My task is to write in a precise position into file.txt with C++
And because my file is static ( it will not be changed) I decided to count the position of curseur where I have to write. ( I know it isn't the best idea to di this)
This is my file and I have to write after '=' :
It is clear that I want to over-write "null;"
But I don't understand why "int main" in the otehr line is also over-writed! Have a look to the following please to undersatnd my problem:
My questions are the following:
How Can I avoid this?
Have you other idea to achieve the goal as described above ?
The is my attempt :
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
ofstream monFlux("client.txt",ios::in | ios::ate);
if(monFlux)
{
monFlux.seekp(61, ios::beg);
int position = monFlux.tellp();
monFlux<< "DECryptBlockWithPCRYPT(d);";
}
else
{
cout << "ERROR" << endl;
}
system("pause");
return 0;
}
First, it looks like you're opening an output stream for reading (ios::in) is that correct?
I don't think you can insert characters the way you describe.
Your approach is more like a human would do it in a text editor. The stream object on the other side just gives you access to the bytes on the disk. It has no "select and replace" text function.
I think this approach could work:
Open an input stream for reading the file and an output stream for writing one.
Stream the first N characters directly to the output stream.
Skip the "null" and insert your replacement string.
Stream the rest of the input file into the output stream.
On success, replace the original file with the new one.
Related
I began learning strings yesterday and wanted to manipulate it around by filling it with a text from a text file. However, upon filling it the cstring array only prints out the last word of the text file. I am a complete beginner, so I hope you can keep this beginner friendly. The lines I want to print from the file are:
"Hello World from UAE" - First line
"I like to program" - Second line
Now I did look around and eventually found a way and that is to use std::skipary or something like that but that did not print it the way I had envisioned, it prints letter by letter and skips each line in doing so.
here is my code:
#include <fstream>
#include <iostream>
#include <cstring>
#include <cctype>
using namespace std;
int main() {
ifstream myfile;
myfile.open("output.txt");
int vowels = 0, spaces = 0, upper = 0, lower = 0;
//check for error
if (myfile.fail()) {
cout << "Error opening file: ";
exit(1);
}
char statement[100];
while (!myfile.eof()) {
myfile >> statement;
}
for (int i = 0; i < 30; ++i) {
cout << statement << " ";
}
I'm not exactly sure what you try to do with output.txt's contents, but a clean way to read through a file's contents using C++ Strings goes like this:
if (std::ifstream in("output.txt"); in.good()) {
for (std::string line; std::getline(in, line); ) {
// do something with line
std::cout << line << '\n';
}
}
You wouldn't want to use char[] for that, in fact raw char arrays are hardly ever useful in modern C++.
Also - As you can see, it's much more concise to check if the stream is good than checking for std::ifstream::fail() and std::ifstream::eof(). Be optimistic! :)
Whenever you encounter output issues - either wrong or no output, the best practise is to add print (cout) statements wherever data change is occurring.
So I first modified your code as follows:
while (!myfile.eof()) {
myfile >> statement;
std::cout<<statement;
}
This way, the output I got was - all lines are printed but the last line gets printed twice.
So,
We understood that data is being read correctly and stored in statement.
This raises 2 questions. One is your question, other is why last line is printed twice.
To answer your question exactly, in every loop iteration, you're reading the text completely into statement. You're overwriting existing value. So whatever value you read last is only stored.
Once you fix that, you might come across the second question. It's very common and I myself came across that issue long back. So I'm gonna answer that as well.
Let's say your file has 3 lines:
line1
line2
line3
Initially your file control (pointer) is at the beginning, exactly where line 1 starts. After iterations when it comes to line3, we know it's last line as we input the data. But the loop control doesn't know that. For all it knows, there could be a million more lines. Only after it enters the loop condition THE NEXT TIME will it come to know that the file has ended. So the final value will be printed twice.
I know how to pass in strings from a text file. In a previous project I read in strings and then tested them on either being "t" or "f", which the result of would set a variable to true or false.
Now I am wondering if it is efficiently possible to read numbers from a text file and pass them into an int? All I can think of is checking for the string "1" and returning 1 in a function, but that would have to be done for every possible integer I could expect in my program, which is not an effective solution.
For context, I am trying to make a save system for a game, and ints/floats that are read in would be variables such as player health, how much of an item they have, etc.
If you already know how to read a string str from a text file, reading numbers is not that difficult: jsut read the string as you did and use stoi() to convert the string into an int, or stof() into float.
int i; double d;
i=stroi(str); d=strod(str2);
Another technique is to use file streams to read or write from a file exactly as you would do from cin and cout:
ifstream file("mytext.txt");
file>>i>>d;
The previous method doesn't care so much about lines. So still another technique is to read a string, convert it into a string stream and use the stringstream as you would with cin:
if (getline(file, str)){ // read a full line
stringstream sst(str);
sst>>i>>d;
}
Using std::fstream. You can open a file, and stream input or output based on how you opened the file.
Example:
#include <iostream>
#include <fstream>
int main(int argc, char** argv)
{
// Pretend we are passed the file location as a command-line argument to our program:
std::fstream file { argv[1], std::ios::in };
if (file.is_open())
{
int value;
file >> value;
std::cout << value << std::endl;
}
else
{
std::cout << "Could not open file " << argv[1] << std::endl;
}
}
Provided that the information is correctly formatted in the file, this should work.
I didn't run it, so there might be syntax errors, but the basics are there. Check out cppreference for some help, they will have further examples.
I want to edit a text file, but I'm stuck in finding the correct functions or methods to do so.
So far I'm able to open a text file and look for a certain string, but I have no idea on how to move the cursor, add or replace information, steps 4 - 7 in my pseudocode shown below.
Can you provide some guidance? Which functions should I use (in case they already exist)?
A sample 'easy' code would be appreciated as well.
Pseudocode:
1. Open file.
2. While not eof
3. Read file until string "someString" is found.
4. Position the cursor at the next line (to where the someString was found).
5. If "someString" = A go to step 6. Else go to step 7.
6. Replace the information in whole line with "newString". Go to step 8.
7. Add new information "newString_2", without deleting the existing.
8. Save and close the text file.
Thanks.
I would recommend to put the getline command into the while loop because then it won't stop only because of EOF but when getline is not able to read anymore.
Like when the error bad occurs (which happens when someone deleted the file while your program was reading it).
It seems like you want to search inside a string, so "find" might be quite helpful.
#include <iostream>
#include <fstream>
#include <string>
int main (){
std::fstream yourfile;
std::string line, someString;
yourfile.open("file.txt", ios::in | ios::app); //The path to your file goes here
if (yourfile.is_open()){ //You don't have to ask if the file is open but it's more secure
while (getline(line)){
if(line.find(someString) != string::npos){ //the find() documentation might be helpful if you don't understand
if(someString == "A"){
//code for replacing the line
}
else{
yourfile << "newString_2" << endl;
}
} //end if
} //end while
} //end if
else cerr << "Your file couldn't be opened";
yourfile.close();
return 0;
}
I can't tell you how to replace a single line in a text file but I hope you can work with that little I can give you.
This should be a good start:
// basic file operations
#include <string>
#include <fstream>
int main ()
{
std::fstream myfile;
std::string line;
while (!myfile.eof())
{
std::getline(myfile,line); // Check getline() doc, you can retrieve a line before/after a given string etc.
//if (line == something)
//{
// do stuff with line, like checking for content etc.
//}
}
myfile.close();
return 0;
}
More informations here
Is there a way to reset an ofstream back to the beginning of the stream but not write over the data? I know how to reset the stream to the beginning but then it writes over all of the old data. I realize I could just save the initial data until I'm ready to write it but it is a very large amount that I'm not sure will store very well. Another option I suppose is to create a temp file with my initial large data, write my new data in the actual file then copy the temp file over and erase it. I suppose I'm just looking for something a little more convenient then those two if possible. Changing the way I get the data isn't an option unfortunately. With this code:
#include <iostream>
#include <fstream>
using namespace std;
int main(int argc, const char * argv[])
{
ofstream out_stream("~/Desktop/test_out.txt");
out_stream <<
"This is some test file\n" <<
"I want to reset stream\n" <<
"Start working......now\n";
out_stream.seekp(0, ios_base::beg);//this sets to beginning but erases old data
out_stream <<
"I hope this workssssss\n";
out_stream.close();
return 0;
}
I would want this output:
I hope this workssssss
This is some test file
I want to reset stream
Start working......now
You seem to want to "prepend" text to the file. That's just not possible. The new text will overwrite what is already there.
You'll have to write the new text to a new file and then write the text from the original file to the new file afterwards.
Solved! thanks all of you very much. My day has been made!(well morning, its 4am)
I'm trying to write a program in C++ that opens a .dat file in binary and replaces the first 1840 hex characters with that of another .dat file, while leaving the remaining hex values of the first .dat file the same. I have spent about 12 hours on this today and have had little success. I am a beginner programmer, I have taken one semester worth of c++ courses and we did not get to streams.
(it opens a file and everything, but deletes every thing after the new values have been added)
#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <cmath>
#include <cstring>
using namespace std;
int main (){
string filename;
long size;
char* memblock;
cout << " Enter a file to be modded by Mod.dat ";
cin >> filename;
ofstream infile ( filename ,std::ofstream::binary);
//filename: the file that will be opened and changed)
ifstream modFile ("Mod.dat", ifstream::binary);
// (mod.dat is the file that i get the first 1840 hex values from)
modFile.seekg (0,modFile.end);
size = modFile.tellg();
memblock = new char [size];
modFile.seekg (0, ios::beg);
modFile.read (memblock, size);
infile.write(memblock, 1840);
modFile.close();
infile.close();
cout << endl;
return 0;
}
Any help would be greatly appreciated, I hope there is some simple way to do this.
Solved! thanks all of you very much. My day has been made!(well morning, its 4am)
Edit:
You can modidy your file in place with something like :
std::fstream s(my_file_path, std::ios_base::binary);
s.seekp(position_of_data_to_overwrite, std::ios_base::beg);
s.write(my_data, size_of_data_to_overwrite);
std::fstream will not truncate your input file as std::ofstream does.
The other solution is to not use the same file for reading and writing. Use three files :
One for the output file.
One for the First input file.
One for the second input file.
fstream infile ( filename ,std::ofstream::binary); does not keeps the contents of the original file. Everything you write will erase the contents of the file.
Thus, you should:
open the output file
open the "Mod" file, read the first 1840 bytes from the first file, write them into the output file.
open the "main input file" file, move the cursor to 1840, read the remaining data and write it to the output file.
Depending on the "main input file" size, you may want to buffer you read/write operation.
My preferred fix, although Matthieu Rouget's fix does indeed work, is to just add ofstreeam::in to the opening of the input file:
ofstream infile ( filename.c_str(), std::ofstream::binary | ofstream::in);
(I had to use c_str() in my build, as glibc in my version doesn't take std::string as input).
I tested this on my local system (it took a while to realize that mod.dat is actually "Mod.dat"!)
It is probably a good idea to also check that the files actually opened, so something like this after ofstream infile line:
if (!infile)
{
cout << "Couldn't open " << filename << endl;
}
and similar for the modfile line.
And since you go through the effort of figuring out what the first part of the modfile size is, I would suggest that you also USE that for the writing of the file.