View Compilation Error

0_0_14928869_31251.cpp: In function 'int main()':
0_0_14928869_31251.cpp:10:35: error: 'isalpha' was not declared in this scope
   if (s[0] != '_' && !isalpha(s[0]))
                                   ^
0_0_14928869_31251.cpp:17:28: error: 'strlen' was not declared in this scope
    for (i = 1; i < strlen(s); i++)
                            ^
0_0_14928869_31251.cpp:19:22: error: 'isalnum' was not declared in this scope
     if (!isalnum(s[i]) && s[i] != '_')
                      ^
0_0_14928869_31251.cpp:25:21: error: 'strlen' was not declared in this scope
    if (i == strlen(s))
                     ^