C++ how to remove all chars and special characters from a file - c++

Question

I have seen how to remove specific chars from a string but I am not sure how to do it with a file open or if you can even do that. Basically a file will be open with anything in it, my goal is to remove all the letters a-z, special characters, and whitespace that may appear so that all that is left is my numbers. Can you easily remove all chars rather than specifying a,b,c etc when the file is open or would I have to convert it to a string? Also would it be better to do this in memory?
My code this far as is follows:
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main() {
string filename;
cout << "Enter the name of the data file to open" << endl;
cin >> filename >> endl;
ofstream myfile;
myfile.open(filename);
if (myfile.is_open()) { //if file is open then
while(!myfile.eof()){ //while not end of file
//remove all chars, special and whitespace
}
}
else{
cout << "Error in opening file" << endl;
}
return 0;
}

Answer 1

Preliminary remarks
If I understand well, you want to keep only the numbers. Maybe it's easier to retain chars that are ascii numbers and eliminate the others rather than eliminate a lot of other chars classes and hope that the remainder is only numbers.
Also never loop on eof to read a file. Loop on the stream instead.
finally, you should read from an ifstream and write to an ofstream
First approach: reading strings
You can read/write the file line by line. You need enough memory to store the largest line, but you benefit from buffering effect.
if (myfile.is_open()) { //if file is open then
string line;
while(getline(myfile, line)){ //while succesful read
line.erase(remove_if(line.begin(), line.end(), [](const char& c) { return !isdigit(c); } ), line.end());
... // then write the line in the output file
}
}
else ...
Online demo
Second approach: reading chars
You can read/write char by char, which gives very flexible option for handling individual characters (toggle string flags, etc...). You also benefit from buffering, but you have function call overhaead for every single char.
if (myfile) { //if file is open then
int c;
while((c = myfile.get())!=EOF){ //while succesful read
//remove all chars, special and whitespace
if (isdigit(c) || c=='\n')
... .put(c); // then write the line in the output file
}
}
else ...
Online demo
Other approaches
You could also read a large fixed size buffer, and operate similarly as with the strings (but don't eliminate LF then). The advantage is that the memory need is not impacted by some very large lines in the file.
You could also determine the file size, and try to read the full file at once (or in very large chunks). You'd then maximize performance at the cost of memory consumption.

Answer 2

This is just an example in order to extract all chars you want from a file with a dedicated filter:
std::string get_purged_file(const std::string& filename) {
std::string strbuffer;
std::ifstream infile;
infile.open(filename, std::ios_base::in);
if (infile.fail()) {
// throw an error
}
char c;
while ((infile >> c).eof() == false) {
if (std::isdigit(c) || c == '.') {
strbuffer.push_back(c);
}
}
infile.close();
return strbuffer;
}
Note: this is just an example and it has to be subject to optimizations. Just to give you an idea:
Read more than one char at time, (with a proper buffer).
Reserve memory in string.
Once you have the buffer "purged" you can overwrite your file on save the content into another file.