Language: C++

#include <bits/stdc++.h>
using namespace std;
int main() {
  //入力
  string H, W;
  cin >> H >> W;
  //一つの文字列に
  string s;
  for(int i = 0; i < H; i++) {
    string f;
    cin >> f;
    s += f;
  }
  //二重ループでＡ、数える Ｂ、入れ替え
  for(int i = 0; i < H; i++) {
    for(int j = 0; j < W; j++) {
      //Ａ、数える
      //１、四隅 ２、国構え ３、中身  s.at(i*W+j)が基本
      int count = 0;
      //１、四隅 左上 ３か所をチェック
      if(i==0 && j==0) {
        if(s.at(1) == '#') count++;
        if(s.at(W) == '#') count++;
        if(s.at(W+1) == '#') count++;
      }
      //１、四隅 右上
      if(i==0 && j==W-1) {
        if(s.at(j-1) == '#') count++;
        if(s.at(W+j-1) == '#') count++;
        if(s.at(W+j) == '#') count++;
      }
      //１、四隅 左下
      if(i==H-1 && j==0) {
        if(s.at((i-1)*W) == '#') count++;
        if(s.at((i-1)*W+1) == '#') count++;
        if(s.at(i*W+1) == '#') count++;
      }
      //１、四隅 右下
      if(i==H-1 && j==W-1) {
        if(s.at((i-1)*W+j-1) == '#') count++;
        if(s.at((i-1)*W+j) == '#') count++;
        if(s.at(i*W-1) == '#') count++;
      }
      //２、国構え 上 ５か所をチェック s.at(i*W+j)が基本
      if(i==0 && j!=0 && j!=W-1) {
        if(s.at(j-1) == '#') count++;
        if(s.at(j+1) == '#') count++;
        if(s.at(W+j-1) == '#') count++;
        if(s.at(W+j) == '#') count++;
        if(s.at(W+j+1) == '#') count++
      }
      //２、国構え 下
      if(i==H-1 && j!=0 && j!=W-1) {
        if(s.at((i-1)*W+j-1) == '#') count++;
        if(s.at((i-1)*W+j) == '#') count++;
        if(s.at((i-1)*W+j+1) == '#') count++;
        if(s.at(i*W+j-1) == '#') count++;
        if(s.at(i*W+j+1) == '#') count++
      }
      //２、国構え 左
      if(j==0 && i!=0 && i!=H-1) {
        if(s.at((i-1)*W) == '#') count++;
        if(s.at((i-1)*W+1) == '#') count++;
        if(s.at(i*W+1) == '#') count++;
        if(s.at((i+1)*W) == '#') count++;
        if(s.at((i+1)*W+1) == '#') count++;
      }
      //２、国構え 右
      if(j==W-1 && i!=0 && i!=H-1) {
        if(s.at((i-1)*W+j-1) == '#') count++;
        if(s.at((i-1)*W+j) == '#') count++;
        if(s.at(i*W+j-1) == '#') count++;
        if(s.at((i+1)*W+j-1) == '#') count++;
        if(s.at((i+1)*W+j) == '#') count++;
      }
      //３、中身 ８か所をチェック s.at(i*W+j)が基本
      if(i!=0 && i!=H-1 && j!=0 && j!=W-1) {
        if(s.at((i-1)*W+j-1) == '#') count++;
        if(s.at((i-1)*W+j) == '#') count++;
        if(s.at((i-1)*W+j+1 == '#') count++;
        if(s.at(i*W+j-1) =='#') count++;
        if(s.at(i*W+j+1) =='#') count++;
        if(s.at((i+1)*W+j-1) =='#') count++;
        if(s.at((i+1)*W+j) =='#') count++;
        if(s.at((i+1)*W+j+1) =='#') count++;
      }
      //Ｂ、入れ替え   ＃、１～８、０の順番
      if(s.at(i*W+j) == '#') s.at(i*W+j) = '#';
      else if(count == 1) s.at(i*W+j) = '1';
      else if(count == 2) s.at(i*W+j) = '2';
      else if(count == 3) s.at(i*W+j) = '3';
      else if(count == 4) s.at(i*W+j) = '4';
      else if(count == 5) s.at(i*W+j) = '5';
      else if(count == 6) s.at(i*W+j) = '6';
      else if(count == 7) s.at(i*W+j) = '7';
      else if(count == 8) s.at(i*W+j) = '8';
      else if(s.at(i*W+j) == '.') s.at(i*W+j) = '0';
    }
  }
  //改行して出力
  for(int i = 0; i < H; i++) {
    for(int j = 0; j < W; J++) {
      cout << s.at(i*W+j);
      if(j == W-1)
        cout << endl;
    }
  }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:9:20: error: no match for 'operator<' (operand types are 'int' and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
    9 |   for(int i = 0; i < H; i++) {
      |                  ~ ^ ~
      |                  |   |
      |                  int std::string {aka std::__cxx11::basic_string<char>}
In file included from /usr/include/c++/14/regex:68,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:181,
                 from a.cc:1:
/usr/include/c++/14/bits/regex.h:1143:5: note: candidate: 'template<class _BiIter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const sub_match<_BiIter>&)'
 1143 |     operator<(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1143:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/regex.h:1224:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&, const sub_match<_BiIter>&)'
 1224 |     operator<(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1224:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/regex.h:1317:5: note: candidate: 'template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(const sub_match<_BiIter>&, __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&)'
 1317 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1317:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/regex.h:1391:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type*, const sub_match<_BiIter>&)'
 1391 |     operator<(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1391:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/regex.h:1485:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)'
 1485 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1485:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/regex.h:1560:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type&, const sub_match<_BiIter>&)'
 1560 |     operator<(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1560:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::__cxx11::sub_match<_BiIter>'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/regex.h:1660:5: note: candidate: 'template<class _Bi_iter> bool std::__cxx11::operator<(const sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)'
 1660 |     operator<(const sub_match<_Bi_iter>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/regex.h:1660:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/usr/include/c++/14/bits/stl_pair.h:1045:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator<(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1045 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1045:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'const std::pair<_T1, _T2>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:67:
/usr/include/c++/14/bits/stl_iterator.h:448:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  448 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:448:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/stl_iterator.h:493:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  493 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1694 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1760 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52:
/usr/include/c++/14/string_view:673:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  673 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:673:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/string_view:680:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  680 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:680:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/string_view:688:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  688 |     operator< (__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:688:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/basic_string.h:3874:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3874 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3874:5: note:   template argument deduction/substitution failed:
a.cc:9:22: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'int'
    9 |   for(int i = 0; i < H; i++) {
      |                      ^
/usr/include/c++/14/bits/basic_string.h:3888:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3888