While in python does not print all the rows

Question

Dear I want display the three rows contained in txt file but I don't know why the following code does not works. The code is

f=open("dati.txt","r")
riga=f.readline()
while riga!="":
    print(f.readline())
    riga=f.readline()
f.close()

Answer 1

because you are reading two lines in a loop. The readline moves the cursor one down each time you call it. So what happens there with the second call of readline() you actually skip it(in the print log)

Also checking for end of file should not be done on empty string, because you may hit an empty line before the end of the file. Try this instead:

with open('somefile') as openfileobject:
    for line in openfileobject:
        do_something()

and or/check this thread(where I copied the snippet from): What is the perfect counterpart in Python for "while not EOF"

Answer 2

The reason why your program is not printing all the rows in the file, but rather only every even numbered row, is because you use f.readline() multiple times in the while statement.

f=open("dati.txt","r")
riga=f.readline() # This line means that the variable "riga" contains the first line in your file
while riga!="":
    print(f.readline()) # here you do f.readline() again, which means that what you are printing is the second line
    riga=f.readline() # This line reads in the third line into the "riga" variable.
f.close()

What I think you are looking for, is to print the contents of the riga variable instead, like this

while riga != "":
    print(riga)
    riga = f.readline()

I should also mention that tstoev's answer also has a good approach at printing each line in a file. It does solve the same problem, but it does not point out why your code doesn't work.

Answer 3

Your code reads three lines and prints only one:

f=open("dati.txt","r")
riga=f.readline()          # Reads a line into riga
while riga!="":
    print(f.readline())    # Reads a line and prints it
    riga=f.readline()      # Reads a line into riga
f.close()

So the problem seems that you read lines into riga, but never print riga.