Decimal to BCD to ASCII - c

Question

Perhaps this task is a bit more complicated than what I've written below, but the code that follows is my take on decimal to BCD. The task is to take in a decimal number, convert it to BCD and then to ASCII so that it can be displayed on a microcontroller. As far as I'm aware the code works sufficiently for the basic operation of converting to BCD however I'm stuck when it comes to converting this into ASCII. The overall output is ASCII so that an incremented value can be displayed on an LCD.
My code so far:
int dec2bin(int a){ //Decimal to binary function
int bin;
int i =1;
while (a!=0){
bin+=(a%2)*i;
i*=10;
a/=2;
}
return bin;
}
unsigned int ConverttoBCD(int val){
unsigned int unit = 0;
unsigned int ten = 0;
unsigned int hundred = 0;
hundred = (val/100);
ten = ((val-hundred*100)/10);
unit = (val-(hundred*100+ten*10));
uint8_t ret1 = dec2bin(unit);
uint8_t ret2 = dec2bin((ten)<<4);
uint8_t ret3 = dec2bin((hundred)<<8);
return(ret3+ret2+ret1);
}

Answer 1

The idea to convert to BCD for an ASCII representation of a number is actually the "correct one". Given BCD, you only need to add '0' to each digit for getting the corresponding ASCII value.
But your code has several problems. The most important one is that you try to stuff a value shifted left by 8 bits in an 8bit type. This can never work, those 8 bits will be zero, think about it! Then I absolutely do not understand what your dec2bin() function is supposed to do.
So I'll present you one possible correct solution to your problem. The key idea is to use a char for each individual BCD digit. Of course, a BCD digit only needs 4 bits and a char has at least 8 of them -- but you need char anyways for your ASCII representation and when your BCD digits are already in individual chars, all you have to do is indeed add '0' to each.
While at it: Converting to BCD by dividing and multiplying is a waste of resources. There's a nice algorithm called Double dabble for converting to BCD only using bit shifting and additions. I'm using it in the following example code:
#include <stdio.h>
#include <string.h>
// for determining the number of value bits in an integer type,
// see https://stackoverflow.com/a/4589384/2371524 for this nice trick:
#define IMAX_BITS(m) ((m) /((m)%0x3fffffffL+1) /0x3fffffffL %0x3fffffffL *30 \
+ (m)%0x3fffffffL /((m)%31+1)/31%31*5 + 4-12/((m)%31+3))
// number of bits in unsigned int:
#define UNSIGNEDINT_BITS IMAX_BITS((unsigned)-1)
// convert to ASCII using BCD, return the number of digits:
int toAscii(char *buf, int bufsize, unsigned val)
{
// sanity check, a buffer smaller than one digit is pointless
if (bufsize < 1) return -1;
// initialize output buffer to zero
// if you don't have memset, use a loop here
memset(buf, 0, bufsize);
int scanstart = bufsize - 1;
int i;
// mask for single bits in value, start at most significant bit
unsigned mask = 1U << (UNSIGNEDINT_BITS - 1);
while (mask)
{
// extract single bit
int bit = !!(val & mask);
for (i = scanstart; i < bufsize; ++i)
{
// this is the "double dabble" trick -- in each iteration,
// add 3 to each element that is greater than 4. This will
// generate the correct overflowing bits while shifting for
// BCD
if (buf[i] > 4) buf[i] += 3;
}
// if we have filled the output buffer from the right far enough,
// we have to scan one position earlier in the next iteration
if (buf[scanstart] > 7) --scanstart;
// check for overflow of our buffer:
if (scanstart < 0) return -1;
// now just shift the bits in the BCD digits:
for (i = scanstart; i < bufsize - 1; ++i)
{
buf[i] <<= 1;
buf[i] &= 0xf;
buf[i] |= (buf[i+1] > 7);
}
// shift in the new bit from our value:
buf[bufsize-1] <<= 1;
buf[bufsize-1] &= 0xf;
buf[bufsize-1] |= bit;
// next bit:
mask >>= 1;
}
// find first non-zero digit:
for (i = 0; i < bufsize - 1; ++i) if (buf[i]) break;
int digits = bufsize - i;
// eliminate leading zero digits
// (again, use a loop if you don't have memmove)
// (or, if you're converting to a fixed number of digits and *want*
// the leading zeros, just skip this step entirely, including the
// loop above)
memmove(buf, buf + i, digits);
// convert to ascii:
for (i = 0; i < digits; ++i) buf[i] += '0';
return digits;
}
int main(void)
{
// some simple test code:
char buf[10];
int digits = toAscii(buf, 10, 471142);
for (int i = 0; i < digits; ++i)
{
putchar(buf[i]);
}
puts("");
}
You won't need this IMAX_BITS() "magic macro" if you actually know your target platform and how many bits there are in the integer type you want to convert.