How to search an index with a variable?

Question

How do I set a user input variable that will print the value of a corresponding index number?

This is the code, please help because I have been texting and emailing my professor for 2 days and and haven't received a reply.

fileName = input("Enter the name of the file, e.g. 'numbers.txt': ")

#Coutning number of lines
lineCount=0
with open(fileName, 'r') as userData:
    for i in userData:
        lineCount=lineCount+1

#converting index count to match linecount
blankIndex = [None]
userList = [blankIndex] + [userData]

print(lineCount)

while True:
    inputTarget = int(input("Enter the number of the line you wish to see, or press enter twice to exit: "))
    if inputTarget == "":
        print("Enjoy your data.  Goodbye!")
        break
    elif inputTarget == 0:
        print("Great, you broke it...*slow clap*")
        break
    else:
        print(userList[inputTarget])

Its just that with questions it is customary to write a sentence followed by a question mark. — quamrana, Jun 16 '21 at 19:41
Please give a example of the file, a example input and the expected output. You want to see content of one file, you do not save the file content at any moment — azro, Jun 16 '21 at 19:42
[How to read specific line from file python](https://stackoverflow.com/questions/2081836/how-to-read-specific-lines-from-a-file-by-line-number) — Yoshikage Kira, Jun 16 '21 at 19:43
@azro Example of how it would work, you have a plaintext file, it imports the lines from that file and converts to a list. A input variable is used to pick a number of the line you wish to see, and then Python prints the value of that index. — Kyle Campbell, Jun 16 '21 at 19:51
Yeah page didnt update until I finished writing the comment, but my kid pulled me away from the computer for a minute lol. Thank you! — Kyle Campbell, Jun 16 '21 at 19:52

score 0 · Answer 1 · answered Jun 16 '21 at 19:44

The stuff in your while loop looks mostly good. The problem is, you're not populating the list in the first place!

fileName = input("Enter the name of the file, e.g. 'numbers.txt': ")

# we don't need to know the number of lines
# we can just open the file and put it into a list of lines with `.readlines()`
with open(fileName, 'r') as f:
    userList = [None] + f.readlines()
    # alternatively, the following commented-out code is a long-form way to accomplish essentially the same thing:
    # userList = []
    # for line in f:
    #    userList.append(line)

Using this approach, we can now treat userList as what it is - a list of strings, which corresponds exactly with the line in the file (to make it 1-indexed instead of 0-indexed, we added None as the first element, like you're trying to do in your snippet). Given an index, we can take out the string at that index.

while True:
    try:
        inputTarget = int(input("Enter the number of the line you wish to see, or press enter twice to exit: "))
    except ValueError: # this is the correct way to handle not-a-number inputs  print("Enjoy your data. Goodbye!")
        break
    if inputTarget == 0:
        print("Great, you broke it...*slow clap*")
        break
    else:
        print(userList[inputTarget])

azro · Answer 2 · 2021-06-16T20:07:41.417

0

While reading the file, you may store the lines, to be able to access them later. Here's code with some adjustement on the if/else conditions

userList = []
with open(fileName, 'r') as userData:
    for i in userData:
        userList.append(i.rstrip())

while True:
    inputTarget = input("Enter the number of the line you wish to see, or press enter twice to exit: ")
    if inputTarget == "":
        print("Enjoy your data.  Goodbye!")
        break
    indexTarget = int(inputTarget)
    if 0 > indexTarget or indexTarget >= len(userList):
        print("Great, you broke it...*slow clap*")
        break
    else:
        print(userList[indexTarget])

To type 1 for first line, add a -1

indexTarget = int(inputTarget) - 1

edited Jun 16 '21 at 20:07

answered Jun 16 '21 at 19:45

azro

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Had to make a small edit so that the input number matched the index number (the old 0, 1 nonsense) but otherwise this worked great. Thank you so much. – Kyle Campbell Jun 16 '21 at 20:02
@KyleCampbell Have you tried a `-1` ? see my edit. You shouldn't expect alll the answer from someone else, try by yourself ;) – azro Jun 16 '21 at 20:08

How to search an index with a variable?

2 Answers2