C/C++ for Python programmer [closed]

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Closed 2 years ago.

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!:)

回答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.

回答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.

回答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)