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%load_ext cython


    

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%%cython

# General purpose pointer - no type given
cdef void* p

# Oh hey, let's save the address to an int
cdef int x = 123
p = &x

# How to use the void pointer?  You need casting:
cdef int* px = <int*>p
print(px[0])

# Can also SAME POINTER for a double
cdef double y = 0.1
p = &y

# And out again
cdef double* py = <double*>p
print(py[0])


    

    




123
0.1

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%%cython

x = ['a', 'b', 'c']

cdef void* p = <void*>x  # Get a void pointer to the Python object

# Surprise! The memory address is the same as the object id()
print(<long>p)
print(id(x))

# By type-casting to <object>, you can get the original 
# Python object back
print(<object>p)


    

    




4424853192
4424853192
['a', 'b', 'c']

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%%cython

# Sometimes "unsigned long long" is too much to type
# Let's rather call it "uint64"
ctypedef unsigned long long uint64

# Now functions are easier to write
cdef uint64 f(uint64 x):
    return x // 2

print(f(2**16))


    

    




32768

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%%cython

# Original function
cdef double f(int x):
    return x / 10.0

# "Type" of the function
ctypedef double(*function_type)(int)

# Declare new function pointer
cdef function_type x
x = &f  # x now points at function f

# Test it out
print(x(2))


    

    




0.2

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