Infix Evaluation

#include<stdio.h>
#include<stdlib.h>
#include<ctype.h>
#include<string.h>
//global variables
int numbers[50],tn=-1,to=-1;
char op[50];
//function to push digits
void push_num(int n)
{
    numbers[++tn] = n;
}
//function to push operators
void push_op(char ch)
{
    op[++to]=ch;
}
//function to pop digits
int pop_num()
{
    return numbers[tn--];
}
//function to pop operators
char pop_op()
{
    return op[to--];
}
//evaluating the expression
int infix_eval(int numbers[50],char op[50])
{
    int x,y;
    char ope;
//taking first two operands
    x=pop_num();
    y=pop_num();
//taking the operators between them
    ope=pop_op();
//executing the operation
    switch(ope)
    {
    case '+':
        return x+y;
    case '-':
        return y-x;
    case '*':
        return x*y;
    case '/':
        if(x==0)
        {
            printf("\nCan not divide by 0");
            exit(0);
        }
        else
            return y/x;
    }
    return 0;
}
//function to check whether the character is an operator or no
int is_operator(char ch)
{
    return (ch=='+' || ch =='-' || ch=='*' || ch=='/' || ch=='^');
}

int precedence(char c)
{
    switch(c)
    {
    case '+':
        return 1;
    case '-':
        return 1;
    case '*':
        return 2;
    case '/':
        return 2;
    case '^':
        return 3;
    }
    return -1;
}

int eval(char exp[20])
{
    int i,num,output,r;
    char c;
    for(i=0; exp[i]!='\0'; i++)
    {
//taking each character from the expression
        c = exp[i];
//checking if it is number
        if(isdigit(c)!=0)
        {
            num = 0;
            while (isdigit(c))
            {
                num = num*10 + (c-'0');
                i++;
                if(i <strlen(exp))
                    c = exp[i];
                else
                    break;
            } i
            --;
//pushing the number into stack of numbers
            push_num(num);
        }
        else if(c=='(')
        {
//pushing the operators into the stack
            push_op(c);
        }
        //If we get close bracket, evaluate the entire bracket
        else if(c==')')
        {
            while(op[to]!='(')
            {
                r = infix_eval(numbers, op);
//pushing the result back to stack
                push_num(r);
            }
            pop_op();
        }
        // if the current character is operator
        else if(is_operator(c))
        {
            while(to!=-1 && precedence(c)<=precedence(op[to]))
            {
                output = infix_eval(numbers, op);
                push_num(output);
            }
            push_op(c);
        }
    }
    while(to!=-1)
    {
        output = infix_eval(numbers, op);
//pushing it back to stack
        push_num(output);
    }
    return pop_num();
}


int main()
{
    char exp[50];
    printf("Enter the infix expression to evaluate: ");
    gets(exp);

    char g;
    printf("Enter the value of X: ");
    scanf("%c", &g);

    for (int i=0; exp[i]!='\0'; i++)
    {
        if (exp[i] == 'x' || exp[i] == 'X')
            exp[i] = g;
    }
    printf("%d",eval(exp));
}