If the first 3 characters are the same, delete the line in a text file?

Question

Goal: Open the text file. Check whether the first 3 characters of each line are the same in subsequent lines. If yes, delete the bottom one.

The contents of the text file:

cat1
dog4
cat3
fish
dog8

Desired output:

cat1
dog4
fish

Attempt at code:

line = open("text.txt", "r")
for num in line.readlines(): 
    a = line[num][0:3] #getting first 3 characters

    for num2 in line.readlines():
        b = line[num2][0:3]
        
        if a in b:
            line[num2] = ""

Answer 1

Open the file and read one line at a time. Note the first 3 characters (prefix). Check if the prefix has been previously observed. If not, keep that line and add the prefix to a set. For example:

with open('text.txt') as infile:
    out_lines = []
    prefixes = set()
    for line in map(str.strip, infile):
        if not (prefix := line[:3]) in prefixes:
            out_lines.append(line)
            prefixes.add(prefix)
    print(out_lines)

Output:

['cat1', 'dog4', 'fish']

Note:

Requires Python 3.8+

Answer 2

You can use a dictionary to store the first 3 char and then check while reading. Sample check then code below

line = open("text.txt", "r")
first_three_char_dict = {}
for num in line.readlines():
    a = line[num][0:3]  # getting first 3 characters

    if first_three_char_dict.get(a):
        line[num] = ""
    else:
        first_three_char_dict[a] = num
        pass;

Answer 3

try to read line and add the word (first 3 char)into a dict. The key of dict would be the first 3 char of word and value would be the word itself. At the end you will have dict keys which are unique and their values are your desired result.

Answer 4

You just need to check if the data is already exist or not in temporary list

line = open("text.txt", "r")
result = []
for num in line.readlines():
    data = line[num][0:3]  # getting first 3 characters

    if data not in result: # check if the data is already exist in list or not
        result.append(data) # if the data is not exist in list just append it