Stack calculator - c++

Question

I found this code online so I could study stack calculators, but when I try to enter an expression for the program to evaluate, it gives me 78. Every time. I've combed through it to see if I could debug it by myself, but I have no clue how to fix this. I've added the comments myself to make things more readable. Could anyone help?
#include<iostream>
#include<stack>
#include<cmath>
using namespace std;
int pri(char ch) //precedence checking
{
switch (ch)
{
case '-':
case '+': return 1;
case '*':
case '/': return 2;
case '^': return 3;
case '(':
case ')': return 4;
default: return -1;
}
}
int calculate(char op, int l , int r) //actual calculations
{
if(op == '+') return l + r;
if(op == '-') return l - r ;
if(op == '*') return l * r;
if(op == '/')
{
if(r > 0)
{
return l/r;
}
return 0;
}
if(op == '^') return pow(l,r);
return -1;
}
int main(void)
{
string equ;
cout << "Enter an expression: ";
getline(cin, equ);
char math[equ.size()+1];
int l = sizeof(math)/sizeof(char);
stack<char> s;
stack<int> op_s;
int i = 0;
while(math[i] != '\0')
{
if(math[i] == '(')
{
s.push('(');
}
else if(math[i] == ')')
{
while(s.top() != '(')
{
int r = op_s.top();
op_s.pop();
int l = op_s.top();
op_s.pop();
int re = calculate(s.top(),l,r);
op_s.push(re);
s.pop();
}
s.pop();
}
else if(math[i] == '+' || math[i] == '-' || math[i] == '*' || math[i] == '/' || math[i] == '^')
{
int pC = pri(math[i]);
while(!s.empty() && pri(s.top()) >= pC)
{
int r = op_s.top();
op_s.pop();
int l = op_s.top();
op_s.pop();
int re = calculate(s.top(),l,r);
op_s.push(re);
s.pop();
}
s.push(math[i]);
}
else
{
op_s.push(int(math[i])- 48);
}
i++;
}
while(!s.empty())
{
int r = op_s.top();
op_s.pop();
int l = op_s.top();
op_s.pop();
int re = calculate(s.top(),l,r);
op_s.push(re);
s.pop();
}
cout <<"Result: " << op_s.top() << endl;
return 0;
}

Answer 1

OK So I didn't go through everything with a fine toothed combed, but one obvious issue is that the char values from equ are never copied into the math array.
Below I just removed the C style math array and instead loop through the char values in equ (I renamed the equ variable to math).
Also note I only tested this with 5*5, I did not do any further testing beyond that.
/** Removed for brevity**/
int main( )
{
string math{ "5*5" };
//cout << "Enter an expression: ";
//getline(cin, math);
stack<char> s;
stack<int> op_s;
for ( auto i = 0; i < math.size( ); i++ )
{
if( math[ i ] == '(' )
{
s.push( '(' );
}
else if(math[ i ] == ')')
{
while( s.top( ) != '(' )
{
int r = op_s.top();
op_s.pop();
int l = op_s.top();
op_s.pop();
int re = calculate(s.top(),l,r);
op_s.push(re);
s.pop();
}
s.pop();
}
else if(math[i] == '+' || math[i] == '-' || math[i] == '*' || math[i] == '/' || math[i] == '^')
{
int pC = pri(math[i]);
while(!s.empty() && pri(s.top()) >= pC)
{
int r = op_s.top();
op_s.pop();
int l = op_s.top();
op_s.pop();
int re = calculate(s.top(),l,r);
op_s.push(re);
s.pop();
}
s.push(math[i]);
}
else
{
op_s.push(int( math[ i ] ) - 48 );
}
}
while(!s.empty())
{
int r = op_s.top();
op_s.pop();
int l = op_s.top();
op_s.pop();
int re = calculate(s.top(),l,r);
op_s.push(re);
s.pop();
}
cout <<"Result: " << op_s.top() << endl;
return 0;
}