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I am trying to implement postfix-expression evaluation, here is my code:

#include<iostream> #include<string.h> using namespace std; template < class T > class Stack { private: T * s; int n; public: Stack(int maxn) { s = new T[maxn]; n = 0; } int empth() const { return n == 0; } void push(T item) { s[n++] = item; } int pop() { return s[--n]; } }; int main() { string a = "598+46**7+*"; int n = a.length(); Stack < int >save(n); for (int i = 0; i < n; i++) { if (a[i] == "+") save.push(save.pop() + save.pop()); if (a[i] == "*") save.push(save.pop() * save.pop()); if ((a[i] >= '0') && (a[i] <= '9')) save.push(0); while ((a[i] >= '0') && (a[i] <= '9')) save.push(10 * save.pop() + (a[i++] - '0')); } cout << save.pop() << endl; return 0; }

But I get a compile error (I am implementing it in linux (ubuntu 11.10)):

postfix.cpp:35:13: error: ISO C++ forbids comparison between pointer and integer [-fpermissive] postfix.cpp:37:10: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]

How do I fix this?

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    You are on SO long enough to know how to indent code. Also, please mark the point in your code where the error occurs. Commented Nov 23, 2011 at 7:55

2 Answers 2

Reset to default
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for(int i=0;i<n;i++){ if(a[i]=="+") save.push(save.pop()+save.pop()); if(a[i]=="*")

You need to use single quotes when you are comparing characters

for(int i=0;i<n;i++){ if(a[i]=='+') save.push(save.pop()+save.pop()); if(a[i]=='*')
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Here is link to evaluate postfix expression.

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