I have written this program for infix to postfix conversion

Question

#include<iostream>
#include<stdio.h> 
#define MAX 20
using namespace std;

char stk[MAX];
int top=-1;

void push(char c)
{
    if(top==MAX-1)
        cout<<"Overflow";
    else
    {
        stk[++top]=c;
    }
}

char pop()
{
    if(top==-1)
    {
        return '\0';
    }
    else
        return stk[top--];
}

int priority(char ch)
{
    if(ch=='(')
        return 1;
    if(ch=='+'||ch=='-')
        return 2;
    if(ch=='*'||ch=='/')
        return 3;
    if(ch=='^')
        return 4;
}

int main()
{
    char exp[35],*t,x;
    cout<<"Enter expression: ";
    fgets(exp,35,stdin);
    t=exp;
    while(*t)
    {
        if(isalnum(*t))
            cout<<*t;
        else if(*t=='(')
            push(*t);
        else if(*t==')')
        {
            while((x=pop())!='(')
                cout<<x;
        }
        else
        {
            if(priority(stk[top])>=priority(*t))
                cout<<pop();
            push(*t);
        }
        t++;
    } 
    while(top!=-1)
        cout<<pop();
    return 0;
}

The output for input:

a+b-(c+d/e) 

is

ab+cde/+
-

I don't understand why - is on a newline. I have just started learning c++ and I am trying to implement some programs I did in c using c++. The same code in c works fine. I think there are some holes in my basic c++ knowledge and I would like to fill them up.

Answer 1

std::fgets does not discard the newline in the input stream like getline would. That means exp contains "a+b-(c+d/e)\\n" and not "a+b-(c+d/e)" . You either need to remove the newline from exp , switch to cin.getline() , or stop your processing loop when it hits the newline.

Answer 2

Try to change fgets to std::cin . And use std::string instead of char* :

#include <iostream>
#include <string>

int main()
{
    string exp;
    cout << "Enter expression: ";
    std::cin >> exp;
    auto t = exp.data();
    char x;

    for(auto &ch: exp)
    {
        if(isalnum(ch))
            cout << ch;
        else if(ch == '(')
            push(ch);
        else if(ch == ')')
        {
            while((x = pop()) != '(')
                cout << x;
        }
        else
        {
            if(priority(stk[top]) >= priority(ch))
                cout << pop();
            push(ch);
        }
    }
    while(top != -1)
        cout << pop();
    return 0;
}

Answer 3

除了NathanOliver提到的'\\n'处理之外，当用户输入了if语句中未检查的任何其他字符时，函数priority()不会返回值，因此该行为可能是不确定的。