execute_code

public class execute_code {
	public static int[] variables=new int[26];//need to initialize to nulls (-10000)
	public static int curr_line;
	public static String stream;

	//assumption: st is a valid cmd
	public static int determine_type_cmd(String st){
		//print
		if(st.startsWith("print(") && st.endsWith(")")){
			return 4;
		}
		//goto
		if(st.startsWith("goto ")){
			return 3;
		}
		if(st.startsWith("if(")){
			return 2;
		}
		//else - assignment
		return 1;
	}

	public static int run_line(String st, int cmd){
		cmd(st.substring(st.indexOf(':')+2, st.length()-2),cmd);
		return curr_line;
	}

	public static void cmd(String st, int cmd){
		curr_line=Integer.MIN_VALUE;
		switch(cmd){
		case(1): assignment_cmd(st, variables); break;
		case(2): if_cmd(st); break;
		case(3): curr_line=goto_cmd(st); break;
		case(4): print_cmd(st); break;
		}
	}

	//#######################################################################

	public static int compute_exp(String st){
		stream=st;
		return compute_exp_helper();
	}

	public static int compute_exp_helper(){

		String token=get_token();
		if(myclass.is_Basic_num(token))
			return Integer.parseInt(token);

		if(myclass.is_Basic_var(token)){
			int ch=token.charAt(0);
			if(variables[ch-'a']==Integer.MAX_VALUE ){
				curr_line=Integer.MAX_VALUE;///////////////runtime error
				return Integer.MAX_VALUE;
			}
			else
				return variables[ch-'a'];
		}

		//else, it is 'binary exp'
		char op=token.charAt(0);
		int left_operand, right_operand;

		left_operand=compute_exp_helper();
		right_operand=compute_exp_helper();

		switch(op){
		case('+'):return left_operand+right_operand;
		case('-'):return left_operand-right_operand;
		case('*'):return left_operand*right_operand;
		case('\\'):return left_operand/right_operand;
		default: return -1;
		}

	}

	public static String get_token(){
		int pos=stream.indexOf(" ");
		String st;

		if(pos>=0){
			st=stream.substring(0,pos);
			stream=stream.substring(pos+1);
		}

		else{
			st=stream;
			stream="";
		}
		return st;
	}

	public static int goto_cmd(String st){
		return Integer.parseInt(st.substring(st.indexOf(" ")+1));
	}

	public static void if_cmd(String st){
		boolean cond=compute_boolean_exp(st.substring(3,st.indexOf(')')));
		String cmd_st="";

		if(!cond)
			return;

		cmd_st=st.substring(st.indexOf(')')+2);
		cmd(cmd_st, determine_type_cmd(cmd_st));
	}

	static boolean compute_boolean_exp(String st) {
		String[] ops={"<=",">=","<",">","==","!="};
		int op_pos=0;
		int i;
		int left_operand,right_operand;

		i=0;
		while(i<6){
			op_pos=st.indexOf(ops[i]);
			if(op_pos>=0)// we found it
				break;
			++i;
		}

		left_operand=compute_exp(st.substring(0,op_pos-1));
		right_operand=compute_exp(st.substring(op_pos+2));

		switch(i){
		case(0): return left_operand<=right_operand;
		case(1): return left_operand>=right_operand;
		case(2): return left_operand<right_operand;
		case(3): return left_operand>right_operand;
		case(4): return left_operand==right_operand;
		case(5): return left_operand!=right_operand;
		default: return false;
		}
	}

	public static void assignment_cmd(String st, int[] variables){
		int index=st.indexOf(" := ");
		String var=st.substring(0,index);
		String exp=st.substring(index + 4);// 4 is reliant upon " := "
		variables[var.charAt(0)-'a']=compute_exp(exp);
	}

	public static void print_cmd(String st){
		String exp= st.substring(6,st.length()-1);//6 because of 'print('
		int result= compute_exp(exp);
		if(result == Integer.MAX_VALUE){
			return;
		}
		Print(compute_exp(exp));
	}

	//#######################################################################

	public static void Print( int val ) {
		System.out.println (""+val);
	}

	public static void initial_Array(){
		int i=0;
		while(i!=26){
			variables[i]=Integer.MAX_VALUE;
			++i;
		}

	}


}