foo.cpp: In function 'int read()':
foo.cpp:13:29: error: 'isdigit' was not declared in this scope
  while(!isdigit(ch=getchar()));
                             ^
foo.cpp:14:18: error: 'isdigit' was not declared in this scope
  while(isdigit(ch))
                  ^
foo.cpp: At global scope:
foo.cpp:18:6: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
 main(){
      ^