I am trying to make an expression calculator and it works ok, but only for single digit numbers. I take in the expression in infix notation and then convert it to postfix notation. I just am not sure how to allow it calculate the correct answer with double digits. I though about having a while loop on the isdigit(*p) part and combing all the numbers to a string but if the input was 15+3 postfix would be 153+ and therefore it would make the number 153 instead of 15. Hopefully some one can help me or point me in the right direction. Thanks.

```
#include <iostream>
#include <stack>
#include <string>
using namespace std;
int priority(char c) {
int rank;
if (c == '*' || c == '/')
rank = 1;
else if (c == '+' || c == '-')
rank = 2;
return rank;
}
string infixPostfix(string infix){
stack<char> st;
int i = 0;
string out;
while(infix.length() != i){
if (infix[i] == '+' || infix[i] == '-' || infix[i] == '*' || infix[i] == '/'){
while (!st.empty()){
if(priority(st.top()) <= priority(infix[i])){
out.push_back(st.top());
st.pop();
}
else break;
}
st.push(infix[i]);
}
else if (infix[i] == '(')
st.push(infix[i]);
else if (infix[i] == ')') {
while (st.top() != '(') {
out.push_back(st.top());
st.pop();
}
st.pop();
}
else
out.push_back(infix[i]);
i++;
}
while (!st.empty()){
out.push_back(st.top());
st.pop();
}
return out;
}
int evaluate(char *postfix){
char *p, t,*q;
stack <int> st;
int num1, num2, result;
p=&postfix[0];
while(*p != NULL){
while(*p == ' ')
p++;
if(isdigit(*p)){
t= *p;
q =&t;
st.push(atoi(q));
}
else{
num1 = st.top();
st.pop();
num2 = st.top();
st.pop();
switch(*p){
case '+':
result = num2 + num1;
break;
case '-':
result = num2 - num1;
break;
case '/':
result = num2 / num1;
break;
case '*':
result = num2 * num1;
break;
default:
return 0;
}
st.push(result);
}
p++;
}
result = st.top();
st.pop();
return result;
}
void main() {
string infix, postfix;
cout << "Enter an infix expression: " << endl;
getline(cin, infix);
cout << endl;
postfix = infixPostfix(infix);
cout << "Infix: " << infix << endl;
cout << "Prefix: " << endl;
cout << "Postfix: " << postfix << endl;
cout << endl;
cout << "Answer: " << evaluate(&postfix[0]) << endl;
cout << endl;
}
```