howto: Read input and store it in another file - c++

Question

I want to make a program that reads the highest value from one file and stores it in another. I've read about ifstream and ofstream but how do I let the ofstream store the highest value from the instream in another file? Here is what I have so far:
#include <iostream>
#include <fstream>
#include <string>
#include <algorithm>
#include <iterator>
#include <vector>
using namespace std;
struct CsvWhitespace : ctype<char> {
static const mask* make_table() {
static vector<mask> v{classic_table(), classic_table() + table_size};
v[','] |= space; // comma will be classified as whitespace
return v.data();
}
CsvWhitespace(size_t refs = 0) : ctype{make_table(), false, refs} {}
} csvWhitespace;
int main() {
string line;
ifstream myfile ("C:/Users/Username/Desktop/log.csv");
ofstream myfile2 ("C:/Users/Username/Desktop/log2.csv");
return 0;
}
auto v = vector<int>{};
myfile.imbue(locale{myfile.getloc(), &csvWhitespace});
copy(istream_iterator<int>{myfile}, istream_iterator<int>{}, back_inserter(v));
myfile2 << *max_element(begin(v), end(v));
}
Thanks in advance :)

Answer 1

You could just copy from the one file in the other, without having to worry about the format, by treating them in binary mode. Here is an example:
#include <stdio.h>
#include <string.h>
#define bufSize 1024
int main(int argc, char *argv[])
{
FILE *ifp, *ofp;
char buf[bufSize];
if (argc != 3)
{
fprintf(stderr,
"Usage: %s <soure-file> <target-file>\n", argv[0]);
return 1;
}
if ((ifp = fopen(argv[1], "rb")) == NULL)
{ /* Open source file. */
perror("fopen source-file");
return 1;
}
if ((ofp = fopen(argv[2], "wb")) == NULL)
{ /* Open target file. */
perror("fopen target-file");
return 1;
}
while (fgets(buf, sizeof(buf), ifp) != NULL)
{ /* While we don't reach the end of source. */
/* Read characters from source file to fill buffer. */
/* Write characters read to target file. */
fwrite(buf, sizeof(char), strlen(buf), ofp);
}
fclose(ifp);
fclose(ofp);
return 0;
}
which was given as an example in IP, source. You just need to specify the cmd arguments as the desired files.

Answer 2

You can do it like this. Live example using cin and cout rather than files.
#include <iostream>
#include <fstream>
#include <string>
#include <algorithm>
#include <iterator>
#include <vector>
using namespace std;
struct CsvWhitespace : ctype<char> {
static const mask* make_table() {
static vector<mask> v{classic_table(), classic_table() + table_size};
v[','] |= space; // comma will be classified as whitespace
return v.data();
}
CsvWhitespace(size_t refs = 0) : ctype{make_table(), false, refs} {}
};
int main() {
string line;
ifstream myfile("log.csv");
ofstream myfile2("log2.csv");
auto v = vector<int>{};
myfile.imbue(locale{myfile.getloc(), new CsvWhitespace{}});
copy(istream_iterator<int>{myfile}, istream_iterator<int>{}, back_inserter(v));
myfile2 << *max_element(begin(v), end(v));
}