How do i make my BASIC interpreter in C know whats been inputed

I am trying to create a BASIC interpreter in C. I started with an interpreter for math calculation (just like a calculator except here i can give the variable X the value 2, for example). My problem is that I don't know how to make my interpreter find the difference between some inputs. For example: 10 LET x = 10, This should be stored in an array for later use. LET x = 10, this should be performed instantly 10 + 1 , this should be performed instantly.

How do i modify my interpreter so it knows these things? I don't know where to make the changes, but I think it should be made in the parser so I'm posting it here. If you want to see other code, just ask.

//diddi

    /*
     * File: parser.c
     * --------------
     * This file implements a version of ReadExp that uses
     * conventional precedence rules.  Thus, the expression
     *
     *        x = 2 * x + y
     *
     * is interpreted as if it had been written
     *
     *        x = ((2 * x) + y))
     *
     * This language can be parsed using the following ambiguous
     * grammar:
     *
     *       E  ->  T
     *       E  ->  E op E
     *
     *       T  ->  integer
     *       T  ->  identifier
     *       T  ->  ( E )
     *
     * Unfortunately, this gram开发者_运维百科mar is not sufficient by itself.  The
     * parser must also provide some way to determine what operators
     * take precedence over others.  Moreover, it must avoid the
     * problem of going into an infinite recursion of trying to read
     * an expression by reading an expression, and so on forever.
     *
     * To solve these problems, this implementation passes a numeric
     * value to the ReadE function that specifies the precedence
     * level for the current subexpression.  As long as ReadE finds
     * operators with a higher precedence, it will read in those
     * operators along with the following subexpression.  If the
     *

 precedence of the new operator is the same or lower than
 * the prevailing precedence, ReadE returns to the next higher
 * level in the recursive-descent parsing and reads the operator
 * there.
 */

#include <stdio.h>
#include <ctype.h>
#include "genlib.h"
#include "strlib.h"
#include "simpio.h"
#include "scanadt.h"
#include "parsering.h"
#include "exp.h"
#include "cmddisp.c"

/*
 * Implementation notes: ParseExp
 * ------------------------------
 * This function just calls ReadE to read an expression and then
 * checks to make sure no tokens are left over.
 */

expressionADT ParseExp(scannerADT scanner)
{
    expressionADT exp;

    exp = ReadE(scanner, 0);
    if (MoreTokensExist(scanner)) {
        Error("ParseExp: %s unexpected", ReadToken(scanner));
    }
    return (exp);
}

/*
 * Implementation notes: ReadE
 * Usage: exp = ReadE(scanner, prec);
 * ----------------------------------
 * This function reads an expression from the scanner stream,
 * stopping when it encounters an operator whose precedence is
 * less that or equal to prec.
 */

expressionADT ReadE(scannerADT scanner, int prec)
{
    expressionADT exp, rhs;
    string token;
    int newPrec;

    exp = ReadT(scanner, 0);
    while (TRUE) {
        token = ReadToken(scanner);
        newPrec = Precedence(token);
        if (newPrec <= prec) break;
        rhs = ReadE(scanner, newPrec);
        exp = NewCompoundExp(token[0], exp, rhs);
    }
    SaveToken(scanner, token);
    return (exp);
}

/*
 * Function: ReadT
 * Usage: exp = ReadT(scanner);
 * ----------------------------
 * This function reads a single term from the scanner by matching
 * the input to one of the following grammatical rules:
 *
 *       T  ->  integer
 *       T  ->  identifier
 *       T  ->  ( E )
 *
 * In each case, the first token identifies the appropriate rule.
 */

expressionADT ReadT(scannerADT scanner, int prec)
{
    expressionADT exp, rhs;
    string token;
    int newPrec;

    exp = ReadF(scanner);
    while (TRUE) {
        token = ReadToken(scanner);
        newPrec = Precedence(token);
        if (newPrec <= prec) break;
        rhs = ReadT(scanner, newPrec);
        exp = NewCompoundExp(token[0], exp, rhs);
    }
    SaveToken(scanner, token);
    return (exp);
}

int Precedence(string token)
{
    if (StringLength(token) > 1) return (0);
    switch (token[0]) {
      case '=': return (1);
      case '+': case '-': return (2);
      case '*': case '/': return (3);
      default:  return (0);
    }
}

expressionADT ReadF(scannerADT scanner)
{
    expressionADT exp;
    string token;

    token = ReadToken(scanner);
    if (StringEqual(token, "(")) {
        exp = ReadE(scanner, 0);
        if (!StringEqual(ReadToken(scanner), ")")) {
            Error("Unbalanced parentheses");
        }
    } else if (isdigit(token[0])) {
        exp = NewIntegerExp(StringToInteger(token));
    } else if (isalpha(token[0])) {
        exp = NewIdentifierExp(token);
    } else {
        Error("Illegal term in expression");
    }
    return (exp);

}

---

The easiest way is to do a step before parsing using the grammar which looks to see if there is a line number (i.e. an integer followed by a space) and then removes (remembering what it was so you can store it in your array) it and hands the remainder of the line to the parser. In most interpreters there is more than one parser working.