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# STC [cspan](../include/stc/cspan.h): Multi-dimensional Array View
The **cspan** is templated non-owning multi-dimensional view of an array.
See the c++ classes [std::span](https://en.cppreference.com/w/cpp/container/span) and
[std::mdspan](https://en.cppreference.com/w/cpp/container/mdspan) for similar functionality.
## Header file and declaration
```c
#include <stc/cspan.h>
using_cspan(SpanType, ValueType, Rank); // define SpanType with ValueType elements.
// Rank is number of dimensions (max 4)
// 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
Note that `cspan_make()`, `cmake_from*()`, `cspan_atN()`, `and cspan_subspanN()` require a (safe) cast to its span-type
on assignment, but not on initialization of a span variable. All functions are type-safe, and arguments are side-effect safe, except for SpanType arg. which must not have side-effects.
```c
SpanTypeN cspan_make(ValueType* data, size_t xdim, ...); // make N-dimensional cspan
SpanType cspan_from(STCContainer* cnt); // create a 1D cspan from a compatible STC container
SpanType cspan_from_array(ValueType array[]); // create a 1D cspan from a C array
void cspan_resize(SpanTypeN* self, size_t xdim, ...); // change the extent of each dimension
size_t cspan_size(const SpanTypeN* self); // return number of elements
unsigned cspan_rank(const SpanTypeN* self); // return number of dimensions
size_t cspan_index(const SpanTypeN* self, size_t x, ...); // index of element
ValueType* cspan_at(SpanTypeN* self, size_t x, ...); // at(): num of args decides input SpanTypeN.
ValueType* cspan_front(SpanTypeN* self);
ValueType* cspan_back(SpanTypeN* self);
SpanType cspan_subdim2(SpanType2* self, size_t x); // return a cspan from a 2D SpanType.
SpanTypeN cspan_subdim3(SpanType3* self, size_t x, ...); // subdimN(): N decides input SpanType,
SpanTypeN cspan_subdim4(SpanType4* self, size_t x, ...); // and num of args decides returned SpanTypeN.
// return a subspan of same rank:
SpanType cspan_subspan(const SpanType* self, size_t offset, size_t count);
SpanType2 cspan_subspan2(const SpanType2 self, size_t offset, size_t count);
SpanType3 cspan_subspan3(const SpanType3 self, size_t offset, size_t count);
SpanType4 cspan_subspan4(const SpanType4 self, size_t offset, size_t count);
SpanTypeN_iter SpanType_begin(const SpanTypeN* self);
SpanTypeN_iter SpanType_end(const SpanTypeN* self);
void SpanType_next(SpanTypeN_iter* it);
```
## Types
| Type name | Type definition | Used to represent... |
|:------------------|:-----------------------------------------------|:---------------------|
| SpanTypeN | `struct { ValueType *data; uint32_t dim[N]; }` | SpanType with rank N |
| SpanTypeN`_value` | `ValueType` | The ValueType |
| SpanTypeN`_iter` | `struct { ValueType *ref; ... }` | Iterator type |
## Example
```c
#include <stdio.h>
#include <stc/cspan.h>
#define i_val float
#include <stc/cstack.h>
using_cspan3(Span, float); // Shorthand to define span types Span, Span2, and Span3.
int main()
{
int xd = 6, yd = 4, zd = 3;
c_AUTO (cstack_float, vec)
{
c_FORRANGE (i, xd*yd*zd)
cstack_float_push(&vec, i);
// define "span3[xd][yd][zd]"
Span3 span3 = cspan_make(vec.data, xd, yd, zd);
*cspan_at(&span3, 4, 3, 2) = 3.14159f;
printf("index: %d", (int)cspan_index(&span3, 4, 3, 2));
Span span1 = cspan_subdim3(&span3, 4, 3);
printf("\niterate span1: ");
c_FOREACH (i, Span, span1)
printf("%g ", *i.ref);
Span2 span2 = cspan_subdim3(&span3, 4);
printf("\niterate span2: ");
c_FOREACH (i, Span2, span2)
printf("%g ", *i.ref);
puts("\niterate span3 by dimensions:");
c_FORRANGE (i, span3.dim[0]) {
c_FORRANGE (j, span3.dim[1]) {
c_FORRANGE (k, span3.dim[2])
printf(" %2g", *cspan_at(&span3, i, j, k));
printf(" |");
}
puts("");
}
printf("%g\n", *cspan_at(&span3, 4, 3, 2));
printf("%g\n", *cspan_at(&span2, 3, 2));
printf("%g\n", *cspan_at(&span1, 2));
}
}
```
Output:
```
index: 59
iterate span1: 57 58 3.14159
iterate span2: 48 49 50 51 52 53 54 55 56 57 58 3.14159
iterate span3 by dimensions:
0 1 2 | 3 4 5 | 6 7 8 | 9 10 11 |
12 13 14 | 15 16 17 | 18 19 20 | 21 22 23 |
24 25 26 | 27 28 29 | 30 31 32 | 33 34 35 |
36 37 38 | 39 40 41 | 42 43 44 | 45 46 47 |
48 49 50 | 51 52 53 | 54 55 56 | 57 58 3.14159 |
60 61 62 | 63 64 65 | 66 67 68 | 69 70 71 |
3.14159
3.14159
3.14159
```
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