I've created some data in numpy that I would like to use in a separate C++ program. Therefore I need to save the data using python and later load it in C++. What is the best way of doing this?
My numpy ndarray is float 32 and of shape [10000 x 18 x 5]. I can save it for example using
numpy.save(filename, data)
Is there an easy way to load such data in C++? Target structure could be an Eigen::Matrix
for example.
After searching for hours I found my year-old example files.
Caveat:
Write numpy array to ascii file with header specifying nrows, ncols:
def write_matrix2D_to_ascii(filename, matrix2D):
nrows, ncols = matrix2D.shape
with open(filename, "w") as file:
# write header [rows x cols]
nrows, ncols = matrix2D.shape
file.write(f"{nrows} {ncols}")
file.write("\n")
# write values
for row in range(nrows):
for col in range(ncols):
value = matrix2D[row, col]
file.write(str(value))
file.write(" ")
file.write("\n")
Example output data-file.txt looks like this (first row is header specifying nrows and ncols):
2 3
1.0 2.0 3.0
4.0 5.0 6.0
Cpp function to read matrix from ascii file into OpenCV matrix:
#include <iostream>
#include <fstream>
#include <iomanip> // set precision of output string
#include <opencv2/core/core.hpp> // OpenCV matrices for storing data
using namespace std;
using namespace cv;
void readMatAsciiWithHeader( const string& filename, Mat& matData)
{
cout << "Create matrix from file :" << filename << endl;
ifstream inFileStream(filename.c_str());
if(!inFileStream){
cout << "File cannot be found" << endl;
exit(-1);
}
int rows, cols;
inFileStream >> rows;
inFileStream >> cols;
matData.create(rows,cols,CV_32F);
cout << "numRows: " << rows << "\t numCols: " << cols << endl;
matData.setTo(0); // init all values to 0
float *dptr;
for(int ridx=0; ridx < matData.rows; ++ridx){
dptr = matData.ptr<float>(ridx);
for(int cidx=0; cidx < matData.cols; ++cidx, ++dptr){
inFileStream >> *dptr;
}
}
inFileStream.close();
}
Driver code to use above mentioned function in cpp program:
Mat myMatrix;
readMatAsciiWithHeader("path/to/data-file.txt", myMatrix);
For completeness, some code to save the data using C++:
int saveMatAsciiWithHeader( const string& filename, Mat& matData)
{
if (matData.empty()){
cout << "File could not be saved. MatData is empty" << endl;
return 0;
}
ofstream oStream(filename.c_str());
// Create header
oStream << matData.rows << " " << matData.cols << endl;
// Write data
for(int ridx=0; ridx < matData.rows; ridx++)
{
for(int cidx=0; cidx < matData.cols; cidx++)
{
oStream << setprecision(9) << matData.at<float>(ridx,cidx) << " ";
}
oStream << endl;
}
oStream.close();
cout << "Saved " << filename.c_str() << endl;
return 1;
}
Future work:
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