Question

Write a program to prompt the user for a file name and a line number and then print only that line from the file. Consider possible difficulties and include code to address them. [Hints: it is important to clearly define what you think “line” means. Also, there are a number of errors to consider trapping.]

Answer 1

ANS)

For example let us consider my program that allows the user to navigate through the lines of text in any text file. The program prompts the user for a filename and copies the lines of text from the file into a list. The program then prints the number of lines in the file and prompts the user for a line number. Actual line numbers range from 1 to the number of lines in the file. If the input is 0, the program quits. Otherwise, the program prints the text in that line number.

code
enterfile = input("Enter the file name: ")
file = open(enterfile, 'r')
linecount = 0

for line in file:
linecount = linecount + 1

print("The number of lines in this txt. file is", linecount)

while True:
linenum = 0

num = int(input("Please enter a line number or press 0 to quit: "))
if num >=1 and num <= linecount:
file = open(enterfile, 'r')
for lines in file:
linenum = linenum + 1
if linenum == num:
print(lines)
else:
if num == 0:
print("Thanks for using the program")
break
Move line linenum = 0 inside the While True: loop.

The linenum variable must be reset to 0 (linenum = 0) when the program re-enters the loop. Otherwise the linenum variable will always keep being incremented and have a value that is greater than num and will never trigger the if statement to print the line at that number.

Alternative method:

enterfile = input("Enter the file name: ")

with open(enterfile) as f:
lines = [line.rstrip() for line in f]

print("The number of lines in this txt. file is", len(lines))

while True:
num = int(input("Please enter a line number or press 0 to quit: "))

if num > 0 and num < len(lines) + 1:
print(lines[num - 1])
elif num == 0:
print('Thanks for using the program.')