PIC16F877A 4x4 keypad interfacing (Simple Calculator)

/*-----------------------------------------------------/
    Name    : Mohd Syahri Fikri Bin Samusi

    [LAB02 Q05] KEYPAD 4x4 interfacing with
    PIC16F877A. A simple calculator, with
    divide by zero check.

    Date modified : 12 April 2012
/-----------------------------------------------------*/

#include <htc.h>
#include <pic16f877a.h>
#include <stdio.h>
#include <stdlib.h>
#include <usart.c>
#define _XTAL_FREQ 20000000
#include <KEYPAD_RB.c>

void init_rs232( )
{
    TRISC = TRISC | 0b10000000;
    BRGH = 1;
    SPBRG = 129;    // set baud rate: Fosc=20Mhz, BR=9600
    TX9  = 0;       // ninebits?1:0,,,8- or 9-bit transmission
    SYNC = 0;       // asynchronous
    SPEN = 1;       // enable serial port pins
    TXEN = 1;       // enable the transmitter & (automatically)TXIF=1
    CREN = 1;       // enable reception
    SREN = 0;       // no effect
    TXIE = 0;       // disable tx interrupts
    RCIE = 0;       // disable rx interrupts
}

int isamong (char a, char *s)
{
    while (*s)
    {
        if (a==*s++)
        {
            return 1;
        }
    }
    return 0;
}

void calc(char *op, long int *a, long int *b)
{
    double ans;
    int f;
    if (*op=='+')       //Performing addition
    {
        f=0;
        ans = *a + *b;
    }
    else if (*op=='-')  //Performing subtraction
    {
        f=0;
        if (*a > *b)
        {
            ans = *a - *b;
        }
        else
        {
            ans = -1*(*b - *a);
        }
    }
    else if (*op=='*')  //Performing multiplication
    {
        f=0;
        ans = *a * *b;
    }
    else if (*op=='/')  //Performing division
    {

        if(*b==0)
        {
            f=1;
        }
        else
        {
            f=0;
            ans = *a  *1.0 / *b;
        }
    }

    if(f==1)
    {
        printf ("\r\n ERROR!! Cannot divide by 0 \n");
    }
    else
    {
        printf ("\r\n%ld %c %ld \n= %f", *a, *op, *b, ans);
    }
}

void main( )
{
    char k=0;
    long int a=0, b=0;
    int c=0,c1=0, d=0, e=0, op;
    kbd_init( );                //Initialize keypad
    init_rs232( );              //Initialize serial I/O
    printf ("\fReady...\r\n");  //Show the ready message
    while (1)                   //Keep running
    {
        if (kbd_getc( )==0)
        {
            do
            {
                k=kbd_getc( );
                __delay_ms(30);
                    if (k>='0'&&k<='9')
                        c1=1;
                    else if (isamong(k,"+-/*C="))
                        c1=1;
                    else
                        c1=0;
            }
            while(c1==0);
        }

        if(k!=0)
        {
            if(k=='C')
            {
                printf ("\r\nAgain...\n");
                c=0;
                d=0;
            }
            else
            {
                if(c==2)
                {
                    printf ("\fReady...\n");
                    c=0;
                }
                if(k!='=')
                {
                    putch(k);
                    if((k>='0')&&(k<='9'))
                    {
                        if(c==0)
                        {
                            if(d==0)
                            {
                                a = k - '0';
                                d++;
                            }
                            else
                            {
                                a = (a*10)+(k-'0');
                                d++;
                            }
                        }
                        else
                        {
                            if(e==0)
                            {
                                b = k - '0';
                                e++;
                            }
                            else
                            {
                                b = (b*10)+(k-'0');
                                e++;
                            }
                        }
                    }
                    else
                    {
                        op = k;
                        c = 1;
                    }
                }
                else
                {
                    calc(&op, &a, &b);
                    c = 2;
                    d = 0;
                    e = 0;
                }
            }
            k=0;
        }
    }
}//End of Program