C array basic operation and manipulation in comparison to Python

2017-06-17 19:48 Joe Xu imported from Stackoverflow

I am switching from Python to C code for the purpose of my project. As I try to manipulate an array, it is very different from what I used to do in python. So I just want to know some "equivalent" operations for an array in C and Python.

So for numpy array in Python, we can directly sum(or other operations) up two arrays elementwise easily. In a sense, I think numpy array is like a virtual "matrix" that you can manipulate with as if it is stored as a whole(at least appear to).

For C array, it looks like a different animal. From what I could search out from the internet, I have to use for loop in order to do these basic operations. I think an array in C is just a set of values that are stored in addresses next to each other in order.

I know that C is much faster than Python, but I also know for loop is a nasty thing that should be avoided as possible in Python. Isn't these for loops in C slow the program down?(though I think when I use numpy module in Python, the module takes care of these for loops implicitly)

I also get a confusion about pointer. Consider this function that takes in a 1D array:

int myfunc(int *myarray);
{
 int sum=0;
 for(int i=0; i<sizeof(myarray); i++)
   {
     sum += myarray[i];
   }
 return sum
 }

Suppose I input some array say "arr", then I let:

*myarray = arr

Thus, later in the function when I am calling lines such as:

sum += myarray[i]

I am actually doing this:

sum += &arr

Which is really confusing. For me, it looks like I am directly adding addresses of variables with values of other variables. Is this code wrong or I misunderstand pointer?

1 answer

answered 2017-06-17 19:48 Michaël Roy

In your function:

int myfunc(int *myarray);             //  The ; at the end of this line will
                                      //  generate a compiler error.
{
 int sum=0;
 for(int i=0; i<sizeof(myarray); i++) // size of int* is the size of a pointer
                                      // 4 for 32bit app, or 8 for 64 bits apps
 {
     sum += myarray[i];
 }
 return sum
}

You should pass the number of elements to your array as a parameter for it to work:

int myfunc(int *myarray, int count)
{
 int sum=0;
 for (int i = 0; i < count; i++)                                           
 {
     sum += myarray[i];
 }
 return sum;
}

[Edit] When you do this:

sum += &arr;

You are incrementing sum with the address of variable arr.

Whereas:

sum += array[i];

increments sum with the int value stored at the (i+1)th position of array.

The following statement:

*myarray = arr;

Stores the value of arr into the value pointed to by myarray, this is equivalent to writing:

myarray[0] = arr;

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error: unknown file type '.pyx' (from 'netCDF4/_netCDF4.pyx')

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haaviston@tuomi13kl:~/.local/lib/python2.7/site-packages/pkg_resources$ pip install --user netCDF4
Downloading/unpacking netCDF4
  Downloading netCDF4-1.3.0.tar.gz (542kB): 542kB downloaded
  Running setup.py (path:/tmp/pip_build_haaviston/netCDF4/setup.py) egg_info for package netCDF4
    Package hdf5 was not found in the pkg-config search path.
    Perhaps you should add the directory containing `hdf5.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'hdf5' found
    using nc-config ...

        HDF5_DIR environment variable not set, checking some standard locations ..
    checking /home/haaviston ...
    checking /usr/local ...
    checking /sw ...
    checking /opt ...
    checking /opt/local ...
    checking /usr ...
    HDF5 library version: 1.8.11 found in /usr
    using netcdf library version 4.1.3
    using Cython to compile netCDF4.pyx...
    netcdf lib does not have group rename capability
    netcdf lib has nc_inq_path function
    netcdf lib does not have nc_inq_format_extended function
    netcdf lib does not have nc_open_mem function
    netcdf lib does not have cdf-5 format capability

Requirement already satisfied (use --upgrade to upgrade): numpy>=1.7 in /usr/lib/python2.7/dist-packages (from netCDF4)
Installing collected packages: netCDF4
  Running setup.py install for netCDF4
    Package hdf5 was not found in the pkg-config search path.
    Perhaps you should add the directory containing `hdf5.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'hdf5' found
    using nc-config ...

        HDF5_DIR environment variable not set, checking some standard locations ..
    checking /home/haaviston ...
    checking /usr/local ...
    checking /sw ...
    checking /opt ...
    checking /opt/local ...
    checking /usr ...
    HDF5 library version: 1.8.11 found in /usr
    using netcdf library version 4.1.3
    using Cython to compile netCDF4.pyx...
    netcdf lib does not have group rename capability
    netcdf lib has nc_inq_path function
    netcdf lib does not have nc_inq_format_extended function
    netcdf lib does not have nc_open_mem function
    netcdf lib does not have cdf-5 format capability
    building 'netCDF4._netCDF4' extension
    error: unknown file type '.pyx' (from 'netCDF4/_netCDF4.pyx')
    Complete output from command /usr/bin/python -c "import setuptools, tokenize;__file__='/tmp/pip_build_haaviston/netCDF4/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-5Mjz_n-record/install-record.txt --single-version-externally-managed --compile --user:
    Package hdf5 was not found in the pkg-config search path.

Perhaps you should add the directory containing `hdf5.pc'

to the PKG_CONFIG_PATH environment variable

No package 'hdf5' found

reading from setup.cfg...

using nc-config ...



    HDF5_DIR environment variable not set, checking some standard locations ..

checking /home/haaviston ...

checking /usr/local ...

checking /sw ...

checking /opt ...

checking /opt/local ...

checking /usr ...

HDF5 library version: 1.8.11 found in /usr

using netcdf library version 4.1.3

using Cython to compile netCDF4.pyx...

netcdf lib does not have group rename capability

netcdf lib has nc_inq_path function

netcdf lib does not have nc_inq_format_extended function

netcdf lib does not have nc_open_mem function

netcdf lib does not have cdf-5 format capability

running install

running build

running build_py

creating build

creating build/lib.linux-x86_64-2.7

creating build/lib.linux-x86_64-2.7/netcdftime

copying netcdftime/__init__.py -> build/lib.linux-x86_64-2.7/netcdftime

creating build/lib.linux-x86_64-2.7/netCDF4

copying netCDF4/__init__.py -> build/lib.linux-x86_64-2.7/netCDF4

copying netCDF4/utils.py -> build/lib.linux-x86_64-2.7/netCDF4

running build_ext

building 'netCDF4._netCDF4' extension

error: unknown file type '.pyx' (from 'netCDF4/_netCDF4.pyx')

----------------------------------------
Cleaning up...
Command /usr/bin/python -c "import setuptools, tokenize;__file__='/tmp/pip_build_haaviston/netCDF4/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-5Mjz_n-record/install-record.txt --single-version-externally-managed --compile --user failed with error code 1 in /tmp/pip_build_haaviston/netCDF4
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C array basic operation and manipulation in comparison to Python

1 answer

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