View Compilation Error

0_0_26334172_19118.cpp: In function 'int to16(char*)':
0_0_26334172_19118.cpp:7:18: error: 'isdigit' was not declared in this scope
  if( isdigit(s[0]) )
                  ^
0_0_26334172_19118.cpp:12:18: error: 'isdigit' was not declared in this scope
  if( isdigit(s[1]) )
                  ^