postfix python code example

Example 1: Postfix evaluation in c

//Assumption -- primary operators '-,+,*,/,%' operand -- a single digit
 
#include<stdio.h>
 
#define MAX 20
 
typedef struct stack
{
	int data[MAX];
	int top;
}stack;
 
void init(stack *);
int empty(stack *);
int full(stack *);
int pop(stack *);
void push(stack *,int);
int evaluate(char x,int op1,int op2);
 
int main()
{
	stack s;
	char x;
	int op1,op2,val;
	init(&s);
	printf("Enter the expression(eg: 59+3*)\nSingle digit operand and operators only:");
	
	while((x=getchar())!='\n')
	{
		if(isdigit(x))
			push(&s,x-48);		//x-48 for removing the effect of ASCII
		else
		{
			op2=pop(&s);
			op1=pop(&s);
			val=evaluate(x,op1,op2);
			push(&s,val);
		}
	}
	
	val=pop(&s);
	printf("\nValue of expression=%d",val);
 
	return 0;
}
 
int evaluate(char x,int op1,int op2)
{
	if(x=='+')
		return(op1+op2);
	if(x=='-')
		return(op1-op2);
	if(x=='*')
		return(op1*op2);
	if(x=='/')
		return(op1/op2);
	if(x=='%')
		return(op1%op2);
}
 
void init(stack *s)
{
	s->top=-1;
}
 
int empty(stack *s)
{
	if(s->top==-1)
		return(1);
	
	return(0);
}
 
int full(stack *s)
{
	if(s->top==MAX-1)
		return(1);
	
	return(0);
}
 
void push(stack *s,int x)
{
	s->top=s->top+1;
	s->data[s->top]=x;
}
 
int pop(stack *s)
{
	int x;
	x=s->data[s->top];
	s->top=s->top-1;
	
	return(x);
}

Example 2: python postfix conversion

"""
Author : ITVoyagers (itvoyagers.in)

Date :31st October 2019

Description : Program to show use of stack in infix to postfix conversion using python.
"""
class infix_to_postfix:
    precedence={'^':5,'*':4,'/':4,'+':3,'-':3,'(':2,')':1}
    def __init__(self):
        self.items=[]
        self.size=-1
    def push(self,value):
        self.items.append(value)
        self.size+=1
    def pop(self):
        if self.isempty():
            return 0
        else:
            self.size-=1
            return self.items.pop()
    def isempty(self):
        if(self.size==-1):
            return True
        else:
            return False
    def seek(self):
        if self.isempty():
            return false
        else:
            return self.items[self.size]
    def isOperand(self,i):
        if i in 'ABCDEFGHIJKLMNOPQRSTUVWXYZ':
            return True
        else:
            return False
    def infixtopostfix (self,expr):
        postfix=""
        print('postfix expression after every iteration is:')
        for i in expr:
            if(len(expr)%2==0):
                print("Incorrect infix expr")
                return False
            elif(self.isOperand(i)):
                postfix +=i
            elif(i in '+-*/^'):
                while(len(self.items)and self.precedence[i]<=self.precedence[self.seek()]):
                    postfix+=self.pop()
                self.push(i)
            elif i is '(':
                self.push(i)
            elif i is ')':
                o=self.pop()
                while o!='(':
                    postfix +=o
                    o=self.pop()
            print(postfix)
                #end of for
        while len(self.items):
            if(self.seek()=='('):
                self.pop()
            else:
                postfix+=self.pop()
        return postfix
s=infix_to_postfix()
expr=input('enter the expression ')
result=s.infixtopostfix(expr)
if (result!=False):
    print("the postfix expr of :",expr,"is",result)