foo.cc: In function 'int main()':
foo.cc:8:20: warning: comparison of integer expressions of different signedness: 'int' and 'size_t' {aka 'long unsigned int'} [-Wsign-compare]
    8 |  for (int i = 0; i < strlen(s); i++)
      |                  ~~^~~~~~~~~~~
foo.cc:10:14: error: 'a' was not declared in this scope
   10 |   if ('a' <= a[i] && a[i] <= 'z')
      |              ^
foo.cc:19:20: warning: comparison of integer expressions of different signedness: 'int' and 'size_t' {aka 'long unsigned int'} [-Wsign-compare]
   19 |  for (int i = 0; i < strlen(s); i++)
      |                  ~~^~~~~~~~~~~

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