I want to return a list of length of lines in a file, but my code isn't working-CodePudding

I want to return the length on each line as an element in a list named lst but my code is not working, the output always comes to be an empty list. Please tell me what's wrong with my code.

# this is the file

f = open("abcd.txt", 'w')
f.write("Hello How Ar3 you?")
f.write("\nHope You're doing fine")
f.write("\nI'm doing okay too.")
f.write("\nSizole!")
f.close()

This is the code I wrote to return a list of length of lines in the file:

f = open("abcd.txt", 'r')
t = f.readlines()
print(t)
lst = []
for line in f.readlines():
    lst.append(len(line))
print(lst)

Output: lst == []

CodePudding user response：

Just make it simple by reading the line once and do the length count. below code is used list comprehension.

texts = f.readlines()
lst = [len(line) for line in texts]
print(lst)

Here's the output of the above code. Hope this helps and most of them had given the correct answers.

[19, 23, 20, 7]

CodePudding user response：

When you read the file back in the second code snippet, the:

t = f.readlines()
...

reads the entire file in to list and assigns it to the variable t.

You then try to read all the lines again with the:

for line in f.readlines():
    ...

Which will not work because they have all been read already.

To fix it, just change the for loop to this:

for line in t:

CodePudding user response：

You don't need to read the lines before the loop (that is the line t = ... is unnecessary).

In fact doing so is likely causing the problem - once you read the lines, the file pointer is at the end of the file so there's nothing left to read.

CodePudding user response：

In your code, you are calling f.readlines() twice. You just need to call it once:

f = open("abcd.txt", 'r')
t = f.readlines()
print(t)
lst = []
# instead of for line in f.readlines(), we can simply use t
for line in t:
    lst.append(len(line))
print(lst)

or if the variable t is not necessary:

f = open("abcd.txt", 'r')
lst = []
for line in f.readlines():
    lst.append(len(line))
print(lst)

f.readlines() will move the file pointer to the end of file. Calling it again will return an empty list, which is not what we want.