C program for converting Infix expression to postfix expression

The Algorithm for converting Infix expression to postfix expression is given here

Program:

#include<stdio.h>
#define SIZE 40
char stack[SIZE];
int top=-1;

void push(char data)
{
    if(top==SIZE-1)
    {
        printf("Stack is full\n");
        return;
    }
    else
    {
        top=top+1;
        stack[top]=data;
        printf("Pushed element is %c\n",data);
    }
}

char pop()
{
    char ch;
    if(top<0)
    {
        printf("stack is empty\n");
        return;
    }
    else
    {
        ch=stack[top];
        printf("poped element is%c\n",ch);
        top=top-1;
        return(ch);
    }
}

int check_pre(char a ,char b)
{
    //operators are arranged in the array based
    //on their priority. from low to high
    char op[]={'-','+','%','/','*','(',')'};
    int i,c1=0,c2=0;
    for(i=0;i<7;i++)
    {
        if(a==op[i])
            c1=i+1;
        else if(b==op[i])
            c2=i+1;
    }
    if(c1>c2)
        return(1);
    else if(c1<c2)
        return(-1);
    else
        return(0);
}
        
                    
int main()
{
    char in_str[50],out_str[50];
    char ch,temp;
    int x=0,y=0,pre;
    printf("Enter the infix string\n");
    scanf("%s",in_str);
    ch=in_str[x];
    while(ch!='\0')
    {
        //for operand
        if((ch>='a' && ch<='z') || 
(ch<='A' && ch>='Z') || 
(ch>='0' && ch<='9'))
               out_str[y++]=ch;
        //for '(' paranthesis
        else if(ch=='(')
            push(ch);
        //for ')' parenthesis
        else if(ch==')')
        {
            temp=pop();
            while(temp!='(')
            {
                out_str[y++]=temp;
                temp=pop();
            }
          //  if(temp=='(')
               // pop();         
                
        } 
        //for operator
        else
        {
        //if the stack is empty or
        // the stack top element is '('
        //just push the operator in to the stack    
            if (top==-1 || stack[top]=='(')
                push(ch);
            else
            {
                temp=stack[top];
                //check the precedence
                pre=check_pre(ch,temp);
                if(pre<0 )
                {
                    do{                    
                    out_str[y++]=pop();
                    temp=stack[top];
}while(top!=-1 && temp!='(' &&  (check_pre(ch,temp)<0));
                    push(ch);
                }
                else
                {
                    push(ch);
                }
            }
        }
        x++;
        ch=in_str[x];
    } 
    while(top!=-1)
    {
        out_str[y++]=pop();
    }
    out_str[y]='\0';
    printf("Postfix string=%s\n",out_str);
    return;
}

Output1:
Enter the infix string
((a+b)*c-(d-e))%(f+g)
Pushed element is (
Pushed element is (
Pushed element is +
poped element is+
poped element is(
Pushed element is *
poped element is*
Pushed element is -
Pushed element is (
Pushed element is -
poped element is-
poped element is(
poped element is-
poped element is(
Pushed element is %
Pushed element is (
Pushed element is +
poped element is+
poped element is(
poped element is%
Postfix string=ab+c*de--fg+%
 Output 2:
Enter the infix string
(5*(((9+8)*(4*6))+7))
Pushed element is (
Pushed element is *
Pushed element is (
Pushed element is (
Pushed element is (
Pushed element is +
poped element is+
poped element is(
Pushed element is *
Pushed element is (
Pushed element is *
poped element is*
poped element is(
poped element is*
poped element is(
Pushed element is +
poped element is+
poped element is(
poped element is*
poped element is(
Postfix string=598+46**7+*