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PostfixEvaluation.cpp
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PostfixEvaluation.cpp
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#include<iostream>
#include<stack>
#include<math.h>
using namespace std;
int postfixEvaluation(string s)
{
stack<int> st;
// we will traverse the string from starting index
for(int i=0;i<s.length();i++)
{
if(s[i]>='0' && s[i]<='9')
{
// To convert a character that is integer simply - 0 from it.
// why? - because the ASCII value of the character - the ASCII value of 0 will be the integer
st.push(s[i]-'0');
}
else{
// Suppose that postfix is 46+29-9*
// 4 will be stored in stack first then 6
// therefore the value 6 is at top of the stack but in above postfix expression it is the second operand
// that's why it is stored in "op2" - Do not get confused here
int op2=st.top(); // 6 is stored in op2
st.pop(); // 6 is popped now top() has 4 in the stack
int op1=st.top(); // stored 4 in op1
st.pop(); // popped 4
// Here whatever the operator will be it will perform the operation and will push the answer in the stack.
switch(s[i])
{
case '+':
st.push(op1+op2);
break;
case '-':
st.push(op1-op2);
break;
case '*':
st.push(op1*op2);
break;
case '/':
st.push(op1/op2);
break;
case '^':
st.push(pow(op1,op2));
break;
}
}
}
return st.top();
}
int main()
{
cout<<postfixEvaluation("46+2/5*7+")<<endl;
return 0;
}