# STC [cspan](../include/stc/cspan.h): Multi-dimensional Array View ![Array](pics/array.jpg) The **cspan** is templated non-owning *single* and *multi-dimensional* view of an array. It has similarities with Python's numpy array slicing and C++ [std::span](https://en.cppreference.com/w/cpp/container/span) / [std::mdspan](https://en.cppreference.com/w/cpp/container/mdspan), and others. ## Header file and declaration **cspan** types are defined by the *using_cspan()* macro after the header is included. This is different from other containers where template parameters are defined prior to including each container. This works well mainly because cspan is non-owning. ```c #include using_cspan(SpanType, ValueType); // define a 1-d SpanType with ValueType elements. using_cspan(SpanTypeN, ValueType, RANK); // define multi-dimensional span with RANK. // RANK is the literal number of dimensions // Shorthands: using_cspan2(S, ValueType); // define span types S, S2 with ranks 1, 2. using_cspan3(S, ValueType); // define span types S, S2, S3 with ranks 1, 2, 3. using_cspan4(S, ValueType); // define span types S, S2, S3, S4 with ranks 1, 2, 3, 4. ``` ## Methods All functions are type-safe. NOTE: the span argument itself is generally **not** side-effect safe - it may be expanded multiple times. However, all index arguments are safe, e.g. `cspan_at(&ms3, i++, j++, k++)` is safe, but `cspan_at(&spans[n++], i, j)` is an error! If the number of arguments does not match the span rank, a compile error is issued. Runtime bounds checks are enabled by default (define `STC_NDEBUG` or `NDEBUG` to disable). ```c SpanType cspan_init(TYPE SpanType, {v1, v2, ...}); // make a 1-d cspan from values SpanType cspan_from(STCContainer* cnt); // make a 1-d cspan from a cvec, cstack, cpque (heap) SpanType cspan_from_n(ValueType* ptr, intptr_t n); // make a 1-d cspan from a pointer and length SpanType cspan_from_array(ValueType array[]); // make a 1-d cspan from a C array intptr_t cspan_size(const SpanTypeN* self); // return number of elements intptr_t cspan_rank(const SpanTypeN* self); // dimensions; compile time constant intptr_t cspan_index(const SpanTypeN* self, intptr_t x, ..); // index of element ValueType* cspan_at(const SpanTypeN* self, intptr_t x, ...); // #args must match input span rank ValueType* cspan_front(const SpanTypeN* self); ValueType* cspan_back(const SpanTypeN* self); SpanTypeN_iter SpanType_begin(const SpanTypeN* self); SpanTypeN_iter SpanType_end(const SpanTypeN* self); void SpanType_next(SpanTypeN_iter* it); SpanTypeN cspan_md(ValueType* data, d1, d2, ...); // make a multi-dim cspan, row-major order. SpanTypeN cspan_md_order(char order, ValueType* data, d1, d2, ...); // order='C': row-major, 'F': column-major (FORTRAN). // transpose a md span (inverse axes). No changes to the underlying array. void cspan_transpose(const SpanTypeN* self); bool cspan_is_order_F(const SpanTypeN* self); // create a subspan of input span rank. Like e.g. cspan_slice(Span3, &ms3, {off,off+count}, {c_ALL}, {c_ALL}); SpanType cspan_subspan(const SpanType* span, intptr_t offset, intptr_t count); SpanType2 cspan_subspan2(const SpanType2* span, intptr_t offset, intptr_t count); SpanType3 cspan_subspan3(const SpanType3* span, intptr_t offset, intptr_t count); // create a sub md span of lower rank. Like e.g. cspan_slice(Span2, &ms4, {x}, {y}, {c_ALL}, {c_ALL}); OutSpan cspan_submd2(const SpanType2* parent, intptr_t x); // return a 1d subspan from a 2d span. OutSpanN cspan_submd3(const SpanType3* parent, intptr_t x, ...); // return a 1d or 2d subspan from a 3d span. OutSpanN cspan_submd4(const SpanType4* parent, intptr_t x, ...); // number of args decides rank of output span. // general slicing of an md span. // {i}: reduce rank. {i,c_END}: slice to end. {c_ALL}: use full extent. OutSpanN cspan_slice(TYPE OutSpanN, const SpanTypeM* parent, {x0,x1}, {y0,y1}.., {N0,N1}); ``` ## TypesPd | Type name | Type definition / usage | Used to represent... | |:------------------|:----------------------------------------------------|:---------------------| | SpanTypeN | `struct { ValueType *data; uint32_t shape[N]; .. }` | SpanType with rank N | | SpanTypeN`_value` | `ValueType` | The ValueType | | `c_ALL` | Use with `cspan_slice()`. | Full extent | | `c_END` | " | End of extent | ## Example 1 Dimension slicing in python, C, and C++: ```py import numpy as np if __name__ == '__main__': ms3 = np.array((1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24), int) ms3 = np.reshape(ms3, (2, 3, 4)) ss3 = ms3[:, 1:3, 2:] ss2 = ss3[1] for i in range(ss2.shape[0]): for j in range(ss2.shape[1]): print(" {}".format(ss2[i, j]), end='') print('') for i in ss2.flat: print(" {}".format(i), end='') # 19 20 23 24 # 19 20 23 24 ``` ... can be written in C using cspan: ```c #include #include using_cspan3(myspan, int); // define myspan, myspan2, myspan3. int main(void) { int arr[] = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24}; myspan3 ms3 = cspan_md(arr, 2, 3, 4); // C-order, i.e. row-major. myspan3 ss3 = cspan_slice(myspan3, &ms3, {c_ALL}, {1,3}, {2,c_END}); myspan2 ss2 = cspan_submd3(&ss3, 1); c_forrange (i, ss2.shape[0]) c_forrange (j, ss2.shape[1]) printf(" %d", *cspan_at(&ss2, i, j)); puts(""); c_foreach (i, myspan2, ss2) printf(" %d", *i.ref); } ``` ... and (almost) in C++23: ```c++ #include #include #include int main(void) { int arr[] = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24}; std::mdspan ms3(arr, 2, 3, 4); auto ss3 = std::submdspan(ms3, std::full_extent, std::tuple{1,3}, std::tuple{2,4}); auto ss2 = std::submdspan(ss3, 1, std::full_extent, std::full_extent); for (std::size_t i = 0; i < ss2.extent(0); ++i) for (std::size_t j = 0; j < ss2.extent(1); ++j) std::print(" {}", ss2[i, j]); std::println(); // std::mdspan can't be iterated joined/flat! } ``` ## Example 2 Slicing cspan without and with reducing the rank: ```c #define i_implement #include #include #include using_cspan3(Span, int); // Shorthand to define Span, Span2, and Span3 int main(void) { // c_init() can create any STC container/span from an initializer list: Span span = c_init(Span, {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24}); // create a 3d cspan: Span3 span3 = cspan_md(span.data, 2, 4, 3); // reduce rank: (i.e. span3[1]) Span2 span2 = cspan_submd3(&span3, 1); puts("\niterate span2 flat:"); c_foreach (i, Span2, span2) fmt_print(" {}", *i.ref); puts(""); // create span on-the-fly int array[] = {3, 65, 4, 3, 7, 87, 45}; c_forfilter (i, ISpan, (ISpan)cspan_from_array(array), c_flt_skip(i, 2) && c_flt_take(i, 3)) fmt_print(" {}", *i.ref); puts(""); // slice without reducing rank: Span3 ss3 = cspan_slice(Span3, &span3, {c_ALL}, {3,4}, {c_ALL}); puts("\niterate ss3 by dimensions:"); c_forrange (i, ss3.shape[0]) { c_forrange (j, ss3.shape[1]) { c_forrange (k, ss3.shape[2]) fmt_print(" {:2}", *cspan_at(&ss3, i, j, k)); fmt_print(" |"); } } // slice and reduce rank: Span2 ss2 = cspan_slice(Span2, &span3, {c_ALL}, {3}, {c_ALL}); puts("\niterate ss2 by dimensions:"); c_forrange (i, ss2.shape[0]) { c_forrange (j, ss2.shape[1]) fmt_print(" {:2}", *cspan_at(&ss2, i, j)); fmt_print(" |"); } puts("\niterate ss2 flat:"); c_foreach (i, Span2, ss2) fmt_print(" {:2}", *i.ref); puts(""); } ``` Output: ``` iterate span2 flat: 13 14 15 16 17 18 19 20 21 22 23 24 iterate ss3 by dimensions: 10 11 12 | 22 23 24 | iterate ss2 by dimensions: 10 | 11 | 12 | 22 | 23 | 24 | iterate ss2 flat: 10 11 12 22 23 24 ```