
    return(nums)

def main():
    filename = open("numbers.txt", 'r')
    largest = reader(filename)
    for line in filename:
        number = reader(filename)
        if number > largest:
            largest = number

    print("The largest number is:", largest)

main()

But when I put the readline() in the main() function and change the parameter inside the loop things work fine:

def helper(filename):

    nums = filename
    return(nums)

def main():

    filename = open("numbers.txt", 'r')
    largest = helper(filename.readline())
    for line in filename:
        number = helper(line)
        if number > largest:
            largest = number

    print("The largest number is:", largest)

main()

There's obviously something that I'm missing but I don't know exactly what it is.

Answer 1

for line in filename: reads each line in the file.

Calling readline() while inside such a loop, as you do in the first code sample, will cause the for loop to miss that line, as it was consumed by readline() and is no longer available to be consumed by the for loop.

The second code sample does not have this problem, because you are no longer calling readline() inside of a for line in file: loop.

Answer 2

You are skipping all the even lines. First you read a line:

largest = reader(filename)

then you read another one

for line in filename:

note line contains now the second line. Finally, a 3d time:

nums = filename.readline()

you completely disregard the second read, as you make no use of line . This of course will continue in the next loop - read the 4th line, disregard it and get the 5th etc.

Also note you did not strip newlines (reading a line leaves a \\n ) or converting to int , so you are comparing strings.

Answer 3

def main():
    filename = open("numbers.txt", 'r')
    largest = -1
    for line in filename:
        if int(line) > int(largest):
            largest = line


    print("The largest number is:", largest)

main()