Header only

.gitignore

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+.vscode
+build
+path.hpp

CMakeLists.txt

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+project(save_and_load_eigen_csv CXX)
+cmake_minimum_required(VERSION 3.0)
+find_package(Eigen3 REQUIRED)
+
+include_directories(${CMAKE_SOURCE_DIR})
+
+configure_file(${CMAKE_SOURCE_DIR}/path.hpp.in ${CMAKE_SOURCE_DIR}/path.hpp)
+
+add_executable(double_test double_test.cpp)
+target_link_libraries(double_test Eigen3::Eigen)
+
+add_executable(float_test float_test.cpp)
+target_link_libraries(float_test Eigen3::Eigen)
+
+add_executable(int_test int_test.cpp)
+target_link_libraries(int_test Eigen3::Eigen)

double_test.cpp

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+// ConsoleApplication3.cpp : This file contains the 'main' function. Program execution begins and ends there.
+//
+
+#include "save_load_eigen_csv.hpp"
+#include "path.hpp"
+
+int main()
+{
+	// test matrix to be saved
+	// Eigen::MatrixXd matrix_test(4, 4);
+	Eigen::Matrix<double, 4, 4> matrix_test;
+	// define the test matrix
+	matrix_test << 1.2, 1.4, 1.6, 1.8,
+		1.5, 1.7, 1.9, 1.10,
+		0.8, 1.2, -0.1, -0.2,
+		1.3, 99, 100, 112;
+
+	// save test matrix
+	std::string matrixFile = build_path + std::string("matrix.csv");
+	saveData<double>(matrixFile, matrix_test);
+
+	// matrix to be loaded from a file
+	Eigen::MatrixXd matrix_test2;
+
+	// load the matrix from the file
+	matrix_test2 = openData<double>(matrixFile);
+
+	// print the matrix in console
+	std::cout << matrix_test2 << std::endl;
+
+	// test the load function on a matrix defined outside this environment.
+	// make sure that the "matrix_outside.csv" file exhists
+	/*MatrixXd matrix_test3;
+
+	matrix_test3 = openData("matrix_outside.csv");
+
+	cout <<"\n\n"<< matrix_test3<<"\n\n";
+
+	cout << 10*matrix_test3 << "\n";*/
+}

float_test.cpp

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+// ConsoleApplication3.cpp : This file contains the 'main' function. Program execution begins and ends there.
+//
+
+#include "save_load_eigen_csv.hpp"
+#include "path.hpp"
+
+int main()
+{
+	// test matrix to be saved
+	Eigen::MatrixXf matrix_test(4, 4);
+
+	// define the test matrix
+	matrix_test << 1.2, 1.4, 1.6, 1.8,
+		1.5, 1.7, 1.9, 1.10,
+		0.8, 1.2, -0.1, -0.2,
+		1.3, 99, 100, 112;
+
+	// save test matrix
+	std::string matrixFile = build_path + std::string("matrix.csv");
+	saveData<float>(matrixFile, matrix_test);
+
+	// matrix to be loaded from a file
+	Eigen::MatrixXf matrix_test2;
+
+	// load the matrix from the file
+	matrix_test2 = openData<float>(matrixFile);
+
+	// print the matrix in console
+	std::cout << matrix_test2 << std::endl;
+
+	// test the load function on a matrix defined outside this environment.
+	// make sure that the "matrix_outside.csv" file exhists
+	/*MatrixXd matrix_test3;
+
+	matrix_test3 = openData("matrix_outside.csv");
+
+	cout <<"\n\n"<< matrix_test3<<"\n\n";
+
+	cout << 10*matrix_test3 << "\n";*/
+}

int_test.cpp

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+// ConsoleApplication3.cpp : This file contains the 'main' function. Program execution begins and ends there.
+//
+
+#include "save_load_eigen_csv.hpp"
+#include "path.hpp"
+
+int main()
+{
+	// test matrix to be saved
+	Eigen::MatrixXi matrix_test(4, 4);
+
+	// define the test matrix
+	matrix_test << 1, 2, 3, 4,
+		5, 6, 7, 8,
+		9, 10, 11, 12,
+		13, 14, 15, 16;
+
+	// save test matrix
+	std::string matrixFile = build_path + std::string("matrix.csv");
+	saveData<int>(matrixFile, matrix_test);
+
+	// matrix to be loaded from a file
+	Eigen::MatrixXi matrix_test2;
+
+	// load the matrix from the file
+	matrix_test2 = openData<int>(matrixFile);
+
+	// print the matrix in console
+	std::cout << matrix_test2 << std::endl;
+
+	// test the load function on a matrix defined outside this environment.
+	// make sure that the "matrix_outside.csv" file exhists
+	/*MatrixXd matrix_test3;
+
+	matrix_test3 = openData("matrix_outside.csv");
+
+	cout <<"\n\n"<< matrix_test3<<"\n\n";
+
+	cout << 10*matrix_test3 << "\n";*/
+}

path.hpp.in

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+#ifndef PATH_HPP
+#define PATH_HPP
+
+const auto build_path= "${CMAKE_CURRENT_BINARY_DIR}/";
+
+#endif /* PATH_HPP */

save_load_eigen_csv.hpp

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+#ifndef SAVE_LOAD_EIGEN_CSV_HPP
+#define SAVE_LOAD_EIGEN_CSV_HPP
+
+#include <iostream>
+#include <Eigen/Dense>
+#include <fstream>
+#include <vector>
+
+template<typename T>
+void saveData(std::string fileName, Eigen::Matrix<T,-1,-1> matrix)
+{
+  // https://eigen.tuxfamily.org/dox/structEigen_1_1IOFormat.html
+  const static Eigen::IOFormat CSVFormat(Eigen::FullPrecision, Eigen::DontAlignCols, ", ", "\n");
+
+  std::ofstream file(fileName);
+  if (file.is_open())
+  {
+    file << matrix.format(CSVFormat);
+    file.close();
+  }
+}
+
+
+template<typename T>
+Eigen::Matrix<T,-1,-1> openData(std::string fileToOpen)
+{
+
+	// the inspiration for creating this function was drawn from here (I did NOT copy and paste the code)
+	// https://stackoverflow.com/questions/34247057/how-to-read-csv-file-and-assign-to-eigen-matrix
+
+	// the input is the file: "fileToOpen.csv":
+	// a,b,c
+	// d,e,f
+	// This function converts input file data into the Eigen matrix format
+
+
+
+	// the matrix entries are stored in this variable row-wise. For example if we have the matrix:
+	// M=[a b c 
+	//	  d e f]
+	// the entries are stored as matrixEntries=[a,b,c,d,e,f], that is the variable "matrixEntries" is a row vector
+	// later on, this vector is mapped into the Eigen matrix format
+	std::vector<T> matrixEntries;
+
+	// in this object we store the data from the matrix
+	std::ifstream matrixDataFile(fileToOpen);
+
+	// this variable is used to store the row of the matrix that contains commas 
+	std::string matrixRowString;
+
+	// this variable is used to store the matrix entry;
+	std::string matrixEntry;
+
+	// this variable is used to track the number of rows
+	int matrixRowNumber = 0;
+
+
+	while (std::getline(matrixDataFile, matrixRowString)) // here we read a row by row of matrixDataFile and store every line into the string variable matrixRowString
+	{
+		std::stringstream matrixRowStringStream(matrixRowString); //convert matrixRowString that is a string to a stream variable.
+
+		while (std::getline(matrixRowStringStream, matrixEntry, ',')) // here we read pieces of the stream matrixRowStringStream until every comma, and store the resulting character into the matrixEntry
+		{
+			matrixEntries.push_back(std::stod(matrixEntry));   //here we convert the string to double and fill in the row vector storing all the matrix entries
+		}
+		matrixRowNumber++; //update the column numbers
+	}
+
+	// here we convet the vector variable into the matrix and return the resulting object, 
+	// note that matrixEntries.data() is the pointer to the first memory location at which the entries of the vector matrixEntries are stored;
+	return Eigen::Map<Eigen::Matrix<T, -1, -1,Eigen::RowMajor>>(matrixEntries.data(), matrixRowNumber, matrixEntries.size() / matrixRowNumber);
+
+}
+
+
+#endif /* SAVE_LOAD_EIGEN_CSV_HPP */