converting ASCII to Packed BCD format

on the previous program 06_BCDtoASCII.md we converted a packed BCD to ASCII. now we are going to do the reverse the conversion, we are going to take 2 ASCII values and combine them together to create a packed BCD

 
#include <avr/io.h>
#include <util/delay.h>
 
int main(void) {
    DDRA = 0xFF;
    unsigned char x = '4';
    unsigned char y = '7';
    unsigned char z;
    
    x = x - '0'; x = x << 4;
    y = y - '0';
     z = x|y;
    
    for(;;) {
        PORTA = z;
        _delay_ms(1000);
    }
    return 0;
}
 

a shorter version for this is: \

#include <avr/io.h>
#include <util/delay.h>
 
int main(void) {
    DDRA = 0xFF;
 
    for(;;) {
        PORTA = (('4'-'0') << 4) | ('7'-'0');  // compute 0x47 directly
        _delay_ms(1000);
    }
}