Calculator Program using stack - c

Question

I am writing a calculator program in c using stack, In below program I used concept of infix to postfix conversion and next postfix evaluation.
I am getting correct answer for 1+2 answer is 3 but for 11+1 or any two and more digit i am getting wrong answer.
Can anyone help me what I will include in my code so that it work for more than two digit like 28+25 or any?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define SIZE 50 /* Size of Stack */
int top = -1;
char pofx[50];
char s[SIZE];
int infix_to_postfix() {
char infx[50], ch;
int i = 0, k = 0;
void push(char elem) { /* Function for PUSH operation */
s[++top] = elem;
}
char pop() { /* Function for POP operation */
return (s[top--]);
}
int pr(char elem) { /* Function for precedence */
switch (elem) {
case '#':
return 0;
case '(':
return 1;
case '+':
case '-':
return 2;
case '*':
case '/':
return 3;
}
return -1;
}
printf("\n\nEnter a Value to calculate : ");
gets(infx);
push('#');
while ((ch = infx[i++]) != '\0') {
if (ch == '(') push(ch);
else if (isalnum(ch)) pofx[k++] = ch;
else if (ch == ')') {
while (s[top] != '(')
pofx[k++] = pop();
char elem = pop(); /* Remove ( */
} else { /* Operator */
while (pr(s[top]) >= pr(ch))
pofx[k++] = pop();
push(ch);
}
}
while (s[top] != '#') /* Pop from stack till empty */
pofx[k++] = pop();
pofx[k] = '\0'; /* Make pofx as valid string */
printf("\n\nGiven Infix Expn: %s Postfix Expn: %s\n", infx, pofx);
return (int) pofx[k];
}
void postfix_evaluate() {
char ch;
int i = 0, op1, op2;
void pushit(int elem) { /* Function for PUSH operation */
s[++top] = elem;
}
int popit() { /* Function for POP operation */
return (s[top--]);
}
infix_to_postfix();
while ((ch = pofx[i++]) != '\0') {
if (isdigit(ch)) pushit(ch - '0'); /* Push the operand */
else { /* Operator,pop two operands */
op2 = popit();
op1 = popit();
switch (ch) {
case '+':
pushit(op1 + op2);
break;
case '-':
pushit(op1 - op2);
break;
case '*':
pushit(op1 * op2);
break;
case '/':
pushit(op1 / op2);
break;
}
}
}
printf("\n Given Postfix Expn: %s\n", pofx);
printf("\n Result after Evaluation: %d\n", s[top]);
}
int main() {
postfix_evaluate();
return 0;
}

Answer 1

part of my own code that might be useful:
if (isdigit(gi.n.nch))
{
gi.x = chr2num(gi.n.nch);
gi.n= nextchar( gi.n, len, instr);
while(isdigit(gi.n.nch))
{
gi.x *= 10;
gi.x += chr2num(gi.n.nch);
gi.n= nextchar( gi.n, len, instr);
}
}