formatted output: columns - c++

Question

I am desperately trying to produce formatted output by using fstream: columns.
There are two functions (whatever functions Func1, Func2) which produce output (write to the same file, "example.dat"):
#include <fstream>
int main()
{
std::ofstream fout;
fout.open("example.dat");
for (int i=0; i<10; i++)
{
fout << Func1(i) << std::endl;
};
// Func2 depends on Func1, thus them **cannot** be written at the same time:
// fout << Func1() << " " << Func2() << std::endl;
for (int i=0; i<10; i++)
{
fout << Func2(i) << std::endl;
};
return 0;
}
The output will similar to:
Func1(0)
Func1(1)
.
.
.
Func1(9)
Func2(0)
Func2(1)
.
.
.
Func2(9)
My question is: How to produce this output as two columns:
Func1(0) Func2(0)
Func1(1) Func2(1)
.
.
.
While them are not simultaneously written.
I suspect that I need to use seekp(), tellp(), but unfortunately I am not a big expert in this.
Please help!
Thank you in advance.

Answer 1

vector<ostringstream> streams(10);
for (int i=0; i < 10; ++i)
{
streams[i] << Func1(i);
}
for (int i=0; i < 10; ++i)
{
streams[i] << " " << Func2(i);
}
ofstream fout("example.dat");
for (int i=0; i < 10; ++i)
{
fout << streams[i].str() << endl;
}

Answer 2

Why would you need seekp and tellp (and writing to the middle of a file is full of pitfalls for the unwary in any case)
Use a std::vector (syntax is probably wrong), thus:
std::vector<std::string> res1;
std::vector<std::string> res2;
res1.reserve(10);
res2.reserve(10);
for (std::size_t i = 0; i < 10; ++i)
{
std::stringstream s;
s << func1(i);
res1[i] = s.str();
}
//same for res2, func2
for (std::size_t i = 0; i < 10; ++i)
{
cout << res1[i] << " " << res2[i] << "\n";
}
That's probably the wrong formatting and bits will need tuning. And std::list might work better.

Answer 3

Assuming you really cannot keep them in memory, you could write to two files, seek back to the start, read them back in line by line and output to your final file to create your concatenated columns.
Something like this might work (I've not tested it):
#include <fstream>
#include <string>
int main()
{
std::fstream fs_func1;
std::fstream fs_func2;
std::ofstream fout;
fs_func1.open("func1.dat")
for (int i=0;i<10;i++)
{
fs_func1 << Func1(i) << std::endl;
}
fs_func1.seekg(0); // Set get pointer to start of file
fs_func2.open("func2.dat");
for (int i=0;i<10;i++)
{
fs_func2 << Func2(i) << std::endl;
};
fs_func2.seekg(0); // Set get pointer to start of file
fout.open("example.dat");
while (!fs_func1.eof() && !fs_func2.eof())
{
std::string func1_line;
std::string func2_line;
std::getline(fs_func1, func1_line);
std::getline(fs_func2, func2_line);
fout << func1_line << " " << func2_line << std::endl;
}
return 0;
}
There is probably a Unix command line tool that would do this for you in one step.

Answer 4

Untested, but the idea is to treat your file as a grid.
const int COLUMN_COUNT = 2;
const int COLUMN_WIDTH = 8;
const int ROW_WIDTH = COLUMN_COUNT * COLUMN_WIDTH + 1; // + 1 is for endl
void write_cell(std::ostream& out, int row, int column, double value) {
// how many rows is there in the file ?
out.seekp(0, std::ios_base::end);
int row_count = out.tellp() / ROW_WIDTH;
// Do we need to create more rows ?
for(int i = row_count; i <= row; i++) {
out.seekp(ROW_WIDTH - 1, std::ios_base::cur);
out << endl;
}
// compute cell position
int pos = row * ROW_WIDTH + column * COLUMN_WIDTH;
// move to cell position
out.seekp(pos);
// write the cell content
out << std::setw(COLUMN_WIDTH) << value;
}
int main()
{
std::ofstream fout;
fout.open("example.dat");
for (int i=0; i<10; i++)
{
write_cell(fout, i, 0, Func1(i));
};
for (int i=0; i<10; i++)
{
write_cell(fout, i, 1, Func2(i));
};
return 0;
}

Answer 5

for (int i=0;i<10;i++)
{
fout << Func1(i);
};
fout<<endl;
for (int i=0;i<10;i++)
{
fout << Func2(i);
};
fout<<endl;