Mistake in custom implementation of printf - c

Question

I'm trying to implement my own version of printf and I'm having trouble when I need to print an argument of the form %pd, where p is the number of characters to be printed.
#include <stdio.h>
#include <ctype.h>
#include <stdarg.h>
#include <stdlib.h>
void print(char *, ...);
char *convert(unsigned int, int);
char *to_lower(unsigned int, int);
unsigned int count_digits(unsigned int);
int main()
{
char str[12]="World";
char c='A';
int n = 5, x= 1011, y = 12, z = 5, t = 10;
print("Hello s c |%s| |%c| 5 |%d| 5 some random %X %x %b %o words\n", str, c, n, x, y, z, t);
print("%5d\n", y);
}
void print(char *format, ...)
{
int num, precision = 0, nDigits, i, temp;
char *num_deca;
va_list lst;
va_start(lst, format);
while(*format != '\0')
{
if(*format != '%')
{
putchar(*format);
format++;
continue;
}
format++;
if(isdigit(*format))
{
temp = atoi(format);
precision = precision*10+temp;
//precision = atoi(format);
format++;
continue;
}
// format++;
switch(*format)
{
case 'c':
putchar(va_arg(lst, int));
break;
case 'd':
num = (va_arg(lst, int));
//nDigits = count_digits(num);
if(num > 0)
{
num_deca = convert(num, 10);
nDigits = count_digits(num);
for(i = 0; i < (precision - nDigits); i++)
{
putchar(' ');
}
fputs(num_deca, stdout);
// fputs(convert(num,10), stdout);
}
else
{
num = -num;
putchar('-');
fputs(convert(num,10), stdout);
}
break;
case 's':
fputs(va_arg(lst, char *), stdout);
break;
case 'X':
num = va_arg(lst, int);
fputs(convert(num,16), stdout);
break;
case 'x':
num = va_arg(lst, int);
fputs(to_lower(num,16), stdout);
break;
case 'b':
num = va_arg(lst, int);
fputs(convert(num,2), stdout);
break;
case 'o':
num = va_arg(lst, int);
fputs(convert(num,8), stdout);
break;
}
format++;
}
va_end(lst);
}
char *convert(unsigned int num, int base)
{
static char Representation[]= "0123456789ABCDEF";
static char buffer[50];
char *ptr;
ptr = &buffer[49];
*ptr = '\0';
do
{
*--ptr = Representation[num%base];
num /= base;
}while(num != 0);
return(ptr);
}
char *to_lower(unsigned int num, int base)
{
static char Representation[]= "0123456789abcdef";
static char buffer[50];
char *ptr;
ptr = &buffer[49];
*ptr = '\0';
do
{
*--ptr = Representation[num%base];
num /= base;
}while(num != 0);
return(ptr);
}
unsigned int count_digits(unsigned int n)
{
unsigned int counter = 0;
while(n != 0)
{
n = n/10;
counter++;
}
return counter;
}
Output: Hello s c |World| |A| 5 |5| 5 some random 3F3 c 101 12 words
d
So, the basic cases are working fine, the only issue I'm encountering is on the second print. The number 12 with %5d% should be whitespacewhitespacewhitespace12 (since 12 only has 2 digits). I computed the number of digits of the argument and used a for loop that goes until the difference between the precision and the number of digits, but then the program doesn't consider d as being part of the format specifier anymore and just prints the letter 'd'. Any idea why?

Answer 1

The problem is that you print every character until you find a '%'. If you find a '%' you go to the next character and process it. However, if the next character is a digit, you modify precission and make a 'continue' to the start of the loop. When re-starting the loop, the next character (a 'd') is not a '%', so it just get printed as is. You should have a variable that tells you if you are still processing a sequence, or either don't go to the start of the loop before you end the processing of the full sequence ('%5d').