String Evaluation

Working code:

Code assumes that given expression will be valid and there is single digit char only to evaluate.

 

 // Evaluating a string  
 bool isOperator(char op){  
      return ( (op=='+')||( op=='-')||(op=='*')||(op=='/'));  
 }  
 int preference(char op1,char op2){  
      if(op1=='(') return -1;  
      else if(op2=='(') return 1;  
      if(op1=='+' || op1=='-'){  
           if(op2=='+'||op2=='-')  
                return 0;  
           else  
                return -1;  
      }  
      else  
           if(op2=='+'||op2=='-')  
                return 1;  
           else  
                return 0;  
 }  
 int applyOperator(int first,char op,int second)  
 {  
      int f = first;  
      int s = second;  
      switch(op){  
           case '+':  
                return f+s;  
                break;  
           case '-':  
                return f-s;  
                break;  
           case '*':  
                return f*s;  
                break;  
           case '/':  
                return f/s;  
      }  
 }  
 int evaluatePostFix(string s){  
      stack<int> st;  
      for(int i=0;i<s.length();i++){  
           if(s[i]>='0' && s[i]<='9')  
                st.push(s[i]-'0');  
           else{  
                int a = st.top();  
                st.pop();  
                int b = st.top();  
                st.pop();   
                int ans = applyOperator(b,s[i],a);  
                //cout<<ans<<" ";  
                st.push(ans);  
           }       
      }  
      return st.top();  
 }  
 string infixToPostfixConversion(string s)  
 {  
      string postfix = ""; // Final postfix conversion of given string  
      stack<char> st;  
      for(int i=0;i<s.length();i++){  
           // If digit is 0 to 9 then directly print it to output  
           if(s[i]>='0' && s[i]<='9'){  
                postfix.push_back(s[i]);  
           }  
           else   
                if(isOperator(s[i])){  
                     if(st.empty())  
                          st.push(s[i]);  
                     else  
                          if( preference(st.top(),s[i])<0)  
                               st.push(s[i]);  
                          else  
                               if(preference(st.top(),s[i])==0){  
                                    postfix.push_back(st.top());  
                                    st.pop();  
                                    st.push(s[i]);  
                               }  
                               else{  
                                    while(!st.empty() && preference(st.top(),s[i])>0){  
                                         //cout<<"in";  
                                         postfix.push_back(st.top());  
                                         st.pop();  
                                    }  
                                    st.push(s[i]);  
                               }  
                }  
                else{  
                     if(s[i]=='(')  
                          st.push('(');  
                     else{  
                          //cout<<s[i]<<" "<<st.size()<<endl;  
                          while(!st.empty() && st.top()!='('){  
                               //cout<<st.top()<<" ";  
                               postfix.push_back(st.top());  
                               st.pop();  
                          }  
                          st.pop();  
                     }            
                }  
      }  
      while(! st.empty())  
      {  
           postfix.push_back(st.top());  
           st.pop();  
      }  
      //cout<<postfix<<endl;  
      return postfix;  
 }  
 int eval(string s){  
      string s1 = infixToPostfixConversion(s);  
      cout<<"Converted String is "<<s1<<endl;  
      return evaluatePostFix(s1);  
 }  
 int main()  
 {  
      cout<<eval("5*(2+(3*4*5)+5/6)")<<endl;;  
      cout<<eval("1+2+3*(4+5)*9+4-5*2");  
      return 0;  
 }