csv_txt_cpp.md

Data Import & Export - C++ - csv/txt

CSV (Comma-Separated Values) is a simple file format used to store tabular data. Files in the CSV format can be imported to and exported from programs that store data in tables, such as Microsoft Excel or OpenOffice Calc. There are many open-source libraries we can use to read from or write to a CSV file. A CSV file often uses ',' or ' ' as delimiter, and they might come with headers at the first row. The txt files to store data is often similar to csv files.

Typical csv/txt file structure looks like:

index,x,y
1,114,32.124
2,115,20.784
3,116,10.542

or with no header and use ' ' as the delimiter:

1 114 32.124
2 115 20.784
3 116 10.542

In C++, for files with no header, we can use three classes, fstream, ifstream or ostream, to easily read from or write to a txt file. See cppreference or cplusplus for more details. For files with header, there is no difference to writing to files with no header, but we better use existed tools to achieve reading, such as fast-cpp-csv-parser which is a small, easy-to-use and fast header-only library for reading CSV files.

Here are step-by-step instructions for files with no header, you can also find an example cpp file.

1. Include the header file as follows

#include <fstream> // for fstream, ifstream, ofstream classes

2. Choose the right class you need

• ifstream is used to read from a file only
• ofstream is used to write to a file only
• fstream is more flexible, you can configure the mode as read or write and it also has some advanced configuration.

ifstream reader;
ofstream writer;
fstream f;

3. Open or create a file

// for ifstream
reader.open("data01.txt");

// for ofstream
writer.open("output.txt");

// for fstream, open a certain file named `file_name` in `open_mode` mode.
f.open(file_name, open_mode);
f.open("data01.csv", std::ios::in); // read mode
f.open("path/to/file/data01.csv", std::ios::in | std::ios::nocreate); // read mode, return error when file doesn't exist
f.open("path/to/file/output.txt", std::ios::out); // write mode

open_mode is an integer-type number, here is a list:

• std::ios::in = 0x01, read, create new file if not exist (ifstream default mode).
• std::ios::out = 0x02, write (overwrite when file exist), create new file if not exist (ofstream default mode).
• std::ios::ate = 0x04, put the pointer at the end of the file when opened. The pointer can be changed, usually used along with std::ios::in and std::ios::out.
• std::ios::app = 0x08, write (append), create new file if not exist, the pointer is always at the end of the opened file.
• std::ios::trunc = 0x10, any current content is discarded, assuming a length of zero on opening.
• std::ios::nocreate = 0x20, return error when file doesn't exist, usually used along with std::ios::in and std::ios::app.
• std::ios::noreplace = 0x40, return error when file exist, usually used along with std::ios::out.
• std::ios::binary = 0x80, open file in binary format.

4. Read & Write

// Read from file
std::vector<std::vector<data_type>> data;
double tmp[col_num];
while (!reader.eof()) {
    for (size_t i = 0; i < col_num; ++i) {
        reader >> tmp[i];
    }
    std::vector<data_type> row_data(tmp, tmp + col_num);
    data.push_back(row_data);
}
data.pop_back(); // the last row is read twice, remove one record.

// Write to file
char trim_character = ' ';
writer << 1 << trim_character << "string" << "\n";
writer << 2 << trim_character << 22.0 << "\n";

5. Close the file

Remember, before operating another file, close the current file firstly, or you cannot open other files using the same instance.

reader.close();
writer.close();
f.close();