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Difference between revisions of "cpp/container/span/subspan"

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< cpp‎ | container‎ | span
m (~)
(∃:+)
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=== Return value ===
 
=== Return value ===
 
The requested subspan {{tt|r}}, such that {{c|1=r.data() == this->data() + Offset}}. If {{tt|Count}} is {{tt|std::dynamic_extent}}, {{c|1=r.size() == this->size() - Offset}}; otherwise {{c|1=r.size() == Count}}.
 
The requested subspan {{tt|r}}, such that {{c|1=r.data() == this->data() + Offset}}. If {{tt|Count}} is {{tt|std::dynamic_extent}}, {{c|1=r.size() == this->size() - Offset}}; otherwise {{c|1=r.size() == Count}}.
 +
 +
===Example===
 +
{{example |
 +
| code=
 +
#include <algorithm>
 +
#include <cstdio>
 +
#include <ranges>
 +
#include <span>
 +
 +
void display(std::span<const char> abc)
 +
{
 +
    const auto columns{ 20U };
 +
    const auto rows{ abc.size() - columns + 1 };
 +
 +
    for (auto offset{ 0U }; offset < rows; ++offset) {
 +
        std::ranges::for_each(
 +
            abc.subspan(offset, columns),
 +
            putchar
 +
        );
 +
        putchar('\n');
 +
    }
 +
}
 +
 +
int main()
 +
{
 +
    char abc[26];
 +
    std::iota(std::begin(abc), std::end(abc), 'A');
 +
    display(abc);
 +
}
 +
| output=
 +
ABCDEFGHIJKLMNOPQRST
 +
BCDEFGHIJKLMNOPQRSTU
 +
CDEFGHIJKLMNOPQRSTUV
 +
DEFGHIJKLMNOPQRSTUVW
 +
EFGHIJKLMNOPQRSTUVWX
 +
FGHIJKLMNOPQRSTUVWXY
 +
GHIJKLMNOPQRSTUVWXYZ
 +
}}
  
 
=== See also ===
 
=== See also ===

Revision as of 16:05, 1 June 2020

 
C++
 
Containers library
 
std::span
 
template< std::size_t Offset,

          std::size_t Count = std::dynamic_extent >

constexpr std::span<element_type, E /* see below */> subspan() const;
 (1)
constexpr std::span<element_type, std::dynamic_extent>

    subspan( std::size_t Offset,

             std::size_t Count = std::dynamic_extent ) const;
 (2)

Obtains a span that is a view over the Count elements of this span starting at offset Offset. If Count is std::dynamic_extent, the number of elements in the subspan is size() - offset (i.e., it ends at the end of *this.).

(1) is ill-formed if

  • Offset is greater than Extent, or
  • Count is not std::dynamic_extent and Count is greater than Extent - Offset.

The behavior is undefined if either Offset or Count is out of range. This happens if

  • Offset is greater than size(), or
  • Count is not std::dynamic_extent and Count is greater than size() - Offset.

The extent E of the span returned by (1) is determined as follows:

  • If Count is not std::dynamic_extent, Count;
  • Otherwise, if Extent is not std::dynamic_extent, Extent - Offset;
  • Otherwise, std::dynamic_extent.

Return value

The requested subspan r, such that r.data() == this->data() + Offset. If Count is std::dynamic_extent, r.size() == this->size() - Offset; otherwise r.size() == Count.

Example

Run this code
#include <algorithm>
#include <cstdio>
#include <ranges>
#include <span>
 
void display(std::span<const char> abc)
{
    const auto columns{ 20U };
    const auto rows{ abc.size() - columns + 1 };
 
    for (auto offset{ 0U }; offset < rows; ++offset) {
        std::ranges::for_each(
            abc.subspan(offset, columns), 
            putchar
        );
        putchar('\n');
    }
}
 
int main()
{
    char abc[26];
    std::iota(std::begin(abc), std::end(abc), 'A');
    display(abc);
}

Output: 

ABCDEFGHIJKLMNOPQRST
BCDEFGHIJKLMNOPQRSTU
CDEFGHIJKLMNOPQRSTUV
DEFGHIJKLMNOPQRSTUVW
EFGHIJKLMNOPQRSTUVWX
FGHIJKLMNOPQRSTUVWXY
GHIJKLMNOPQRSTUVWXYZ

See also

obtains a subspan consisting of the first N elements of the sequence
(public member function) [edit]
obtains a subspan consisting of the last N elements of the sequence
(public member function) [edit]
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