C++ delete last character in a txt file

Question

I need some help on deleting the last character in a txt file. For example, if my txt file contains 1234567, I need the C++ code to delete the last character so that the file becomes 123456. Thanks guys.

what have you tried? show your code.We can help you to solve a specific problem or understand things which are strange to you but we won't write code instead of you! — shift66, Mar 02 '12 at 06:32
If the file is small enough to that you can read it into a buffer then why not do all the data manipulation you need to before you save it? — Edward Severinsen, Sep 17 '18 at 03:37
Possible duplicate of [Removing Characters from a File](https://stackoverflow.com/questions/5503863/removing-characters-from-a-file) — mt3d, May 08 '19 at 17:57

score 5 · Answer 1 · answered Mar 02 '12 at 06:34

The only way to do this in portable code is to read in the data, and write out all but the last character.

If you don't mind non-portable code, most systems provide ways to truncate a file. The traditional Unix method is to seek to the place you want the file to end, and then do a write of 0 bytes to the file at that point. On Windows, you can use SetEndOfFile. Other systems will use different names and/or methods, but nearly all will have the capability in some form.

score 5 · Answer 2 · edited Jun 06 '18 at 17:22

For a portable solution, something along these lines should do the job:

#include <fstream>

int main(){
    std::ifstream fileIn( "file.txt" );              // Open for reading
    std::string contents;
    fileIn >> contents;                              // Store contents in a std::string
    fileIn.close();
    contents.pop_back();                             // Remove last character
    std::ofstream fileOut( "file.txt", std::ios::trunc ); // Open for writing (while also clearing file)
    fileOut << contents;                             // Output contents with removed character
    fileOut.close();

    return 0;
}

Ð.. · Answer 3 · 2018-08-03T18:42:30.657

Here's a more robust method, going off Alex Z's answer:

#include <fstream>
#include <string>
#include <sstream>

int main(){
    std::ifstream fileIn( "file.txt" );                   // Open for reading

    std::stringstream buffer;                             // Store contents in a std::string
    buffer << fileIn.rdbuf();
    std::string contents = buffer.str();

    fileIn.close();
    contents.pop_back();                                  // Remove last character


    std::ofstream fileOut( "file.txt" , std::ios::trunc); // Open for writing (while also clearing file)
    fileOut << contents;                                  // Output contents with removed character
    fileOut.close(); 
}

The trick is these lines, which allow you to read the entire file efficiently into the string, and not just a token:

    std::stringstream buffer;
    buffer << fileIn.rdbuf();
    std::string contents = buffer.str();

This is inspired from Jerry Coffin's first solution in this post. It is supposed to be the fastest solution there.

score 1 · Answer 4 · edited Jul 18 '18 at 19:52

1

If the input file is not too large, you can do the following:-

1. Read the contents into a character array.
2. Truncate the original file.
3. Write the character array back to the file, except the last character.

If the file is too large, you can possibly use a temporary file instead of a character array. It will be a bit slow though.

edited Jul 18 '18 at 19:52

Ð..

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answered Mar 02 '12 at 06:36

vaisakh

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C++ delete last character in a txt file

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