How would one get a view of a PyArrayObject* similar to the following python code?
# n-column array x
# d is the length of each column
print(x.shape) # => (d, n)
by_column = [x[::,i] for i in range(x.shape[1])]
assert len(by_column) == n
print(by_column[n-1].shape) # => (d,)
So far my code is this:
// my_array is a PyArrayObject*
std::vector<PyArrayObject*> columns = {};
npy_intp* dims = my_array->dimensions;
npy_intp* strides = my_array->strides;
std::vector<int> shape = {};
for (int i = 0; &dims[i] != strides; i++){
shape.push_back(dims[i]);
}
switch (shape.size()) {
case 1: {
// handle 1D array by simply iterating
}
case 2: {
int columns = shape[1];
// What now?
}
}
I'm having trouble finding any reference to do this in C/C++ in both the documentation and the source code, could you give an example of how one would do this?
The C/C++ API for numpy seems really convoluted when compared to something like std::vector, and the documentation isn't very beginner-friendly either, so any references to easier guides would be appreciated too.
You should access the internal structure of PyArrayObject via the PyArray_XXX functions like PyArray_NDIM. To get the contents of a sequence, you use PyObject_GetItem with a tuple key, where in your use case the tuple will have a PySliceObject as the first element.
Related
Tried many things, just cannot get it to work when writing a mex-function.
I have an input from MATLAB which I pass to a method as const matlab::data::Array. This array may contain complex data, sometimes it's only real. So the most straightforward approach should be, in my naive thoughts, that I can simply convert the Array to a TypedArray<std::complex<double>> and I get full complex values if the array contains complex values, and I get complex values with imag=0 if the array contains only real values. It seems to be impossible... This last conversion is not accepted in any case, and MATLAB even simply crashes on trying to cast single elements from a real-valued Array to std::complex<double>.
Anybody a solution how to get a TypedArray<std::complex<double>> in all cases so I can use that in C++ code?
Story of my life, trying for hours and after posting here I find something that works within half an hour... Following code seems to do the job:
void prepareObject(const matlab::data::Array& corners, const matlab::data::Array& facets)
{
size_t N_facet_rows = facets.getDimensions()[0];
size_t N_facet_columns = facets.getDimensions()[1];
matlab::data::TypedArray<std::complex<double>> complex_facets = arrayFactory.createArray<std::complex<double>>(facets.getDimensions());
// Convert the facets to a complex-valued array.
if (facets.getType() == ArrayType::DOUBLE) {
std::complex<double> v;
// Input is DOUBLE, so for each value init a complex<double> and store that in the complex array.
v.imag(0);
for (int i_r = 0; i_r < N_facet_rows; i_r++) {
for (int i_c = 0; i_c < N_facet_columns; i_c++) {
v.real(facets[i_r][i_c]);
complex_facets[i_r][i_c] = v;
}
}
}
else {
// Input is COMPLEX_DOUBLE, so simply copy all values.
for (int i_r = 0; i_r < N_facet_rows; i_r++) {
for (int i_c = 0; i_c < N_facet_columns; i_c++) {
complex_facets[i_r][i_c] = (std::complex<double>) facets[i_r][i_c];
}
}
}
I am trying to create a mask for torch in C++ of type BoolTensor. The first n elements in dimension one need to be False and the rest need to be True.
This is my attempt but I do not know if this is correct (size is the number of elements):
src_mask = torch::BoolTensor({6, 1});
src_mask[:size,:] = 0;
src_mask[size:,:] = 1;
I'm not sure to understand exactly your goal here, so here is my best attempt to convert into C++ you pseudo-code .
First, with libtorch you declare the type of your tensor through the torch::TensorOptions struct (types names are prefixed with a lowercase k)
Second, your python-like slicing is possible thanks to the torch::Tensor::slicefunction (see here and there).
Finally, that gives you something like :
// Creates a tensor of boolean, initially all ones
auto options = torch::TensorOptions().dtype(torch::kBool));
torch::Tensor bool_tensor = torch::ones({6,1}, options);
// Set the slice to 0
int size = 3;
bool_tensor.slice(/*dim=*/0, /*start=*/0, /*end=*/size) = 0;
std::cout << bool_tensor << std::endl;
Please not that this will set the first size rows to 0. I assumed that's what you meant by "first elements in dimension x".
Another way to do it:
using namespace torch::indexing; //for using Slice(...) function
at::Tensor src_mask = at::empty({ 6, 1 }, at::kBool); //empty bool tensor
src_mask.index_put_({ Slice(None, size), Slice() }, 0); //src_mask[:size,:] = 0
src_mask.index_put_({ Slice(size, None), Slice() }, 1); //src_mask[size:,:] = 0
I'm super new to C++ and am trying to build a json file that contains x0...x7 of the files that gets parsed from a pre-defined function so that it can compose a JSON string to give to R. so that it can open a socket to R and send this piece of JSON to it.
however, im kinda stuck here, here is what i have:
std::map<std::string,std::string>::const_iterator qIter;
std::string variable;
std::map<string,string> mymap;
variable = "x";
for (int i=1,i<=7,i++){
float variable+i = ( (qIter = request.getQuery().find(variable+i))
== request.getQuery().end()
)
? 0.0
: atof(qIter->second.c_str());
if ( !isLegalNumber(request.getQuery(),variable+i,variable+i) )
{
strcpy(filePath,"yourErrorFilename.html");
}
else
{
// I want to add the x0 or xn variable here into a json
// The value is now in variable 'x'of something
}
}
Any insights appreciated.
edit: here's my isLegalNumber() method
bool isLegalNumber (const std::map<std::string,std::string>&
map,
const std::string& varName,
float& value
)
{
float temp;
char* cPtr;
std::map<std::string,std::string>::const_iterator
iter = map.find(varName);
if (iter == map.end())
return(false);
temp = strtod(iter->second.c_str(),&cPtr);
if ( (*cPtr != '\0') || (cPtr == iter->second.c_str()) )
return(false);
value = temp;
return(true);
}
im trying to convert a string/ dictionary into a json,
the first question would be how to add it into a dictionary,
and second, how to convert that dictionary into JSON.
basically i want the json to look like
{
x1: value of x1,
x2: value of x2,
....
x7: value of x7
}
I'm not totally clear what you're trying to do in your example code. Specifically, I don't know what the string value variable is used for. I'm guessing you actually want to define an array of floats. You can also skip the first step where you're setting the value to either 0.0 or atof(...) since your isLegalNumber function sets it later anyway. e.g.
float x[8] = {0.0f};
// Note that arrays in C++ are zero-indexed, so your iteration variable should start at 0
for (int i=0; i<=7; i++) {
std::string varName = "x";
varName.push_back(std::to_string(i+1)); // Append the index
if ( !isLegalNumber(request.getQuery(), varName, x[i] ) {
// Error
} else {
// Add to JSON structure
}
}
Once you've got that sorted out, for working with JSON in C++, I would strongly recommend using an existing open-source library such as JSON for Modern C++, rather than rolling your own implementation. This will make it much easier to build the JSON structure you need and ensure that it is properly formatted.
That library has quite thorough documentation, and it allows you to define JSON structures using very similar syntax to the actual JSON you're trying to write, e.g.
json j2 = {
{"pi", 3.141},
{"happy", true},
{"name", "Niels"},
// ...
};
Or in your specific case, define json j; outside the loop, and then in your if case you can do:
j[varName] = x[i];
And then you can convert it to a string using std::string s = j.dump();.
Currently I have this trunk of code: a vector of structs v[myStruct]. The size of it is 3. I named these structs as myStruct_A, myStruct_B, and myStruct_C. Each piece of code is almost same, but with its name as suffix.
SomeMap someMap_A;
for (auto& pair : myStruct_A.hashMap) {
SomeStruct someStruct = foo(pair);
someMap_A[someStruct.key] = someStruct.value;
anotherStruct_A[someStruct.nameX] = someStruct.bundle;
}
someVector.push_back[someMap_A];
All of the three A,B,C need do the above things. The question here is that if using a function, I don't know how to separate and specify the names. I need get several vectors of data which are based on all of those three myStructs' outputs.
You can do this with a macro. For example, here's a simplified version:
#define process(i, suffix) some_vect_ ## suffix.push_back(i); \
some_set_ ## suffix.insert(i);
int main() {
std::vector<int> some_vect_A, some_vect_B, some_vect_C;
std::set<int> some_set_A, some_set_B, some_set_C;
for (int i = 0; i < 100; i++) {
int val = rand();
process(val, A); // adds `val` to `some_vect_A` and `some_set_A`
process(val, B); // adds `val` to `some_vect_B` and `some_set_B`
process(val, C); // adds `val` to `some_vect_C` and `some_set_C`
}
}
This particular demo code is pretty pointless (it produces identical results in all three sets and all three vectors), but does show how to do the manipulation you need.
I've developed a method called "rotate" to my stack object class. What I did was that if the stack contains elements: {0,2,3,4,5,6,7} I would needed to rotate the elements forwards and backwards.
Where if i need to rotate forwards by 2 elements, then we would have, {3,4,5,6,7,0,2} in the array. And if I need to rotate backwards, or -3 elements, then, looking at the original array it would be, {5,6,7,0,2,3,4}
So the method that I have developed works fine. Its just terribly ineffecient IMO. I was wondering if I could wrap the array around by using the mod operator? Or if their is useless code hangin' around that I havent realized yet, and so on.
I guess my question is, How can i simplify this method? e.g. using less code. :-)
void stack::rotate(int r)
{
int i = 0;
while ( r > 0 ) // rotate postively.
{
front.n = items[top+1].n;
for ( int j = 0; j < bottom; j++ )
{
items[j] = items[j+1];
}
items[count-1].n = front.n;
r--;
}
while ( r < 0 ) // rotate negatively.
{
if ( i == top+1 )
{
front.n = items[top+1].n;
items[top+1].n = items[count-1].n; // switch last with first
}
back.n = items[++i].n; // second element is the new back
items[i].n = front.n;
if ( i == bottom )
{
items[count-1].n = front.n; // last is first
i = 0;
r++;
continue;
}
else
{
front.n = items[++i].n;
items[i].n = back.n;
if ( i == bottom )
{
i = 0;
r++;
continue;
}
}
}
}
Instead of moving all the items in your stack, you could change the definition of 'beginning'. Have an index that represents the first item in the stack, 0 at the start, which you add to and subtract from using modular arithmetic whenever you want to rotate your stack.
Note that if you take this approach you shouldn't give users of your class access to the underlying array (not that you really should anyway...).
Well, as this is an abstraction around an array, you can store the "zero" index as a member of the abstraction, and index into the array based on this abstract notion of the first element. Roughly...
class WrappedArray
{
int length;
int first;
T *array;
T get(int index)
{
return array[(first + index) % length];
}
int rotateForwards()
{
first++;
if (first == length)
first = 0;
}
}
You've gotten a couple of reasonable answers, already, but perhaps one more won't hurt. My first reaction would be to make your stack a wrapper around an std::deque, in which case moving an element from one end to the other is cheap (O(1)).
What you are after here is a circular list.
If you insist on storing items in an array just use top offset and size for access. This approach makes inserting elements after you reached allocated size expensive though (re-allocation, copying). This can be solved by using doubly-linked list (ala std::list) and an iterator, but arbitrary access into the stack will be O(n).
The function rotate below is based on reminders (do you mean this under the 'mod' operation?)
It is also quite efficient.
// Helper function.
// Finds GCD.
// See http://en.wikipedia.org/wiki/Euclidean_algorithm#Implementations
int gcd(int a, int b) {return b == 0 ? a : gcd(b, a % b);}
// Number of assignments of elements in algo is
// equal to (items.size() + gcd(items.size(),r)).
void rotate(std::vector<int>& items, int r) {
int size = (int)items.size();
if (size <= 1) return; // nothing to do
r = (r % size + size) % size; // fits r into [0..size)
int num_cycles = gcd(size, r);
for (int first_index = 0; first_index < num_cycles; ++first_index) {
int mem = items[first_index]; // assignment of items elements
int index = (first_index + r) % size, index_prev = first_index;
while (index != first_index) {
items[index_prev] = items[index]; // assignment of items elements
index_prev = index;
index = (index + r) % size;
};
items[index_prev] = mem; // assignment of items elements
}
}
Of course if it is appropriate for you to change data structure as described in other answers, you can obtain more efficient solution.
And now, the usual "it's already in Boost" answer: There is a Boost.CircularBuffer
If for some reason you'd prefer to perform actual physical rotation of array elements, you might find several alternative solutions in "Programming Pearls" by Jon Bentley (Column 2, 2.3 The Power of Primitives). Actually a Web search for Rotating Algorithms 'Programming Pearls' will tell you everything. The literal approach you are using now has very little practical value.
If you'd prefer to try to solve it yourself, it might help to try looking at the problem differently. You see, "rotating an array" is really the same thing as "swapping two unequal parts of an array". Thinking about this problem in the latter terms might lead you to new solutions :)
For example,
Reversal Approach. Reverse the order of the elements in the entire array. Then reverse the two parts independently. You are done.
For example, let's say we want to rotate abcdefg right by 2
abcdefg -> reverse the whole -> gfedcba -> reverse the two parts -> fgabcde
P.S. Slides for that chapter of "Programming Pearls". Note that in Bentley's experiments the above algorithm proves to be quite efficient (among the three tested).
I don't understand what the variables front and back mean, and why you need .n. Anyway, this is the shortest code I know to rotate the elements of an array, which can also be found in Bentley's book.
#include <algorithm>
std::reverse(array , array + r );
std::reverse(array + r, array + size);
std::reverse(array , array + size);