<code lang="c">#include <stdlib.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>

#define DEL		1
#define VAR		2
#define NR		3

char token[80];
char tok_type;
char *prog;

double vars[26]={0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};

void asg(int *ans);
void exp(int *ans);
void atom(int *ans);
void get_token(void);
int isdelim(char c);
int find_var(char *s);


void main(void)
{
	int ans;
	char *p;
	
	p=malloc(100);

	do 
	{ 
		prog=p;
		printf("enter expression: ");
		gets(prog);
		asg(&ans);
		printf("answer is: %d\n",ans);
	} while (*p);
}

void asg(int *ans)
{
	int temp, slot;
	char op;
	
	get_token();
	
	if(tok_type==VAR)
	{
		slot=toupper(*token)-'A';
		get_token();
		get_token();
		exp(ans);
		vars[slot]=*ans;
		return;
	 }
	exp(ans);
}

void exp(int *ans)
{
	char op;
	int temp;
	
	atom(ans);
	while((op=*token)=='+' || op=='-')
	{
		get_token();
		atom(&temp);
		switch(op)
		{ 
			case '-':
				*ans=*ans-temp;
				break;
				
			case '+':
				*ans=*ans+temp;
				break;
		}
	}
}

void atom(int *ans)
{
	switch(tok_type)
	{ 
		case VAR:
			*ans=find_var(token);
			get_token();
			return;
		case NR:
			*ans=atoi(token);
			get_token();
			return;
	}
}

void get_token(void)
{
	char *temp;
	
	tok_type=0;
	temp=token;
	*temp='\0';
	
	if (!*prog) return;
	while(isspace(*prog)) ++prog;
	
	if (strchr ("+-*/=()", *prog))
	{
		tok_type=DEL;
		*temp++=*prog++;}
	else if(isalpha(*prog))
	{
		while(!isdelim(*prog)) *temp++=*prog++;
		tok_type=VAR;}
	else if(isdigit(*prog))
	{
		while(!isdelim(*prog)) *temp++=*prog++;
		tok_type=NR;
	}
	*temp='\0';
}

isdelim(char c)
{
	if (strchr (" +-/*=()", c) || c==9 || c=='\r' || c==0)
		return 1;
	return 0;
}

int find_var(char *s)
{   
	if(!isalpha(*s)) return 0;
    	return vars[toupper(*token)-'A'];
}
</code>