Question

I want to build simple parse tree/but it’s not working. I think that strlen is returning the wrong result.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <conio.h>

int min(int a ,int b){ return (a>b)?b:a;}

class Node
{
    Node* left;
    Node* right;
    char operation;

public:
    float res;

    Node(char* str)
    {
        char* ch = NULL;
        ch = strrchr2(str);
        if (ch==NULL)
        {
            if(str[0]=='(')
            {
                str = &str[1]; 
                int k = strlen(str);
                char *tmp = new char[k-1];

                strncpy(tmp,str,k-1);
                ch = strrchr2(tmp);

                div(tmp,ch);
            }
            else
                res = atof(str);                
        }
        else
        {
            div(str,ch);
        }
    }

    void div(char* str,char* ch)
    {
            //getting opertion
            strncpy(&operation,ch,1);

            //getting position of operation
            int pos = strlen(str)-strlen(ch);

            //right node creation
            ch = &ch[1];
            right = new Node(ch);

            //left node creation
            char* tmp = new char[pos];
            strncpy(tmp,str,pos);
            left = new Node(tmp);

            //applying opertion to to node's results
            switch( operation )
            {
                case '/':this->res = left->res/right->res;break;
                case '*':this->res = left->res*right->res;break;
                case '+':this->res = left->res+right->res;break;
                case '-':this->res = left->res-right->res;break;
                default:break;
            };
    }

    char* strrchr2(char* s)
    {
        char* str = new char[strlen(s)];
        strcpy(str,s);
        int i = search(str);
        if(i==-1) return NULL;
            else
        if(str[i]=='+' || str[i]=='-') return &str[i];
            else if (str[i] == '*' || str[i]=='/')
            {
                char c = str[i+1];
                if(i+1<strlen(str)-1 && search(&c)!=-1) return strrchr2(&c);
                else return &str[i];
            }
    }

    int search(char* str)
    {
        int cnt1 = 0;
        int cnt2 = 0;
        for(int i=0;i<strlen(str);i++)
        {
            if (str[i]=='(') cnt1++;
            else if (str[i]==')') cnt2++;
            if (cnt1!=0)
            {
               if(cnt1==cnt2)
               {
                  int m = strlen(str);
                  return ((i+1)!=m?i+1:-1);
               }
            }
            else
            {
                if (str[i]=='-' || str[i]=='+' || str[i]=='*' || str[i]=='/') return i;
            }
        }
        char* ch = NULL;
        ch = strchr(str,'-');
        if(ch==NULL) ch = strchr(str,'+');
        if(ch==NULL) ch = strchr(str,'*');
        if(ch==NULL) ch = strchr(str,'/');
        if(ch==NULL)return -1;
        else return strlen(str)-strlen(ch);
    }
};

int main()
{
    char* expr = "(1/2)-(1/2)";
    Node* n = new Node(expr);
    printf("%s = %.3f",expr,n->res);



    _getch();
}

Answer 1

It seems like you are doing C with class more than C++.
Try using std::string insted of char *. This way you won’t have any trouble with strlen (if strlen is really the problem) because string has a size() function and it is garantied to work very well. Most of all you’ll be able to use many features that you are trying to re-code there.

More informations about strings : http://www.cplusplus.com/reference/string/string/

M.Dijsktra also created an algorithm to parse mathematical expressions and output an AST (Abstract Syntax Tree). It will allow you to deal with more complex expression. It’s called the ‘shunting yard algorithm’. Here it is on wikipedia : http://en.wikipedia.org/wiki/Shunting-yard_algorithm

I thought you might want to know what’s already been done . 🙂