View Compilation Error

0_0_30432467_13057.cpp: In function 'int main(int, const char**)':
0_0_30432467_13057.cpp:21:58: error: 'a' was not declared in this scope
    if ( (!isalnum(input[i]) && input[i]!='_') || isdigit(a[0]))
                                                          ^
0_0_30432467_13057.cpp:24:5: error: 'flag' was not declared in this scope
     flag = 0;
     ^