Code Fragment for a DFA Scanner

• Both a next state and a current state variable are used. This gives the action code three pieces of information to work with: current state, next state, and the character just read.

• The semantic actions go between the call to the NextState() function and the setting of the current state variable.

• The character ch is read and end-of-file is tested after testing for an accepting state. We want the loop to end as soon as the DFA accepts.

• Since the loop may end without being in an accepting state, code at the end of the loop must tidy up and decide what to return if that happens.

• typedef enum { S_START, S_ACCEPT, S_ERROR, ... } State;
  
  State current, next;
  ...
  current = S_START;
  while( current != S_ACCEPT && (ch=fgetc(fp)) != EOF ){
  next = NextState(current,ch);
  if( next == S_ERROR ){
  … /* handle the failure */ …
  }
  … /* remaining semantic actions go here */ …
  current = next;
  }
  switch( current ){
  case S_START: …
  case S_ACCEPT: …
  case S_ERROR: …
  …
  default: …
  }