Thread: evaluating boolean expressions to get a result

I am writing a program to read a bunch of text and perform boolean operations on it and outputting the result. e.g "T F && T || !" should output false.I have designed my program ,written a parser to get the chars and a method to perform boolean operations.My only problem is that evaluation of expressions is not working.

Code:

struct stack{
    char  *characters;
    int top;
    
    
};
/*This function initialises and returns a stack adt
 the stack size if allocated a default value of 4 because i know that there can 
 * only be at most 2 values on the stack at any time
 
 */
struct stack * newStack(){
    struct stack * p= malloc(sizeof(struct stack));
    if(!p){
        fprintf(stderr,"out of memory\n");
    }
    
    p->top=0;
    p->characters=malloc(sizeof(char)*6); 
    
    return p;
}


/* a void method which is used to push
  * char  onto the stack
  */
 void push(char *c, struct stack *p){
    p->characters[p->top]=*c;
    p->top++;




 }
 
 /*
  a double method called pop used to
  * pop char of the stack
  */
 char * pop(struct stack *p){
     char * c;
     p->top = p->top - 1;
     c=p->characters[p->top];//assing the char the value of the index
     p->characters[p->top]=' ';  //clean up the index
   
   return c;




 }
 
 
 
/*
 takes 4 parameters
 * a=top of stack -1
 * b=top of stack -2
 * result=where the result woud be stored
 * operation= boolean operation to be performed
 
 */
void performoperation(char *a,char *b,char * result,char *operation){
   const char *t="T";
   const char * and="&";
   const  char *or="|";
   const  char *not="!";
    int i,j;
   // if (strcmp(a,t)==0)
        if (*a==*t)
          i=1;
          else
           i=0;
        if (*b==*t)
          j=1;
         else
           j=0;
   
    int jy=0;
   
    if(strcmp(operation,and)==0){
        jy=i && j;
    }
   
        else 
            if(strcmp(operation,or)==0)
          
                jy=i || j;
        
            else
                if(strcmp(operation,not)==0)
                {
                 jy= (!i);
            
        }
    if(jy==1)
        *result='T';
    else
        *result='F';
}


/*
 been stuck in this method for a while
 */


void evaluateExpression(char *expressions,struct stack * stack){
    char *a;
    char *b;
    char *res;
   
    while(*expressions!='\0'){
        if(*expressions=='T' || *expressions=='F') {//if its a t or f
            push(expressions, stack); 
      
       }
        
        else{
           
            a=pop(stack);
            b=pop(stack);
            performoperation(a,b,res,expressions);
            push(res,stack);
         
        }
           expressions++;
    }
 
    
}
void parseexpressionfromcommandline(char * unparsedexpression,char *parsedexpression){
    
    if(unparsedexpression==NULL )
        printf("does not point to any address");
    else {
        char value;  //this would hold my value 
        int pos=0;
        
       
        while(*unparsedexpression!='\0'){
          
            while(!isspace(*unparsedexpression)&& (*unparsedexpression)!='\0'){
             value=(*unparsedexpression++);
         
            }//inneer while loop
            
          
        unparsedexpression++;
        parsedexpression[pos++]=value;
      
      
        }//outer while loop
        parsedexpression[pos]='\0';
     
         
    }//outer while
    
    
}






int main(int argc, char** argv) {
    char result[13];
    char *p="T F && F || !\0";
    char *v;
    char * u;
    //char * q='u';
    struct stack *stack;
    v=result;
    stack= newStack();
   parseexpressionfromcommandline(p,v);
   printf("%s \n",v);
       printf("%u",strlen( v));
  evaluateExpression(v,stack);
 
    return (EXIT_SUCCESS);
}

Code:

performoperation(a,b,res,expressions);

My compiler MinGW GCC says "main.c|129|warning: 'res' may be used uninitialized in this function [-Wuninitialized]|"

Code:

c=p->characters[p->top];//assing the char the value of the index

main.c|49|warning: assignment makes pointer from integer without a cast [enabled by default]|

Tim S.

You've been here long enough to have 115 posts and your only description of this program is "not working"?
You can do better than that, and bumping is never okay IMHO. If you ever want to bump then you either didn't wait long enough or didn't provide enough information, or something like that.

Now, please explain the problem...

Originally Posted by sigur47

When ver i insert an item in evaluate expressionss,It pushes the item onto the stack But the second time it attempts to push onto the stack,Its not able to do so

As evidenced by what?
You're actually stating your conclusion, or assessment of the problem, but for other people what you actually need to do is to state the symptoms. If this seems hard you're probably overthinking it. Just forget you know anything about what the program does, and state I enter "blah" and it outputs "blah", or it goes into an infinite loop, or it outputs nothing, or it crashes, or whatever it does. This perhaps seems overly simple if you've been overthinking it, but it's actually a good common ground starting point to start from and ensure everyone is on the same page.

I'm just trying to help you explain things better as it will help you in the future also.

I too also think that you've quite over-used pointers here. Push for example doesn't need to be given a pointer to a char, it just needs to be given the char. Pop doesn't need to return a pointer to the char, it just needs to be given the char. Stuff like that.