This is a text-only version of the following page on https://raymii.org:

This is a text-only version of the following page on https://raymii.org:
---
Title : C++ create and write to a CSV file
Author : Remy van Elst
Date : 07-06-2019
Last update : 16-06-2019
URL : https://raymii.org/s/snippets/Cpp_create_and_write_to_a_csv_file.html
Format : Markdown/HTML
---

In this quick snippet I'll show you how to create and write to a csv file. It
includes checking if the file is writable, and if it's not there, creates it
with a different first row as header. It's a quick example, I've used it to log
some test data. It can probably be improved. It does use a mutex and `guard_lock`
so it should be thread safe.

I've updated this snippet to use a variadic template. [Click here to view the
new snippet][1].

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### write_csv.cpp

The first method checks if the file exists. The second method is a template, but
with a set amount of columns (since that was enough in my case). The types could
differ which is why I opted for a template.

Note that this is a cut-down simplified example, in the next section I list some
possible improvements. Since this was a one-off user story for some tests, the
time allocated wasn't much, thus making the code bare.

In `main()` there is an example with different types and a loop to show how you
could use the function.

#include <iostream>
#include <mutex>
#include <fstream>

std::mutex logMutex;

bool fileExists(std::string& fileName) {
return static_cast<bool>(std::ifstream(fileName));
}

template <typename filename, typename T1, typename T2, typename T3>
bool writeCsvFile(filename &fileName, T1 column1, T2 column2, T3 column3) {
std::lock_guard<std::mutex> csvLock(logMutex);
std::fstream file;
file.open (fileName, std::ios::out | std::ios::app);
if (file) {
file << "\"" << column1 << "\",";
file << "\"" << column2 << "\",";
file << "\"" << column3 << "\"";
file << std::endl;
return true;
} else {
return false;
}
}

int main() {
std::string csvFile = "logfile2.csv";
std::string naam = "Hallo";
if(!fileExists(csvFile))
writeCsvFile(csvFile, "header1", "header2", "header3");

for (int i = 1; i < 10; ++i) {
if (!writeCsvFile(csvFile, i, naam, static_cast<float>(i * 3.5))) {
std::cerr << "Failed to write to file: " << csvFile << "\n";
}
}
return 0;
}

The example `logfile.csv` looks like this after two runs:

"header1","header2","header3"
"1","Hallo","3.5"
"2","Hallo","7"
"3","Hallo","10.5"
"4","Hallo","14"
"5","Hallo","17.5"
"6","Hallo","21"
"7","Hallo","24.5"
"8","Hallo","28"
"9","Hallo","31.5"
"1","Hallo","3.5"
"2","Hallo","7"
"3","Hallo","10.5"
"4","Hallo","14"
"5","Hallo","17.5"
"6","Hallo","21"
"7","Hallo","24.5"
"8","Hallo","28"
"9","Hallo","31.5"
"1","Hallo","3.5"
"2","Hallo","7"
"3","Hallo","10.5"
"4","Hallo","14"
"5","Hallo","17.5"
"6","Hallo","21"
"7","Hallo","24.5"
"8","Hallo","28"
"9","Hallo","31.5"

### Possible improvements

* Use a `struct` with the column data (making the amount of columns expandable)
* Check if the folder exists
* Check if the file exists but is empty (then write the header)
* Checking and escaping special characters (like `"` or newlines)

[1]: https://raymii.org/s/snippets/Cpp_create_and_write_to_a_CSV_file_with_a_variadic_template.html

---

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