Segmentation error while using stacks in c++ stl?

Question

I was trying to write this code for converting infix expression to postfix expression but I'm getting a segmentation fault for some values. Code is :

    #include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <stack>
#include <bits/stdc++.h> 
using namespace std;
 int prec(char ch){
     if(ch=='^')
         return 3;
     else if(ch=='*' || ch=='/')
         return 2;
     else if(ch=='+' || ch=='-')
         return 1;
     else 
         return 0;
 }

int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    stack <char> st;
    string s;
    cin>>s;
    for(int i=0;i<21;i++){
        if((s[i]>='a' && s[i]<='z') || (s[i]>='A' && s[i]<='Z'))
            cout<<s[i];
        else{
            if( st.empty())
                st.push(s[i]);
           else  if( s[i]=='(' || prec (s[i])> prec(st.top()))
               st.push(s[i]);
            else if( s[i]==')'){
                while(st.top()!='(')
                {cout<<st.top(); st.pop();}
                st.pop();
            }
            else{
                while(prec(st.top())>=prec(s[i]))
                {
                    cout<<st.top(); st.pop();
                }
            }
        }
    }
    return 0;
}

For smaller expressions, it gives an answer but for an expression like a+b*(c^d-e)^(f+g*h)-i it gives segmentation fault.

Iterating fixed 21 times regardless of the length of string looks a bad idea. — MikeCAT
– MikeCAT, Commented Oct 22, 2020 at 19:05
Why not using for(int i=0;i<s.size();i++) in order to avoid that? — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Oct 22, 2020 at 19:08
Thou shalt not top or pop an empty std::stack. And under no circumstances should you hop on pop. — user4581301
– user4581301, Commented Oct 22, 2020 at 19:08
s[i]>='a' && s[i]<='z' -> std::islower(s[i]) and so on. Don't give up portability for no reason. Or do the whole lot in one go with std::isalpha — Bathsheba
– Bathsheba, Commented Oct 22, 2020 at 20:02

MikeCAT · Accepted Answer · 2020-10-22 19:07:46Z

2

Firstly, the iteration should be just in the strings.

This means that

for(int i=0;i<21;i++){

should be

for(int i=0;i<s.size();i++){

Secondly, you forgot to check if the stack is empty in the loop.

This means that

while(prec(st.top())>=prec(s[i]))

should be

while(!st.empty() && prec(st.top())>=prec(s[i]))

answered Oct 22, 2020 at 19:07

MikeCAT

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