Infix_2_postfix

#include<iostream>
#include<stack>                                                          // in-build stack data structure

using namespace std;

bool is_operator(char c){                                               //to check if the parameter is an operator
	if((c=='+') || (c=='-') || (c=='*') || (c=='/'))
	 return true;
	else 
	  return false;
}

int check_pre(char c){                                                  //precedence function in which precedence of the ( and ) is not considered
	
	if(c=='^')  return 3;
	
	else if((c=='*') || (c=='/')) return 2;
	
	else if((c=='+') || (c=='-')) return 1;
	 
	else return -1; 
}

string in_post(stack<char> s,string infix){
	
	string postfix;
	
	for( int i=0 ; i<infix.length() ; i++){
		
		if(( infix[i] >= 'a' && infix[i]<='z') || ( infix[i] >= 'A' && infix[i] <= 'Z'))         //If it is an alpha then we will directly add in postfix
		
		postfix+=infix[i];
		
		else if( infix[i] == '(' )                                                               //If '(' then simply push in stack
		        s.push(infix[i]);
		        
		else if( infix[i] == ')' ){
			while ((!s.empty()) && (s.top() != '(')){                                           //If ')' then pop till '(' 
				
				postfix+=s.top();
				s.pop();
				
			}
			if(s.top() == '('){
				s.pop();
			}
		}
		else if(is_operator(infix[i])){                                                         //check if it is an operator
			
			if(s.empty()){                                                                     //If stack is empty then simply push
				s.push(infix[i]);
			}
			
			else{
				if(check_pre(infix[i]) > check_pre(s.top()))                                 //If precedence is higher then simply push           
				    s.push(infix[i]);
				    
				else if(check_pre(infix[i])==check_pre(s.top()) && infix[i]=='^')            //Associtivity of exponential is right to left 
				    s.push(infix[i]);
				    
				else{
					while((!s.empty() && check_pre(infix[i]) <= check_pre(s.top()))){       //pop out till th top has lower precedence
						postfix+=s.top();
						s.pop();
					}
					s.push(infix[i]);
				}
			}
		}
	}
	while(!s.empty()){                                                                          //pop everything out till the stack is empty
		postfix+=s.top();
		s.pop();
	}
	
	
	return postfix;
}


int main(){
    
    string infix,postfix;
    
    cout<<"-----INFIX_to_POSTFIX-------"<<endl;
    cout<<"Enter an INFIX expression:";
    cin>>infix;
    stack <char> s;
    cout<<endl;
    cout<<"INFIX expression: "<<infix<<endl;
    postfix = in_post(s,infix);
    cout<<"POSTFIX expression:"<<postfix<<endl;
    
	
	return 0;
}