Implement a compiler and interpreter for a programming language where each program consists of a single arithmetic expression preceded by a sequence of assignment statements with arithmetic expressions involving integers and variables named with single lower-case characters. For example, given the input
(x = 1)
(y = (x + 1))
(((x + y) * 3) + (4 * x))
your program should print the value 13.

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Difficulty level

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This exercise is mostly suitable for LEVEL 3 students

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 void printList2(matrix *node) {
	matrix *temp, *curr;
	matrix *head ;
	if(node)
	{
	head=node->right;
	printf("\n%d , %d ;\n", node->data.row, node->data.col);
	for (curr = head->right; curr != head; curr = curr->right)
		printf("%d , %d , %d ;\n", curr->data.row, curr->data.col, curr->data.value);
	printf("\n%d , %d ;\n", node->data.row, node->data.col);
	for (temp = head->down; temp != head; temp = temp->down)
	printf("%d , %d , %d ;\n", temp->data.row, temp->data.col, temp->data.value);
	}
}
int compare(matrix *node, matrix *nodeA) {
   //printList2(node);
    //printList2(nodeA);
	matrix *temp, *curr; 
	matrix *tempA, *currA; 
	matrix *head ;
	matrix *headA ;
	
	if(node)
	{
	    head=node->right;
	    headA=nodeA->right;
	    if(node->data.row!=nodeA->data.row || node->data.col!=nodeA->data.col) return 0;
	    
	    curr = head->right;
	    currA= headA->right;
	    while(1)
	    {
	        if(curr==head && currA==headA) break;

	        if((curr==head && currA!=headA) || (curr!=head && currA==headA) )
	        {
	            return 0;
	        }
	       
	       if((curr->data.row!=currA->data.row) || (curr->data.col!=currA->data.col)|| (curr->data.value!=currA->data.value)) return 0;
		    curr = curr->right;
		    currA = currA->right;
		    
	    }
	    
	     
	    if(node->data.row!=nodeA->data.row || node->data.col!=nodeA->data.col) return 0;
        
 
	    temp = head->down;
	    tempA = headA->down;
	   	 while(1)
	    {
	        if(temp==head && tempA==headA) break;

	        if((temp==head && tempA!=headA) || (temp!=head && tempA==headA) )
	        {
	            return 0;
	        }
	            
	       if((temp->data.row!=tempA->data.row) || (temp->data.col!=tempA->data.col)|| (temp->data.value!=tempA->data.value)) return 0;
		    temp = temp->down;
		    tempA = tempA->down;
	    }
	}

	if((node==NULL && nodeA==NULL) || (node && nodeA) )
	    return 1;

	 return 0;
}
#include <stdio.h>
#include <stdlib.h>
#include<string.h>
#include<limits.h>
#define N 20
 
#include<cstdio>
#include<cstdlib>
#include<string>
typedef char element[LINE_MAX];

typedef struct {
    element data[N];
    int top;
} stack;

stack CreateStack()
{
    stack p;
    p.top = -1;
    return p;
}

int isEmptyStack(stack p)
{
    return (p.top == -1);
}

int isFullStack(stack p)
{
    return (p.top == N - 1);
}

int Push(stack *p, element e)
{
    if (isFullStack(*p)) return 0;
    strcpy(p->data[++p->top] , e);
    return 1;
}

int Pop(stack *p)
{
    if (isEmptyStack(*p)) return 0;
    p->top--;
    return 1;
}

int Top(stack p, element *e)
{
    if (isEmptyStack(p)) return 0;
    strcpy(*e , p.data[p.top]);
    return 1;
}

int toint(element e)
{
    int i;
    int value=0, mult=1;

    for(i=strlen(e)-1;i>0;i--,mult*=10)
        value+=(e[i]-'0')*mult;
    //treat first element maybe it's -
    
    if(e[0]=='-')
        return -value;
    value+=(e[0]-'0')*mult;
    return value;
}

int isvariable(element e)
{
    return e[0]>='a' && e[0]<='z';
}

 
int is_digit(element e)
{
    
    return e[0]>='0' && e[0]<='9';
}

int evaluate()
{
    char str[LINE_MAX];
    stack s;
    int i ,value, mult;
    element e1,op,e2;
    int v[26];
    int v1, v2;
    
    s=CreateStack();

    while (fgets(str,LINE_MAX,stdin) != NULL)
    {
        str[strcspn(str, "\r\n")] = 0;
        //printf("%s\n",str);
        for(i=0;str[i];i++)
        {
        
            if(str[i]==')')
            {
                
                if(Top(s,&e2) && isvariable(e2))
                {
                    Pop(&s);
                }
                else
                {
                    v2=0;
                    mult=1;
                
                    while(Top(s,&e2) ) 
                    {
                        if(is_digit(e2) || is_digit(e2+1))
                        {
                            v2+=toint(e2)*mult; mult*=10;
                            Pop(&s);
                      
                        }
                        else
                            break;
                    }
                    //printf("##v2=%d##\n",v2);
                }
                
 
                Top(s,&op);Pop(&s);
                
                
                if(Top(s,&e1) && isvariable(e1))
                {
                    Pop(&s);
                }
                else
                {
                    v1=0;
                    mult=1;
                
                    while(Top(s,&e1) ) 
                    {
                        if(is_digit(e1) || *e1=='-')
                        {
                            v1+=toint(e1)*mult; mult*=10;
                            Pop(&s);
                      
                        }
                        else
                            break;
                    }
                }
                
            
               
                Pop(&s); // remove (
                //printf("%d  %d\n",v1 , v2);
                
                if(*op=='=') 
                {
                    
                    v[*e1-'a'] = v2;
                    
                    //printf("e1=%c v2=%d \n",e1[0],v2);
                    
                }
                else
                {
                    if(isvariable(e1)) v1=v[*e1-'a']; 
                    if(isvariable(e2)) v2=v[*e2-'a']; 
                  //printf("%d  %d\n",v1 , v2);
                    if(*op=='+') value = v1+v2;
                    if(*op=='-') value =v1-v2;
                    if(*op=='*') value =v1*v2;
    
                     //printf("op=%c v1=%d v2=%d value=%d\n",*op, v1,v2,value);
                    sprintf (e1, "%d",value);  

                    Push(&s,e1);
                    
                }
                
            }
            else
            {
                e1[0]=str[i];
                e1[1]='\0';

               Push(&s,e1);
               
            }
        }
        
        
        
        if(Top(s,&e1) && Pop(&s))
            return toint(e1);
    }
    return 0;
}

int main(){
    printf("%d",evaluate());
    return 0;
    
}

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Looking for a more challenging exercise, try this one !!
Implementation of a set using an array of long integers as bits

Mistakes may occur. If, while solving, you come across any mistakes with the codes, please, don't hesitate to contact me so that we may correct it together, creating a better programming community.
All proposed suggestions are most welcomed. !!!