Replace a line in text file

Question

I want to replace a line of text in a file, but I don't know a functions to do this.

I have this:

ofstream outfile("text.txt");
ifstream infile("text.txt");

infile >> replace with other text;

Any answers for this?

I miss to say, for add text in Some line in the file...

Example

infile.add(text, line);

Does C++ have functions for this?

See if this helps: http://stackoverflow.com/questions/4499095/replace-line-in-a-file-c — Pramod, Feb 29 '12 at 19:08
Is this homework? If so please tag it, otherwise I would suggest a scripting language like perl or python, or just using `sed`. — Antonio Pérez, Feb 29 '12 at 19:10
@AntonioPérez Yeah, SDL, Engines, Games, its a requeried for the Game developed in the industry — Warkanlock, Feb 29 '12 at 19:39

Anton · Answer 1 · 2012-02-29T19:44:45.873

I'm afraid you'll probably have to rewrite the entire file. Here is how you could do it:

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    string strReplace = "HELLO";
    string strNew = "GOODBYE";
    ifstream filein("filein.txt"); //File to read from
    ofstream fileout("fileout.txt"); //Temporary file
    if(!filein || !fileout)
    {
        cout << "Error opening files!" << endl;
        return 1;
    }

    string strTemp;
    //bool found = false;
    while(filein >> strTemp)
    {
        if(strTemp == strReplace){
            strTemp = strNew;
            //found = true;
        }
        strTemp += "\n";
        fileout << strTemp;
        //if(found) break;
    }
    return 0;
}

Input-file:

ONE
TWO
THREE
HELLO
SEVEN

Output-file:

ONE
TWO
THREE
GOODBYE
SEVEN

Just uncomment the commented lines if you only want it to replace the first occurance. Also, I forgot, in the end add code that deletes filein.txt and renames fileout.txt to filein.txt.

You can avoid a potential allocation (and save a line) by replacing `strTemp += "\n"; fileout << strTemp;` with `fileout << strTemp << '\n';`. — ildjarn, Jan 23 '17 at 19:54

score 3 · Answer 2 · answered Feb 29 '12 at 19:08

3

You need to seek to the correct line/char/position in the file and then over-write. There is no function to search and replace as such (that I know of).

answered Feb 29 '12 at 19:08

Kashyap

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Mmm yes, seekg and seekp infile.seekg(20*i, ios::beg); infile.read(cad, 20); No? – Warkanlock Feb 29 '12 at 19:09
12

This won't work if the replacement text is not exactly the same size in bytes as what it replaces. – zwol Feb 29 '12 at 19:10

score 3 · Answer 3 · answered Feb 29 '12 at 19:11

3

The only way to replace text in a file, or add lines in the middle of a file, is to rewrite the entire file from the point of the first modification. You cannot "make space" in the middle of a file for new lines.

The reliable way to do this is to copy the file's contents to a new file, making the modifications as you go, and then use rename to overwrite the old file with the new one.

answered Feb 29 '12 at 19:11

zwol

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1

To follow the strategy to create a modified copy and then rename it with original name it is noteworthy to copy the permissions and file attributes as well. – Duloren Mar 07 '21 at 00:31

Replace a line in text file

3 Answers3

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