C/C++ for Python programmer [closed] - c++

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I have to switch from Python to C/C++.
Do you know a quick "reference tutorial" or something like that to have a reference to how to start? For example something like the Numpy and Scipy tutorials.
I have read a lot of "documentation", for example
C++ for dummies
the K&R C Programming Language
a lot of blog and online documentation such as: http://eli.thegreenplace.net/2010/01/11/pointers-to-arrays-in-c/,
http://newdata.box.sk/bx/c/
tons of Q&A here on StackOverflow
...
but it's still not clear to me even how to do start porting to C/C++ something like:
#!/usr/bin/env python
import time
import numpy as np
import tables as tb
"""Retrieve 3D positions form 1000 files and store them in one single HDF5 file.
"""
t = time.time()
# Empty array
sample = np.array([])
sample.shape = (0,3)
# Loop over the files
for i in range(0, 1000):
filename = "mill2sort-"+str(i)+"-extracted.h5"
print "Doing ", filename
# Open data file
h5f = tb.openFile(filename, 'r')
# Stack new data under previous data
sample = np.vstack((sample, h5f.root.data.read()))
h5f.close()
# Create the new file
h5 = tb.openFile("mill2sort-extracted-all", 'w')
# Save the array
h5.createArray(h5.root, 'data', sample, title='mill_2_sub_sample_all')
h5.flush()
h5.close()
print "Done in ", time.time()-t, " seconds."
in C or C++. In this example I was not even able to understand how to pass a 3D array to a function that find it's dimensions, something like
int getArrayDimensions(int* array, int *dimensions){
*dimensions = sizeof(*array)/sizeof(array[0]);
return 0;
}
With array being
int array[3][3][3] = ...
Thank you for any suggestion!:)

Answer 1

OK, for that particular example:
you can get the time services from the standard library here
you can use eigen for linear algebra. It's an amazing library, I'm in love with it.
check here to learn how to manipulate files
While using C++, you might miss some features from python, but most of them are actually provided by the boost libraries. For instance returning multiple values from a function is very easy with boost.tuple library as in here. You can use boost::shared_ptr if you don't want to bother yourself with memory management. Or if you want to keep using python to play with your c++ classes, you can use boost.python. Boost.parameter helps you define functions with named arguments. There is also Boost.lambda for lambda functions, but if your environment supports it, you can also use C++11 to have language support for lambda functions. Boost is a gold mine, never stop digging. Just assume that it's part of the standard library. I develop C++ in many different platforms, and neither eigen nor boost has let me down yet.
Here's a good FAQ for C++ best practices. This is a very important principle that you have to keep in mind at all times, while working in C++. I extend it a bit, in my mind and think; If you're going to do something dangerous such as: Allocate memory with a raw new, or index a raw C style array, pass around raw pointers, or do static_cast (even worse reinterpret_cast) etc. They should usually happen in a class somehow dedicated to them, and the code to make sure they don't cause trouble lives very close to them, so that you can see at a glance that everything is under control.
Finally, my favourite!!! Do you want to keep using generators in C++? Here's some dark magic.

Answer 2

Alright, lets just start with C for now.
void readH5Data(FILE *file, int ***sample); // this is for you to implement
void writeH5Data(FILE *file, int ***sample); // this is for you to implement
int main(int argc, const char *argv[])
{
#define width 3
#define height 3
#define depth 3
time_t t = time(NULL);
int ***sample = calloc(width, sizeof(*sample));
for (int i = 0; i < width; i++)
{
sample[i] = calloc(height, sizeof(**sample));
for (int j = 0; j < height; j++)
{
sample[i][j] = calloc(depth, sizeof(***sample));
}
}
for (int i = 0; i < 1000; i++)
{
char *filename[64];
sprintf(filename, "mill2sort-%i-extracted.h5", i);
// open the file
FILE *filePtr = fopen(filename, "r");
if (filePtr == NULL || ferror(filePtr))
{
fprintf(stderr, "%s\n", strerror(errno));
exit(EXIT_FAILURE);
}
readH5Data(filePtr, sample);
fclose(filePtr);
}
char filename[] = "mill2sort-extracted-all";
FILE *writeFile = fopen(filename, "w");
if (writeFile == NULL || ferror(writeFile))
{
fprintf(stderr, "%s\n", strerror(errno));
exit(EXIT_FAILURE);
}
writeH5Data(writeFile, sample);
fflush(writeFile);
fclose(writeFile);
printf("Done in %lli seconds\n", (long long int) (time(NULL) - t));
for (int i = 0; i < width; i++)
{
for (int j = 0; j < width; j++)
{
free(sample[i][j]);
}
free(sample[i]);
}
free(sample);
}
As long as you remember that your array is 3x3x3, you should have no problems overstepping the bounds in your 'writeH5Data' method.

Answer 3

This question is getting quite old, but here is a couple of references that have been useful to me:
A Transition Guide: Python to C++ (pdf)
A Brief Introduction to C++ for Python programmers (incomplete but quite good)