在文本文件中查找字符串，删除行和下面的行

Question

I have a bit of code to find a string in a text file, print the line the string is on and then print the 5 lines below it. However, I need to modify it so that instead of printing, it deletes/removes the line after the string is found. How would I go about doing this?

File file = new File("./output.txt");
Scanner in = null;
try {
    in = new Scanner(file);
    while (in.hasNext()) {
        String line = in.nextLine();
        if (line.contains("(1)")) {
            for (int a = 0; in.hasNextLine() && a < 6; a++) {
                System.out.println(line);
                line = in.nextLine();
            }
        }
    }
} catch (Exception e) {
}

Answer 1

Find a small snippet you can start with.

Assuming your question.txt has the following input.

line 1
line 2
line 3 (1)
line 4
line 5
line 6
line 7
line 8
line 9
line 10

This snippet will print all lines and skip the line line 3 (1) as well the five lines after.

List<String> lines = Files.readAllLines(Paths.get("question.txt"), Charset.defaultCharset());
for (int i = 0; i < lines.size(); i++) {
    if (lines.get(i).contains("(1)")) {
        i = i + 6;
    }
    System.out.println(lines.get(i));
}

output

line 1
line 2
line 9
line 10

To store the lines into the file is left for you.

Answer 2

My Suggestion is you first declare and initialise a StringBuilder say output before your above code like:

StringBuilder output = new StringBuilder();

Now after the close of the if statement before the closing of the while loop append the line to the output and add a "\\n" at the end like this:

output.append(line+"\n");

Now finally after your code that you have posted create a FileWriter say writer and then use the writer to write the output as shown below:

try(FileWriter writer = new FileWriter(file, false)){
   writer.write(output);
}catch IOException(e){
   e.printStackTrace();
}

Also don't forget to remove or comment out the following line if you do not want them printed in the output.

System.out.println(line);

Answer 3

SubOtimal has a good, concise answer that will work for most cases. The following is more complex but avoids loading the whole file into memory. That probably isn't an issue for you but just in case...

public void deleteAfter(File file, String searchString, int lineCountToDelete) {
    // Create a temporary file to write to
    File temp = new File(file.getAbsolutePath() + ".tmp");
    try (BufferedReader reader = new BufferedReader(new FileReader(file));
         PrintWriter writer = new PrintWriter(new FileWriter(temp)) ) {

        // Read up to the line we are searching for
        // and write each to the temp file
        String line;
        while((line = reader.readLine()) != null && !line.equals(searchString)){
            writer.println(line);
        }

        // Skip over the number of lines we want to "delete"
        // as well as watching out for hitting the end of the file
        for(int i=0;i < lineCountToDelete && line != null; i++){
            line = reader.readLine();
        }

        // Write the remaining lines to the temp file.
        while((line = reader.readLine()) != null){
            writer.println(line);
        }

    } catch (IOException e) {
        throw new IllegalStateException("Unable to delete the lines",e);
    }

    // Delete the original file
    if(!file.delete()){
        throw new IllegalStateException("Unable to delete file: " + file.getAbsolutePath());
    }

    // Rename the temp file to the original name
    if(!temp.renameTo(file)){
        throw new IllegalStateException("Unable to rename " +
                temp.getAbsolutePath() + " to " + file.getAbsolutePath());
    }
}

I tested this with multiple conditions, including a line that doesn't exist, a line at the end and a line with fewer lines left than the number to skip. All worked and gave the appropriate results.