Remove all the lines that contain the character ‘a’ in a file and write them to another file program in Python, C and Java

What You Will Learn

• The problem statement
• The idea: read, check, write
• The program in Python, C and Java
• Dry run with an example file
• Other ways: ignore case, with open, list comprehension, user-chosen character and more
• Common mistakes, practice questions and FAQ

Problem Statement

Remove all the lines that contain the character ‘a’ in a file and write it to another file.

Suppose story.txt contains these six lines:

Welcome to my website
Learn with fun
Study hard
Apple juice is sweet
Python is easy
Enjoy coding

After the program runs, the new file new.txt must contain only the lines without the letter a:

Welcome to my website
Apple juice is sweet
Enjoy coding

The original file story.txt is not changed. The interactive tool above lets you type up to six lines instead of creating real files.

The Idea: Read, Check, Write

The word “remove” can be confusing, because the program does not delete anything from the original file. It copies only the lines you want to keep. A line is kept when it does not contain the character, so the result is a file without the unwanted lines. The program needs two files open at the same time:

• An input file opened in read mode "r".
• An output file opened in write mode "w". If new.txt does not exist, Python creates it. If it already exists, its old content is erased.

For every line of the input file there are two decisions: "a" not in line is checked, and if it is True, the line is written to the output file with fout.write(line). A line read from a file already ends with a new line character (\n), so writing it back keeps the line structure.

The test "a" in line asks whether the character appears anywhere in the line. It works for a character or a longer piece of text and it is case-sensitive, so a capital A does not count.

The Program

Logic:

• Open story.txt for reading and new.txt for writing.
• Read the input file line by line.
• If the line does not contain “a”, write it to the output file.
• Close both files, so that everything is saved.

Python program

fin = open("story.txt", "r")
fout = open("new.txt", "w")
for line in fin:
    if "a" not in line:
        fout.write(line)
fin.close()
fout.close()

After the program runs, new.txt contains:

Welcome to my website
Apple juice is sweet
Enjoy coding

#include <stdio.h>
#include <string.h>
int main() {
    FILE *fin = fopen("story.txt", "r");
    FILE *fout = fopen("new.txt", "w");
    char line[200];
    while (fgets(line, 200, fin) != NULL) {
        if (strchr(line, 'a') == NULL) {
            fputs(line, fout);
        }
    }
    fclose(fin);
    fclose(fout);
    return 0;
}

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
public class RemoveLines {
    public static void main(String[] args) throws IOException {
        BufferedReader fin = new BufferedReader(new FileReader("story.txt"));
        BufferedWriter fout = new BufferedWriter(new FileWriter("new.txt"));
        String line;
        while ((line = fin.readLine()) != null) {
            if (!line.contains("a")) {
                fout.write(line);
                fout.newLine();
            }
        }
        fin.close();
        fout.close();
    }
}

Dry Run

The program checks each line of the example file:

Only the lines without a are copied, so new.txt has 3 lines and 3 lines have been removed. Look at line 4, Apple juice is sweet. It has a capital A but no lowercase a, so it is copied. That is correct for the question as it is written, because ‘a’ and ‘A’ are different characters.

Other Ways to Write the Program

Method 2: Ignore the case

If “Apple” should count as well, convert a copy of the line to lowercase before the check. The original line is written unchanged.

fin = open("story.txt", "r")
fout = open("new.txt", "w")
for line in fin:
    if "a" not in line.lower():
        fout.write(line)
fin.close()
fout.close()

Now new.txt contains:

Welcome to my website
Enjoy coding

Method 3: with open() closes the files for you

One with statement can open both files. They are closed automatically at the end of the block, even if an error happens.

with open("story.txt", "r") as fin, open("new.txt", "w") as fout:
    for line in fin:
        if "a" not in line:
            fout.write(line)

Method 4: List comprehension and writelines()

Read all lines into a list, build a new list of the lines to keep, and write the whole list at once. This is short, but it keeps the whole file in memory.

with open("story.txt", "r") as fin:
    lines = fin.readlines()
keep = [line for line in lines if "a" not in line]
with open("new.txt", "w") as fout:
    fout.writelines(keep)
print(len(lines) - len(keep), "lines removed")

3 lines removed

Method 5: Count the copied and removed lines

Two counters report what happened. The program then reads new.txt back to show it.

removed = 0
copied = 0
with open("story.txt", "r") as fin, open("new.txt", "w") as fout:
    for line in fin:
        if "a" not in line:
            fout.write(line)
            copied = copied + 1
        else:
            removed = removed + 1
print("Lines copied:", copied)
print("Lines removed:", removed)
with open("new.txt", "r") as f:
    print(f.read())

Lines copied: 3
Lines removed: 3
Welcome to my website
Apple juice is sweet
Enjoy coding

Method 6: Let the user choose the character

Replace the fixed “a” with a variable, so the same program removes any character or even a word.

ch = input("Enter the character to remove: ")
with open("story.txt", "r") as fin, open("new.txt", "w") as fout:
    for line in fin:
        if ch not in line:
            fout.write(line)

For the input e, the new file contains:

Study hard
Enjoy coding

Method 7: Keep the result in the same file

To change the original file itself, read all its lines first, and only then open it again in write mode. Opening it with "w" before reading would erase it.

with open("story.txt", "r") as f:
    lines = f.readlines()
with open("story.txt", "w") as f:
    for line in lines:
        if "a" not in line:
            f.write(line)

Method 8: The opposite: keep only the lines with a

Remove the word not, and the program keeps the lines that contain a:

fin = open("story.txt", "r")
fout = open("new.txt", "w")
for line in fin:
    if "a" in line:
        fout.write(line)
fin.close()
fout.close()

Learn with fun
Study hard
Python is easy

Method 9: Handle a missing file

try:
    fin = open("missing.txt", "r")
    fout = open("new.txt", "w")
    for line in fin:
        if "a" not in line:
            fout.write(line)
    fin.close()
    fout.close()
except FileNotFoundError:
    print("File not found. Check the file name and folder.")

File not found. Check the file name and folder.

#include <stdio.h>
#include <string.h>
int main() {
    FILE *fin = fopen("story.txt", "r");
    FILE *fout = fopen("new.txt", "w");
    char line[200];
    while (fgets(line, 200, fin) != NULL) {
        if (strchr(line, 'a') == NULL && strchr(line, 'A') == NULL) {
            fputs(line, fout);
        }
    }
    fclose(fin);
    fclose(fout);
    return 0;
}

#include <stdio.h>
#include <string.h>
int main() {
    FILE *fin = fopen("story.txt", "r");
    FILE *fout = fopen("new.txt", "w");
    if (fin == NULL || fout == NULL) {
        printf("File error\n");
        return 1;
    }
    char line[200];
    while (fgets(line, 200, fin) != NULL) {
        if (strchr(line, 'a') == NULL) {
            fputs(line, fout);
        }
    }
    fclose(fin);
    fclose(fout);
    return 0;
}

import java.io.File;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Scanner;
public class RemoveLinesScanner {
    public static void main(String[] args) throws IOException {
        Scanner sc = new Scanner(new File("story.txt"));
        PrintWriter pw = new PrintWriter("new.txt");
        while (sc.hasNextLine()) {
            String line = sc.nextLine();
            if (line.indexOf('a') == -1) {
                pw.println(line);
            }
        }
        sc.close();
        pw.close();
    }
}

Common Mistakes

• Using strip() when writing. strip() removes the new line at the end of the line, so the lines run together in the new file:

fin = open("story.txt", "r")
fout = open("new.txt", "w")
for line in fin:
    if "a" not in line:
        fout.write(line.strip())
fin.close()
fout.close()

Welcome to my websiteApple juice is sweetEnjoy coding

• Reversing the test. Writing if "a" in line keeps the lines that should be removed, which is Method 8. Read the problem statement carefully: remove the lines with a means keep the lines without a.
• Opening the same file for reading and writing. Mode "w" erases the file at the moment it is opened. This program ends with an empty story.txt and no output at all:

fin = open("story.txt", "r")
fout = open("story.txt", "w")
for line in fin:
    if "a" not in line:
        fout.write(line)
fin.close()
fout.close()

After this program runs, story.txt is empty, and the original text is lost. Always write to a different file, or read the lines into a list before you open the file for writing.

• Forgetting to close the output file. Written text waits in a buffer and may not reach the disk until the file is closed. Call fout.close() or use with open().
• Forgetting that ‘a’ and ‘A’ are different. A plain "a" in line test does not catch a capital A. Use line.lower() if both should count.
• Opening the output with “r”. Read mode cannot write, and it fails if the file does not exist. Use "w" for a new file or "a" to add to an existing one.
• Wrong file location. A relative file name is searched in the folder from which the program runs. If it is somewhere else, you get a FileNotFoundError (or NULL from fopen() in C).

Practice Questions

1. Remove the lines that contain the word “the” and write them to another file.
2. Write the lines that contain ‘a’ to one file and the other lines to a second file.
3. Remove the lines that start with a vowel. Hint: line[0] in “aeiouAEIOU”.
4. Remove the empty lines of a file. Hint: line.strip() == “”.
5. Remove the lines that are longer than 20 characters.
6. Copy the file and put the line number before every kept line.
7. Display the number of lines of the input file and of the output file.

Frequently Asked Questions

How do you remove lines containing a character from a file in Python?

Open the input file in read mode and a second file in write mode. Loop over the input lines, and write a line to the output file only when "a" not in line. The lines with the character are never written, so they are missing from the new file.

Does the program delete the lines from the original file?

No. It copies the wanted lines into a new file, and the original file stays unchanged. If you want to change the original, read its lines into a list first and write the kept lines back (Method 7).

What does the test “a” in line do?

It checks whether the text "a" appears anywhere inside the string line and returns True or False. It is case-sensitive.

How do I remove lines with both a and A?

Convert a copy of the line to lowercase before the test, as in if "a" not in line.lower():. The line itself is written unchanged.

What is the difference between the file modes “r”, “w” and “a”?

"r" reads an existing file, "w" creates a file or erases an existing one and writes, and "a" adds text at the end without erasing. For the output file of this program use "w".

Why must I close the files?

Closing a file saves any buffered text and frees the file for other programs. If you forget fout.close(), some of the written lines may be missing from the file. The with open() statement closes the files for you.

Why are all the lines joined in my new file?

Each line read from a file ends with a new line character. If you remove it with strip() or rstrip() before writing, the lines are written one after another. Write the line as it is, or add "\\n" yourself.

What happens if no line contains the character?

All lines are copied, and new.txt is an exact copy of the input file. If every line contains the character, new.txt is created but stays empty.

How do I do this in C?

Open both files with fopen(), read each line with fgets(), test it with strchr(line, 'a') == NULL, write the kept lines with fputs(), and close the files with fclose(). Check that fopen() did not return NULL.

How do I do this in Java?

Use a BufferedReader to read lines and a BufferedWriter to write them. Keep a line when !line.contains("a"), call newLine() after each write(), and close both files. Declare throws IOException on main.